Basic knowledge of fault codes

In the fault diagnosis, we often need to read the fault code and diagnose according to the fault code. The following is an explanation of the relevant knowledge of the fault code:

PresentcarBoth provide fault self-diagnosis function. The principle of the self-diagnosis function is: the internal fault diagnosis circuit of the ECU can continuously monitor the signal of each input component of the electronic control system during the operation of the car. When the electronic component is found to be faulty, it can automatically start the fault operation program and store the fault in the form of code. In the RAM of the computer, and this phenomenon does not disappear for a period of time, the ECU judges that this part of the signal circuit is faulty. The ECU stores the fault in the internal random access memory in the form of a code, and at the same time lights up the fault indicator on the dashboard to remind the driver. The development and application of fault codes of automobile fault self-diagnosis system provides convenience for automobile drivers and maintenance personnel to discover and eliminate faults in time during automobile operation.car repairPersonnel can use the instrument or manual method to read the fault code, and generally can identify the possible cause and location of the fault.

1 Determination of fault code
The setting of any fault code of the automobile has certain conditions. When the self-diagnostic system detects that one or several signals exceed its set conditions, the ECU will determine the fault code. Generally, the method for determining the fault code of automobile ECU mainly includes the following methods:
1.1 Value range judgment method
When the input signal exceeds the specified value range, the self-diagnostic system will confirm that the input signal is faulty.For example: water temperaturesensorDesigned to be within the normal temperature range of 30℃～120℃, the output voltage is 0.3～4.7v, when the ECU detects that the signal is less than 0.15 or greater than 4.85v, it will determine the water temperaturesensorSignal short circuit, open circuit or sensor damage failure.
1.2 Time domain judgment method
When the ECU detects that a certain input signal has not changed within a certain period of time or the change does not reach a predetermined number of times, the self-diagnostic system determines that the signal is faulty. E.g:Oxygen SensorThe signal not only requires the change of signal voltage and voltage, but also the change frequency of the signal voltage must exceed a certain number of times in a certain period of time (such as some models require 8 times/10S), when it is less than this value, it will be generated Fault code means that the sensor response is too slow.
1.3 Function Judgment Method
When the ECU sends an instruction to the actuator, it detects the change of the output parameter of the corresponding sensor or feedback signal. If the output signal does not change according to the trend specified by the program, it is determined that there is a fault.For example: in some cars, the ECU sends out a command to open the exhaust gas recirculation (EGR) valve and detects the intake airPressure SensorWhether there is a corresponding change in the MAP output signal is used to determine whether the EGR valve is operating or not. If there is no change, it is considered that the EGR valve and circuit are faulty. Some cars, such as Buick, use EGR position sensors to determine the work of EGR.Another example, Toyota Camry’sIgnition systemThere are IGT and IGF signals. IGF is the ignition feedback signal, which is used to detect whether the ignition control signal IGT sent by the ECU has been executed. If the IGF detects that there is no ignition, the fault code will be memorized.
1.4 Logical decision method
The ECU compares the data of two or more interconnected sensors, and when it finds that the logical relationship between the two sensor signals violates the set conditions, it determines that one or both of them are faulty.For example: ECU detects the engineRotating speedGreater than 3000rmp, andThrottleThe output signal of the position sensor is less than 5%. For the engine, this relationship is impossible, and the ECU determines that theValveThe position sensor is malfunctioning.

2 Reading and clearing fault codes
2.1 Reading fault codes
There are two ways to read the fault code: one is the onboard method (which has been basically eliminated, but many models retain the manual reading method), and the other is the external device method.
The on-board reading method can read the fault code without using any external equipment. It is suitable for reading the fault code of the early on-board diagnostic system (OBD-1). This method usually needs to input a trigger signal to the computer first. After receiving this signal, it starts to flash and display the fault code, the pattern of flashing is the fault code of the memory. The meaning of the trigger signal and the fault code of the fault indicator lamp of each car system is different.For example, when a Toyota car reads the fault code, it is connectedIgnition Switch, Use the special diagnostic cable SST (or other wires) to checkConnectorConnect TE1 and E1 together, and read the fault code according to the blinking pattern of the fault indicator "CHECK" on the dashboard. The read fault code also needs to find the meaning of the fault code from the maintenance manual.
The external device method is to use a computerDiagnostic instrument(Commonly known asdecoder) To read the fault code, it is not only suitable for the first-generation on-board diagnostic system, but also for the second-generation on-board diagnostic system (OBD-2). The decoder is the only computer fault diagnostic instrument that can directly communicate information with the car computer. The fault code can be read under the decoder menu operation, and the meaning of the fault code can also be displayed in Chinese, without the need to search for a maintenance manual.
2.2 Clear fault code
There are 4 methods for clearing fault codes stored in the control computer: automatic clearing method, power-off method, external device method, and triggering program method.
Automatic clearing is after the fault has been completely eliminated, after the ignitionswitchWhen the opening and closing cycle is more than 50 to 80 times, and the fault does not appear again, the control computer will automatically clear the stored fault code.
The power-off method uses unplugging the power sourcefuseOr dismantleAccumulatorIt takes more than 10 seconds to eliminate the fault code memorized in the computer. This method is suitable for most models, but the self-applicable value in the ECU or the memory of other systems, such as anti-theft, audio password, etc., are cleared at the same time after the power is cut off.
External device method, using the external device decoder, can be automatically cleared according to the prompt after connection.
The trigger procedure method can be cleared according to the certain trigger mode and prescribed procedure steps in the maintenance manual.

