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For a long time, fault codes were regarded as a panacea to solve electronically controlled vehicles by many of our maintenance personnel. Whenever there is a fault in the electronic control vehicle, try to call out the fault code. With the trouble code, it seems that the car has been repaired halfway. On the contrary, people often don't know how to start. For a time, many maintenance books about the interpretation of fault codes of various imported cars, the search methods of fault codes, the detailed location of diagnosis seats and other information have become the focus of many maintenance personnel. There are even more rumors in the industry. For example, an auto mechanic has the same manual maintenance strategy as Wulin cheats. Any intractable diseases can be overcome as long as they are turned over, and future generations are fortunate enough to see them occasionally. In fact, the cheats are just a manual fault code set accumulated over the years. In recent years, with the influx of imported cars and the introduction of foreign advanced models by domestic joint venture automobile enterprises, many automobile maintenance enterprises are upgrading from hardware to software, and the maintenance technology of modern electronically controlled vehicles is no longer as unfathomable as before. Various diagnostic instruments and decoders are equipped by many auto repair manufacturers, especially some special after-sales service centers, and a lot of money has been spent on advanced equipment such as original diagnostic instruments such as TECH 2 of SGM, VAG 1552 of SVW and CONSULT of Nissan. With these devices, the retrieval, identification and interpretation of fault codes are not so mysterious, but in actual production, our maintenance work is not easy because of this reason, and even strange things make the maintenance work more complicated because of fault codes. How to face the trouble code? The author will take Shanghai Buick as an example to introduce some knowledge about fault codes and related diagnostic ideas. $-Yt|W
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Buick series cars produced by Shanghai General Motors are equipped with the second generation on-board diagnosis system, namely OBD-II (on-board diagnosis). Due to Buick's pure American ancestry, SGM's diagnosis system is basically the same as other cars produced or sold in North America after 1996, except that it is not equipped with a dioxygen sensor, so it cannot perform the efficiency monitoring function of three-way catalytic converter (mainly limited by domestic use conditions). j(Z5 % 4o & lt； K
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Representation method and type of DTC. d8IP @
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Like the early OBD-I, if a vehicle fault is detected, the powertrain control module will store the corresponding fault code DTC in the memory. Because OBD is a standard regulation required by the federal government of the United States, the fault codes of on-board diagnostic systems that meet this requirement will be standardized in a nearly consistent format, which also enables the DTC system to develop continuously, and the updated OBD-III will be launched soon. SHLyBw{J
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On vehicles equipped with OBD-Ⅱ system, all fault codes DTC begin with English letters, followed by four digits. Such as P0 10 1, C 1234, B2236 and so on. sgmu8`wY
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The letter at the beginning of DTC indicates the monitored fault system: P is the power system; B is the body system; C is the chassis system; U is the fault code of the network or data communication transmission system. The first number is a general code (applicable to all vehicle manufacturers) or a manufacturer-specific code. For example, 0 represents a generic code, and 1 represents a manufacturer-specific code. General Motors has a specific number type code to help you diagnose the technical condition of your vehicle. t 1p2，0xoc\
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The second number indicates the affected fault system type, and the digital range of fuel and air metering system is1-7:1; 2. Fuel and air metering system (especially the loop function of injection system is poor); 3. Ignition system or cylinder missing monitoring system; 4 is the auxiliary discharge system; 5 is the speed control and idle speed control system; 6 is the computer output line system; 7 is the gearbox. O0(5/#x5WM
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The last two digits indicate the faulty parts or parts in the system, which is not enough here. kn3DS@Fvt！ J 1C8")3B
Diagnostic code type DTC fz5EZw_Ic
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According to the requirements of OBD II protocol, the diagnostic execution instruction classifies DTC into different types according to the monitoring system and the severity of the fault. Different types have different requirements for setting codes. The diagnostic execution instruction will only light the engine fault lamp MIL when there is a DTC related to emissions (which may exceed 1.5 times FTP). Diagnostic trouble codes are divided into four categories, including A, B, C and D, in which the diagnostic trouble codes of A and B are related to the engine emission level. m:7iJ； my
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Class A: Monitor the diagnosis in the first trip, report "Fault Detected" to the diagnosis execution instruction, store DTC and light the MIL lamp. eV，rrdNY
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Class B: Run the diagnostic test related to emission level in the second continuous trip, report "Fault Detected" to the diagnostic instruction, store DTC and light MIL. After the fault is detected for the first time, the class B fault code will enter the ready state, and then the class B fault code will be in the alert state or ready to store a history code. If the fault occurs again, make the MIL light on; On the contrary, passing the test will make the system out of the alert state of Class B DTC. Some special conditions will be used for misfire and fuel regulation fault codes DTC. N3peS
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Class C: Run the non-emission related diagnostic test in the first trip, report "Fault Detected" to the diagnostic execution instruction, and light the vehicle maintenance light "Repair the vehicle as soon as possible" (if equipped) when storing DTC. (:%|，
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Class D: Run the non-emission related diagnostic test in the first trip and report "Fault Detected" to the diagnostic execution instruction. Class D will store DTC without lighting MIL lamp. These DTC will be very beneficial to the maintenance and diagnosis of vehicles, especially when the driver points out that some performance is degraded and the MIL is not on. (“o[-，t
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Correctly understand the setting conditions of DTC.
