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Error Code P0804 1-4 Upshift (Skip Shift) Lamp Control Circuit Malfunction. It 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, BMW, Chevrolet, Ford, GMC, and Mercedes-Benz. Specifications on the definition, troubleshooting, and repairs, of course, vary between make, model, and powertrain configuration.
If the vehicle stores the Error Code P0804, this means the PCM (powertrain control module) has determined a malfunction in the transmission upshift (also referred to as skip shift) solenoid control system.
The automatic transmission upshift control solenoid is used for vehicles where the transmission can be manually shifted using the gear range by either pushing or pulling the shift lever in a singular direction. This is especially common among high-performance cars or sports cars. Since the shifter only needs to be slightly moved, the electronic solenoid is required to input a shift signal to the PCM and effect the desired gear range.
The TCM (transmission control module) and TCCS may be stand-alone units, but they are commonly integrated into a single housing with the ECM (engine control module), and this is what is usually referred to as PCM.
If the PCM determines a malfunction in the upshift lamp control circuit, then this error code will be stored and simultaneously activate the malfunction indicator lamp (MIL).
- Partially or totally disabled skip shift
- Inoperative, illuminated, or flashing skip shift indicator lamp
- Transmission may be in limp-mode
- Faulty skip shift lamp bulb
- Open or shorted skip lamp control wiring or connectors
- Programming error in the controller
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.
If the fluid is dark and smells strong of burnt friction material, then it must be completely replaced. You can remove the transmission pan to drain the torque converter or flush the transmission (recommended). You need to replace the filter as well.
When removing the pan from the transmission, preserve whatever debris may have landed therein. If the pan is clear of debris, then you can assume that the clutches are still mechanically intact. If there’s a lot of friction material present in your pant (and stuck to the magnet), you will have to take the vehicle to a qualified technician to rebuild the transmission.
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
- Adjustment or replacement of the solenoid
- Check circuit continuity and resistance between switch and PCM, and repair as needed
- PCM replacement (rare)
Though this code is not always considered serious, the problem must be addressed as soon as possible, as it could lead to other problems if ignored for too long.