3 Classification and identification of fault codes
There are two types of fault codes, historical fault codes and current fault codes. Historical fault code is also called intermittent fault code or soft fault code. It is a fault code that has not been cleared due to a fault that has occurred in the past but has not occurred at present. There are two situations in which historical fault codes are generated. One is that the fault has been eliminated, but the fault code is not cleared, and the fault code will not be generated again after it can be cleared; the other is that the fault has not been eliminated, but it has not occurred currently. After the fault code is cleared, the fault code will appear when the fault occurs again, so the fault code can only be completely cleared after the fault is completely eliminated.
The current fault code is also called the hard fault code, which is the fault code generated by the fault that is occurring; it is the fault that does exist at present, and the fault code also exists. It belongs to the current fault code generated by a continuous fault, and it will not be cleared.
The current fault code is a fault that does exist currently, and it is easier to judge. However, historical fault codes are more difficult to diagnose, because historical fault codes are only faults that have occurred in the past but not now. To reproduce the state of the fault, it may take a long time to capture the recurrence of historical fault codes or artificially create reproducible faults. Conditions such as heating, vibration, etc., and better equipment is needed to capture the changes of various data parameters at the moment of failure. Therefore, the current fault code is generally solved first, and the historical fault code is temporarily used as a reference for fault diagnosis. The historical fault code and the current fault code can be distinguished by the following methods:
3.1 First, use the instrument to read all the fault codes;
3.2 Clear all fault codes;
3.3 Test run (It should be emphasized that the test run is not to run the engine in situ after starting, but to conduct a road test. For certain fault codes, the specified working conditions must be carried out according to the design requirements.);
3.4 Read the fault code again
The fault code read for the second time is the current fault code. The fault codes in the first reading are historical fault codes.

