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Error Code P0802 is defined as Transmission Control System MIL Request Circuit/Open. Error Code P0801 is a generic trouble code, meaning it applies to all vehicles with the OBD-II system, especially those made from 1996 up to present and equipped with automatic transmission. Specifications on the definition and steps for repairs may vary depending on the model and make of the vehicle.
This includes vehicles from, but not limited to, Buick, Cadillac, Chevrolet, Dodge, Peugeot, and Toyota. Specifications on the definition, troubleshooting, and repairs, of course, vary between make, model, and powertrain configuration.
If the Error Code P0801 appears, this means the PCM (powertrain control module) has detected a malfunction in the reverse inhibit control circuit. Depending on the make and model of the vehicle, this code may either pertain to transmission or transfer case.
The PCM calculates the automatic transmission shift strategy by using input signals from multiple sources, such as the engine, transmission, and transfer case sensors. The reverse inhibit switch keeps the transmission from being shifted into reverse while the vehicle is running forward. Usually, there’s an electronically controlled solenoid responsible for the actuation of the plunger that prevents the reverse shifter pawl from engaging the reverse gear selector when the transmission is geared forward.
If the PCM determines a voltage that exceeds the maximum allowable parameters from the reverse inhibit circuit, it activates this code along with the malfunction indicator lamp (MIL).
Sometimes, this code may not exhibit any symptoms at all. The common symptoms, however, include:
- Shifter goes to reverse while the vehicle is running forward
- The shifter is unable to switch to reverse
- Blown fuses in transmission control-related circuits
- Faulty reverse inhibit solenoid
- Open or shorted reverse inhibit control solenoid
- Bad reverse inhibit sensor
- Defective or programming error in PCM
How to Check
Before diagnosing this code, make sure the fluid is clean, and the transfer case is filled to the appropriate level. If there is not enough fluid (level too low), then there must be a leak. Find the source of the leak and do the necessary repair. Then, refill the transfer case with the require fluid before proceeding with the diagnosis.
You will need a diagnostic scanner, DVOM (digital volt/ohmmeter), and specific diagnostic information source to diagnose this code (i.e., Technical Service Bulletin or TSB) that matches your vehicle make, model, as well as engine size, code/s stored, and symptoms of the problem.
After filling the transfer case with the appropriate fluid at the recommended level, you can now start with the diagnosis.
Retrieve all stored codes and the pertinent freeze frame data by connecting the diagnostic scanner. You can write the information down before clearing the codes and take the vehicle for a test-drive until the PCM either enters readiness mode or the code is reset.
If the PCM sets to readiness mode, the code must be intermittent. This can be more difficult to diagnose, as the condition that caused the code may need to worsen before you can proceed with the diagnosis.
If the code immediately resets, however, then you can get to the next step. Read your vehicle information source and familiarize yourself with the diagnostic flow-chart. Locate the connector pin-out charts, connector face views, and the specifications for the component testing procedures.
Test the voltage and ground circuits at the TCCS controller by using the DVOM. If you can detect both voltage and ground, use the DVOM to check the controller output. Test the components and sensors related to the specific symptoms. Any components that fail to comply with the recommended specifications are probably defective.
If the TCCS components are in good condition, then test the system circuits using the DVOM. You can use the voltage drop testing method for this step. Make sure you disconnect all controllers before testing for the resistance with your DVOM.
How to Fix
Common repairs for this code are:
- Replacement of reverse inhibitor switch
- Repair of damaged wiring for the switch
- Replacement of damaged shifter
When this code is present, that means the reverse inhibitor may be disabled. Conditions that caused this code must be addressed as soon as possible.