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The history of fault codes in electronic control system is the history of continuous improvement of modern automobile computer-controlled self-diagnosis system. From the initial simple monitoring of the line voltage of the input and output elements, for example, when the low potential of the monitored voltage is in a short circuit state, the high potential of the monitored voltage is in an open circuit state, and the sudden change of the line voltage exceeds the voltage threshold set inside the self-diagnosis system, the self-diagnosis system sets the corresponding fault code according to the line port corresponding to the monitored voltage and the code number originally set in the rom corresponding to the fault symptom. Due to the limitation of microprocessor's information processing ability, the early self-diagnosis system can only identify or set a small number of fault codes, and the content of fault codes is limited to the open circuit and short circuit of the line, the loss and incompleteness of the signal, the abnormal change of the current of the working execution element, etc. Because the fault content is intuitive, it is not difficult to solve it by using a multimeter after reading the fault information. In addition, the early imported electronically controlled vehicles have just entered China, and the maintenance information is relatively small. Therefore, at that time, how to obtain the diagnostic trouble code and find the meaning of the diagnostic trouble code was indeed the most important task in the maintenance of electronically controlled vehicles. emz)-'EaZ
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With the great progress of microelectronics and computer technology, as well as the strict requirements of countries for engine emission level, the self-diagnosis ability of modern electronically controlled vehicles is getting stronger and stronger. In other words, the monitoring requirements, quantity and control accuracy of electronic control system will be greatly improved, and the types and quantity of fault codes will also increase. Taking Shanghai GM Buick as an example, the number of fault codes of the optical powertrain (engine and gearbox) is about 90, so many fault codes make the description of fault content more detailed and complete. This is helpful for automobile maintenance technicians to diagnose vehicles. However, due to the different requirements of various monitoring and diagnosis systems for control accuracy and diagnosis, the content of fault codes is no longer limited to simple expressions such as high voltage, low voltage or no signal, and new fault codes such as long-term over-concentration of fuel fine-tuning system, degradation of MAP performance and position deviation of EGR system appear, which are difficult to understand at first glance. Moreover, the setting of these faults often has its specific conditions and special requirements of the monitoring system. According to the author's experience, it is very important to read the maintenance manual carefully after retrieving the fault diagnosis code, and find out the corresponding fault information, fault setting conditions and strategies after fault setting. For example, when the vehicle speed is greater than 8km/h, TPS is greater than 10% and A/C is turned off, a certain fault condition is set. If you stop, idle and turn on the air conditioner to find the fault, you may never be able to reproduce it. The consequence of this is often that after you think the maintenance is over, you hastily clear the fault code (and the fault code does not appear again under the conditions at that time) and hand over the vehicle to the customer. Unfortunately, it didn't take long for the car to return to the pit, and the fault code mysteriously appeared again. Such maintenance cases can be said to be numerous. In fact, the trouble codes that are difficult for many maintenance personnel to understand do not appear after maintenance, and the situation that the vehicle reappears immediately after being taken away by the customer is also caused by the unclear conditions for setting the trouble codes. Personally, I have seen the maintenance case of replacing three oxygen sensors 1 week continuously because of the fault code of related oxygen sensors, the maintenance case of replacing EGR valve and PCM because of the fault code of an EGR system, and even the maintenance case of replacing almost the entire engine harness. CDlw_U5Dc
