Technical Description of OBD-II Trouble Code
Malfunction in the Reverse Inhibit Control Circuit
What does that signify?
This diagnostic trouble code (DTC) is a generic powertrain code that is applicable to many OBD-II vehicles (1996-newer). This includes vehicles from various manufacturers such as Subaru, Dodge, Buick, Cadillac, Peugeot, Chevrolet, Toyota, etc. Although it is a generic code, the specific repair steps may vary depending on the year, make, model, and powertrain configuration of the vehicle.
If your vehicle has stored a code P0801, it means that the powertrain control module (PCM) has detected a malfunction in the reverse inhibit control circuit. Depending on the make and model of the vehicle, this code may pertain to either the transmission or transfer case.
The transmission control module (TCM) or transfer case control module (TSCM) may be a separate unit, but it is commonly integrated into a single housing with the engine control module (ECM). This combined unit is known as the PCM.
The PCM uses input signals from various engine and transmission sensors to calculate the automatic transmission shift strategy. The reverse inhibit switch is designed to prevent the transmission from being shifted into reverse while the vehicle is moving forward. Typically, an electronically controlled solenoid is responsible for activating a plunger that prevents the reverse shifter pawl from engaging the reverse gear selector when the transmission is already engaged in a forward gear.
If the PCM detects a level of voltage in the reverse inhibit circuit that exceeds the maximum allowable parameters, it may store a code P0801 and illuminate the malfunction indicator lamp (MIL).
What is the severity of this DTC?
The presence of a stored code P0801 indicates that the reverse inhibitor may be disabled. It is important to rectify any conditions that have contributed to the storage of this code as quickly as possible.
What are some of the symptoms of the code?
Symptoms of a P0801 trouble code may include:
What are some of the common causes of the code?
Possible causes for this code may include:
What are some P0801 troubleshooting steps?
When diagnosing automatic transmission codes, it is important to start by ensuring that the transmission fluid is clean and at the appropriate level. If the fluid level is low, locate the source of the leak and repair it. Refill the transmission with the recommended fluid and proceed with the diagnosis.
To diagnose a code P0801, you will need a diagnostic scanner, a digital volt/ohmmeter (DVOM), and access to vehicle-specific diagnostic information.
You can use your source of vehicle information to find a technical service bulletin (TSB) that matches the year, make, and model of your vehicle, as well as the stored codes and exhibited symptoms. If you find a relevant TSB, it may provide helpful diagnostic information.
With the transmission filled to the appropriate level with the recommended fluid, you can proceed with the first step of the diagnosis.
Step 1
Use the scanner (connected to the vehicle diagnostic connector) to retrieve all stored codes and relevant freeze frame data. It is advisable to write down this information before clearing the codes and then test-drive the vehicle until the PCM either enters readiness mode or the code is reset.
If the PCM enters readiness mode at this time, it indicates that the code is intermittent and may be more challenging to diagnose. In such cases, the conditions that led to the storage of the code may need to worsen before an accurate diagnosis can be made.
Step 2
If the code is immediately reset, the next step of the diagnosis will involve searching your vehicle information source for diagnostic flowcharts, connector pin-out charts, connector face views, and component testing procedures/specifications.
Step 3
Use the DVOM to test the voltage and ground circuits at the reverse inhibit solenoid and sensor (if equipped). If voltage and ground are detected, test the reverse inhibit solenoid and sensor according to the manufacturer’s specifications. Any components that fail to meet the recommended specifications should be considered defective. If the components are functional, test the signal circuit that is input to the PCM.
Step 4
If the reverse inhibit solenoid and sensor are functional, and the signal input is present, use the DVOM to test the system circuits. The voltage drop testing method is effective for this task. Before testing the resistance with the DVOM, disconnect all controllers from the circuit.