4 Limitations of fault codes
Although the fault code can help the maintenance personnel to solve the fault in time, it is very simple to think that the fault code of the electronic control car can solve the various faults of the car. There is no need to have the principle structure of the electronic control car Too much mastery can also repair various modern electronically controlled cars is wrong, because the fault code has certain limitations.
4.1 The self-diagnostic system is difficult to play a role in the mechanical system
The self-diagnosis system generally can only monitor the faults of the electronic control system, but it is difficult for the self-diagnosis system of the mechanical system to play a role. When a large amount of natural wear, deformation, aging, damage, fatigue, and corrosion occur in various parts of the assembly or mechanism of the automobile, it is difficult for the self-diagnostic system to generate fault codes, nor can it play a diagnostic role.
4.1.1 Engine: The valve phase is abnormal, the cylinder pressure drops, and the air and fuel supply system is not well sealed.
4.1.2 Automatictransmission: The planetary gear mechanism works abnormally; the hydraulic control system is blocked, leaks, the pressure is incorrect, various valves work poorly, the shift actuator is poorly moved, etc.; the pump wheel, turbine and lock of the hydraulic torque converterclutchFailure, etc.
4.1.3 Electronically controlled actuators: various functional failures due to mechanical wear and tear, such as idle speed control valves, fuel injectors, and electric fuel pumps. For example: due to poor sealing of the engine intake pipe and poor sealing of the fuel supply system, the fuel pressure will be too low, which will cause engine "gasp" or poor acceleration failure. At this time, the self-diagnosis system can detect that the fuel pressure is too low, but it cannot Determine where the air intake line and fuel line are not properly sealed. When the idle speed control valve is unstable due to a mechanical failure, the fault self-diagnosis system cannot detect that the idle speed control valve is faulty.
4.2 Uncertain relationship between fault and fault code and fault phenomenon
When a car has a fault code, it will have different symptoms.Like a carCrankshaftThe position sensor has a fault and generates a fault code. Its fault phenomenon is very obvious, which directly leads to the failure of the car to start. However, some sensors have faults and generate fault codes, and the fault phenomenon is very unobvious. If the fault code of the intake air temperature sensor appears, it indicates that the line of the intake air temperature sensor or itself is faulty, but this fault is often difficult to find by feeling. Sometimes there is a fault code, but there is not necessarily a fault. This is mainly caused by external interference, misoperation of maintenance personnel, false faults and other influences.
Sometimes, when there is a fault phenomenon, there must be a fault, but there may not necessarily be a fault code. This is because the fault code is defined by the self-diagnostic system of the control computer, and any fault point that is not restricted by the computer control cannot generate a fault code. In addition, when there is a mechanical failure, the failure phenomenon is more obvious, but the self-diagnostic system does not necessarily generate a failure code. Therefore, a fault code does not necessarily have a fault, and no fault code does not necessarily have a fault.
4.3 The fault code is only a general reference to the fault, but not the specific location of the fault
The entire control system is composed of many subsystems (each sensor, actuator, power supply, and various parts of the computer circuit, etc.) circuits. The content of the fault code does not only mean that the sensor (or actuator) is faulty, but that the signal of the subsystem is abnormal. As for the abnormality, the cause may appear in any part of the subsystem— ——On the device, connector, line or computer.
For example, the fault code No. 22 of Toyota Camry refers to the fault of the water temperature sensor, but the fault scope should include: the fault of the water temperature sensor itself, the line fault between the water temperature sensor and the ECU, and the fault of the ECU itself and the abnormal reception and processing of the signal, so At this time, further diagnosis is required.For example: can be simulated by signalDetectorThe signal of the simulated water temperature sensor replaces the water temperature sensor to input the signal to the computer. If the working condition of the engine improves and the malfunction symptom disappears, it can be judged as a malfunction of the water temperature sensor; if the malfunction symptom does not improve, the computer can directly respondTerminalInput the signal, if the fault symptoms disappear, it means that the water temperature sensor causes the computer ECU wiring fault, otherwise, it can be judged that the ECU itself is faulty.
Therefore, the fault code only provides the maintenance personnel with the general direction for further inspection, it is only a general reference to the fault, and cannot and does not tell us where and what has failed. In order to really determine where and what the problem is, it is necessary to use other instruments or multimeters to further conduct circuit analysis and component testing based on the corresponding technical data (including circuit diagrams, device locations, standard values, etc.), and use your own expertise Knowledge can accurately judge and solve faults. This is why you should not think that you can fix the car after reading the trouble code. When using the fault code provided by the self-diagnostic system, it is necessary to comprehensively analyze the working principle of the engine, the fault phenomenon, the nature of the components and the design parameters, etc., to grasp the law of changes in the automotive technology status, and to correctly identify the locations and causes of various faults in modern vehicles. And get rid of it in time.

5. The specific meaning of the fault code
OBD fault codes usually start with P (powertrain), B (body), C (chassis), U (computer communication), followed by "1, 2, 3, 4, 5, 6, 7, 8 , 9, A, B, C, D, E, F" in the four digits. Generally speaking, OBD-II fault code consists of one letter and four numbers. For example, P0279, the following figure shows the meaning of each code:

Common abbreviations of OBD-II fault codes:
5.1 Circuit/Open Circuit
"Circuit/open circuit" is used most of the time when the detection of a fixed value or specific circuit high or low is not feasible. It can also be used with circuit low and high fault codes at the same time, provided that all three circuit conditions can be detected.
5.2 Scope/performance
"Range/Performance" means that the circuit is within the normal operating range, but is incorrect for the current operating conditions. It can be used to indicate stuck or distorted values, to show poor performance of circuits, components, or systems.
5.3. Low circuit
"Circuit low" means that the circuit voltage, frequency, or other characteristics measured at the input of the control module are below the normal operating range.
5.4 Circuit height
"Circuit high" means that the circuit voltage, frequency, or other characteristics measured at the input of the control module are higher than the normal operating range.
5.5. Row
"Row" refers to a combination of cylinders that share a common control sensor. The first row always contains cylinder 1, and the second row refers to the opposite row.
5.6 Location of the sensor
The "position of the sensor" is either relative to the engine air flow: from the intake system to the exhaust system, or relative to the engine fuel flow: fromtankTo the engine, follow the order of 1, 2, 3, etc. 7. Left/right and front/rear
The "left/right and front/rear" positions of the components assume that you are sitting in the driver's seat to watch.
5.7 A, B
If the component is represented by a letter (for example, A, B, C, etc.), this is a value defined by the manufacturer.
5.8 Intermittent/Unstable
"Intermittent/unstable" means that the signal is temporarily discontinuous, but the fault duration is not long enough to be considered as an open circuit or short circuit, or the rate of change is too high. Other: One more thing please note. The OBD-II system uses the hexadecimal system. It uses 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, A, B, C, D, E, F to count. For example, the next fault code of P029F is P02A0.