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The significance of carefully checking the setting conditions of DTC lies in: as an automobile maintenance worker, you must know the actual time when the fault of this car occurred. What triggered the fault code? How do I simulate fault conditions? K J8B} {
How to determine that the fault has really been eliminated *qC ka! z
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The fault code tells you that a certain system of the car has a certain fault now or in the past, so as long as the fault code does not appear after maintenance, it means that the fault has been eliminated. But remember, the fault code can disappear because the fault does not exist. Similarly, if the fault monitoring system program is suspended due to certain conditions, the fault code will not appear. Take the OBD-Ⅱ system of Shanghai GM Buick as an example. It takes at least 12 minutes to complete all the projects of the whole system, and within this 12 minutes, it must be carried out according to the detection requirements of OBD-II: idle speed, acceleration, cruise, re-acceleration, high-speed cruise again, non-braking taxi and other specific working conditions. This continuous working condition detection is called OBD-II driving cycle or 15 working condition. Only after such a driving cycle, all the monitors are started and passed the detection, and I/M marks are recorded in the control module, indicating the detection situation of the whole system. Due to personal driving habits, road conditions and other reasons, if there is no monitor to start the system during the test run after maintenance, the fault code will naturally not appear. Therefore, after the repair work is completed, it is necessary to carry out trial operation according to the original code setting conditions and avoid various factors that limit the start of the monitor. If there is SGM TECH 2 diagnostic instrument, it is very necessary to observe the fault monitoring status by entering the specific DTC menu. Z fj{。 %
Maintenance check example: i|KdP)5Ga
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Example 1: Buick GL8 commercial vehicle cannot accelerate, with a top speed of 80 km/h ... yzacoqds.
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A 200 1 Buick GL8 7-seater commercial vehicle came into the station for maintenance. The symptoms of customer failure are: normal idle speed but weak acceleration, the maximum throttle speed will not exceed 80 km/h, and the failure indicator light will sometimes flash. The repairman reads the PCM fault code as P0 13 1 with TECH 2 diagnostic instrument, that is, the voltage of the HO2S circuit is too low. According to the prompt of fault code and circuit diagram, the corresponding circuit of HO2S heated oxygen sensor is overhauled by multimeter, which eliminates the possibility of short circuit to ground due to low voltage of signal circuit. When idling, the signal of HO2S can change up and down at 450mV, but the change range is not large. The repairman suspected that the oil pressure was low and took the oil pressure gauge for testing. The idle oil pressure is about 2.7kg, and the oil pressure change is basically normal when the accelerator is empty. According to customer feedback, this is what happened after the car went to other places several times. Considering that the thermal oxygen sensor may be poisoned by adding lead or low-grade fuel, the repairman replaced the suspected thermal oxygen sensor. After replacement, the fault code stored in PCM is cleared by TECH 2. In order to confirm that the fault has been eliminated, run at idle speed and enter the DTC menu again, and no DTC will be displayed on the screen. Just one day later, the customer came back, complaining that the fault appeared again soon and the exhaust pipe would roar when the car accelerated. After receiving the car, the author read the fault code of PO 13 1 again, and for the sake of prudence, he also carefully consulted the diagnostic instructions of the fault code. 36JqcAug A
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1. Conditions for running DTC sWPV!
