What does it mean?
This diagnostic code (DTC) is a generic powertrain code and applies to many OBD-II vehicles (1996-newer). This may include, but is not limited to, vehicles from Chevrolet, Mercedes-Benz, BMW, Ford, GMC, etc. Although generic, the exact repair steps may vary depending on the year, make, model, and powertrain configuration.
If your vehicle has stored a P0804 code, it means that the Powertrain Control Module (PCM) has detected a malfunction in the shift lamp control system (also known as skip shift).
The automatic transmission shift control solenoid is used in vehicle applications where the transmission can be manually shifted within the gear range by pushing or pulling the gear lever in a single direction. This feature is particularly appreciated in high-performance or sports cars. Since the gear lever only needs to be moved slightly, an electronic solenoid is required to input a shift signal into the TCM and perform the desired gear shift. The shift indicator light illuminates when the driver selects the shift function as opposed to the conventional automatic gear lever. It may also be illuminated (or flash) if a problem is detected.
The Transmission Control Module (TCM) can be a standalone unit but is most often integrated into a single housing with the Engine Control Module (ECM). This is referred to as the PCM.
If the PCM detects a malfunction in the shift lamp control circuit, a P0804 code will be stored and a Malfunction Indicator Lamp (MIL) will illuminate.
How severe is this DTC?
A stored P0804 code indicates that a malfunction in the shift lamp control circuit has been detected. Since the upshift (or skip shift) system is an auxiliary or high-performance shifting function, it is not a critical transmission control system, and a P0804 code should not be classified as severe.
What are some of the symptoms of the code?
Symptoms of a P0804 fault code may include:
Skip shift is partially or fully disabled
The skip shift indicator light is inoperative, illuminated, or flashing
The transmission may be placed in limp mode
What are some common causes of the code?
Causes
of this code may include:
Faulty shift lamp bulb
Shorted or open shift lamp control wiring or connectors
Controller programming error
What are the troubleshooting steps for P0804?
When diagnosing automatic transmission codes, you should start by ensuring the transmission fluid is clean and the transmission is filled to the proper level. If the fluid level is low, locate the source of the leak and repair the condition. Refill the transmission with the recommended fluid and proceed.
If the fluid is excessively dark and has a strong burnt friction material smell, it will need to be replaced. To do this, remove the transmission pan (and drain the torque converter) or flush the transmission (recommended). In either case, you will also want to replace the filter at this time. When removing the transmission pan, be sure to save any debris that may have landed in it. If the pan is relatively free of debris, you can assume the clutches are mechanically intact. If there is an excessive amount of friction material in the pan (and stuck to the magnet), the transmission will require rebuilding by a qualified technician.
A diagnostic scanner, a digital volt/ohmmeter (DVOM), and a source of vehicle-specific diagnostic information will be necessary to diagnose a P0804 code.
You can use your vehicle information source to locate a Technical Service Bulletin (TSB) that matches the vehicle’s year, make, and model; as well as engine size, stored codes, and presented symptoms. If you find one, it may provide useful diagnostic information.
With the transmission filled to the proper level with the recommended fluid, proceed to the first step of diagnosis.
Step 1
Use the scanner (connected to the vehicle’s diagnostic connector) to retrieve all stored codes and relevant freeze frame data. It is a good idea to record this information before clearing the codes, then test drive the vehicle until the PCM enters readiness mode or the code resets.
If the PCM enters readiness mode at this point, the code is intermittent and may be much more difficult to diagnose. If this is the case, the conditions that contributed to the code being stored may need to worsen before an accurate diagnosis can be made.
Step 2
If the code resets immediately, the next step in your diagnosis will require you to search your vehicle information source for diagnostic flowcharts, connector pinout diagrams, connector face views, and component testing procedures/specifications.
Step 3
Use the DVOM to test the voltage and ground circuits at the shift indicator light. If voltage and ground are detected, use the DVOM to test the lamp bulb.
Step 4
If no skip shift component has failed, use the DVOM to test the system circuits. The voltage drop testing method works well for this task. Disconnect all controllers from the circuit before testing resistance with the DVOM.
Test the skip shift fuse system with the circuit loaded to avoid misdiagnosis