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Technical Description of OBD-II Trouble Code
Malfunction in the Control Circuit of the 1-4 Upshift (Skip Shift) Lamp
What does that signify?
This diagnostic trouble code (DTC) is a generic powertrain code that is applicable to various OBD-II vehicles (1996-newer). This includes vehicles from Chevrolet, Mercedes-Benz, BMW, Ford, GMC, and others. Although it is a generic code, the specific repair steps may vary depending on the year, make, model, and powertrain configuration.
If your vehicle has stored a code P0804, it means that the powertrain control module (PCM) has detected a malfunction in the transmission upshift (also known as skip shift) lamp control system.
The automatic transmission upshift control solenoid is used in vehicles where the transmission can be manually shifted through the gear range by moving the shift lever in a single direction. This feature is particularly popular in high-performance or sports cars. Since the shifter only needs to be moved slightly, an electronic solenoid is required to send a shift signal to the TCM and achieve the desired gear change. The skip shift lamp is illuminated when the driver selects the skip shift function instead of the conventional automatic shifter. It may also illuminate or flash if a problem is detected.
The transmission control module (TCM) may be a standalone unit, but it is commonly 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 upshift lamp control circuit, a code P0804 will be stored and a malfunction indicator lamp (MIL) will illuminate.
What is the severity of this DTC?
A stored code P0804 indicates that a malfunction has been detected in the upshift lamp control circuit. Since the upshift (or skip shift) system is an auxiliary or high-performance shifter feature, it is not a critical transmission control system, and a code P0804 should not be considered severe.
What are some of the symptoms of the code?
Symptoms of a P0804 trouble code may include:
What are some of the common causes of the code?
Possible causes for this code may include:
What are some P0804 troubleshooting steps?
When diagnosing automatic transmission codes, it is important to start by checking that the transmission fluid is clean and at the correct level. If the fluid level is low, locate and repair the source of the leak. Refill the transmission with the recommended fluid and proceed.
If the fluid is excessively dark and has a strong burnt friction material odor, it will need to be replaced. This can be done by removing the transmission pan (and draining the torque converter) or by flushing the transmission (recommended). In either case, the filter should also be replaced. When removing the pan, be careful to preserve any debris that may have accumulated. If the pan is relatively free of debris, it can be assumed that the clutches are mechanically intact. If there is a significant amount of friction material in the pan (and stuck to the magnet), the transmission will require rebuilding by a qualified technician.
To diagnose a code P0804, you will need a diagnostic scanner, a digital volt/ohmmeter (DVOM), and vehicle-specific diagnostic information.
Consult your vehicle information source to find a technical service bulletin (TSB) that matches the vehicle’s year, make, and model, as well as the engine size, stored codes, and exhibited symptoms. If you find one, it may provide helpful diagnostic information.
With the transmission filled to the appropriate level with the recommended fluid, 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, the code is intermittent and may be more difficult to diagnose. In such cases, the conditions that led to the code being stored may need to worsen before an accurate diagnosis can be made.
Step 2
If the code is immediately reset, the next step in 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 skip shift indicator lamp. If voltage and ground are detected, use the DVOM to test the lamp bulb.
Step 4
If no skip shift components have failed, use the DVOM to test the system circuits. The voltage drop testing method is effective for this task. Disconnect all controllers from the circuit before testing resistance with the DVOM.
Professional Service
While we always recommend seeking mechanical solutions for this issue, if that is not possible, we offer the option to permanently delete this specific DTC. All other codes will continue to function. If you are interested in this option, please upload your ECU file to our portal.