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* There are no fault codes DTCeGe}Y= for throttle position, intake manifold absolute pressure, IAT, engine coolant temperature, MAF, CKP sensor, misfire, fuel nozzle circuit, evaporative emission and EGR} y =
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* Closed-loop command air-fuel ratio is between 14.4 and 14.9; 8soXl Z]R '
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* Throttle opening is between 5% and 40%. DNQW-G(B)
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2. Conditions for setting DTC qTpzdE%-
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* During normal closed-loop operation, the signal voltage of HO2S is always lower than 175mV, or during fuel control operation in power enhancement mode (mixture enrichment), the signal voltage of HO2S is always lower than 600mV；; & amp-4wM_LG7f
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* Each condition lasts for 5s. }=-K)g '
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Judging from the DTC conditions of P0 13 1 above, it can be clear that there is basically no problem with the engine electronic control system of the automobile. Because, in order to start the HO2S monitor of Buick system to run the detection of DTC PO 13 1, the following working conditions must be met first: no throttle position, intake manifold absolute pressure, IAT, engine coolant temperature, MAF, CKP, evaporative emission, EGR, misfire and fault code of injector circuit; Moreover, I also found that the fault code was cleared after the last maintenance, and DTC did not reappear immediately, which made the maintenance personnel have the illusion that the fault was eliminated-to run the diagnosis of PO 13 1, it must be in a non-idle state. In order to further understand the working condition of the engine when the fault occurs, check the 18 engine parameters when setting the fault code from the frozen fault status item of TECH 2 (Note: the frozen frame is also the requirement of OBD-II, which records the relevant engine parameters when DTC is stored, which is convenient for maintenance personnel to check). Among them, the throttle opening is 88%, the engine speed is 2478r/min, the engine load is 90%, and the vehicle speed is 75 km/h. These parameters just confirm the symptoms described by customers, such as weak acceleration, poor acceleration and MIL flashing. In order to get the most authentic data as possible, please connect the fuel pressure gauge and TECH 2 diagnostic instrument and test drive with the customer. When the speed is lower than 60km/h, the performance of this GL8 is ok, but when trying to overtake quickly or climb a slightly steep slope, the vehicle will show fatigue. No matter how the customer refuels the valve, the speed just won't rise. On the slope of the last bridge, the pointer of the fuel pressure gauge fluctuates constantly and can only be maintained at around 1.5 ~ 2 kg. TECH 2 powertrain data shows that the throttle opening is 84%, the vehicle speed is 70km/h, the engine speed is 2 100r/min, and the HO2S is continuously lower than 150mV. The fault symptom reappears. According to these data, when the engine is running under heavy load, the fuel pressure of the system can't meet the requirements, and the oil pressure is too low, which leads to the lean mixture, which triggers the condition that the HO2S monitor sets PO 13 1. After returning to the maintenance station and replacing the fuel pump assembly and oil filter, retest and check.
Enter the specific DTC menu of the diagnostic instrument and display P0 13 1 fault code passed the test, and the whole maintenance work is over. bdn#W3|u
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Example 2. 2 gearbox. Buick GLX car is impacted by gear shifting, and its fuel consumption is high. ，k & gtRz0-
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A Buick GLX car, when shifting gears, the gearbox obviously feels the car stall, which has a serious impact on shifting gears and increases fuel consumption. By reading the DTC, it shows that the fault is P 1860, that is, the circuit fault of the torque converter clutch pulse width modulation (TCC PWM) solenoid valve. Because the fault indication is an obvious fault of the electronic control system of the gearbox, some conventional mechanical detection steps such as automatic gearbox pressure test and stall test are omitted, and customers say that all electrical components in the gearbox, such as control harness, TCC solenoid valve, shift solenoid valve and even powertrain harness, have been replaced, so the fault diagnosis seems to be deadlocked. I found the explanation about setting this DTC again, hoping to find the answer from it. wPCeK{ 1
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1. Conditions for running DTC /4t? h
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* the system voltage is 9 ~18v; [Yf NEFD！
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* The engine speed is higher than 500 rpm for 5s, and the fuel is not disconnected. ^rL8Dl%3
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*PCM indicates that the solenoid valve is opened to a duty ratio above 90%, and the high voltage (b+) is maintained; Jq natural gas hydrate
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*PCM instructs the solenoid valve to open the duty ratio less than 10% and keep the low voltage (0v); & amp& gtZ-#D"mk
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* One of the above conditions exists for at least 4.3 seconds. x=LEs+
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In order to reproduce the fault code again, the maintenance personnel cleared DTC P 1860 previously stored in PCM and turned off the ignition switch unintentionally. When I turned on the ignition switch again, the TECH 2 diagnostic instrument that lost communication returned to the original interface for diagnosing fault codes, and the original prompt without fault codes suddenly turned into fault codes P 1860. Didn't the fault code be completely cleared just now? The author repeatedly cleared DTC. Strangely, as soon as the ignition switch is turned on and the engine does not start, the fault code appears. How is this inconsistent with the operating fault code P 1860 described in the maintenance manual? The manual can't be wrong, and obviously, it should be meaningless to monitor the TCC system of automatic transmission when the engine is not started. Is there a problem with the powertrain control module PCM? It happened that a big accident car was undergoing sheet metal repair, so I borrowed its PCM to try to troubleshoot it. Moreover, after the PCM found the fault code setting, it took some countermeasures, such as forbidding the transmission to shift into 4th gear, forbidding the TCC to work, and freezing the shift adaptation. The problem of high fuel consumption mentioned by customers is naturally understood.