P0804: 1-4 Upshift (Skip Shift) Lamp Control Circuit Malfunction

What Does The P0804 Code Mean?

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 called skip shift) lamp control system.

The automatic transmission upshift control solenoid is used in vehicle applications where the transmission may be manually shifted through the gear range by pushing or pulling the shift lever in a singular direction. This feature is especially popular in high-performance or sports cars. Since the shifter only needs to be moved slightly, an electronic solenoid is required to input a shift signal to the TCM and effect the desired gear change. The skip shift lamp is illuminated when the driver selects the skip shift function as opposed to the conventional automatic shifter. It may also be illuminated (or flash) if a problem is detected.

The transmission control module (TCM) may be stand alone unit but is most commonly integrated into a single housing with the engine control module (ECM). This is what 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) illuminated

What Are The Symptoms Of The P0804 Code?

Symptoms of a P0804 trouble code may include:

• Skip shift is partially or totally disabled
• Skip shift indicator lamp is inoperative, illuminated, or flashing
• The transmission may be placed in limp-in mode

What Are The Potential Causes Of The P0804 Code?

Causes for this code may include:

• Defective skip shift lamp bulb
• Shorted or open skip shift lamp control wiring or connectors
• Controller programming error

How Serious Is This P0804 Code?

A stored code P0804 indicates that an upshift lamp control circuit malfunction has been detected. 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 classified as severe.

How Can You Fix The P0804 Code?

When diagnosing automatic transmission codes, you should begin by making sure that the transmission fluid is clean and that the transmission if filled to the appropriate 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 smells strongly of burnt friction material, it will need to be replaced. This can be accomplished by removing the transmission pan (and draining the torque converter) or flushing the transmission (recommended). Either way, you will want to replace the filter at this time too. When removing the pan from the transmission, take care to preserve whatever debris may have landed therein. If the pan is relatively clear of debris you can assume that 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 a rebuild from a qualified technician.

A diagnostic scanner, a digital volt/ohmmeter (DVOM), and a source of vehicle specific diagnostic information will be required to diagnose a code P0804.

You may use your source of vehicle information to locate a technical service bulletin (TSB) that matches the vehicle year, make, and model; as well as the engine size, code/s stored, and symptoms exhibited. If you find one, it could yield 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 pertinent freeze frame data. It is a good idea to write this information down before clearing the codes 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 much more difficult to diagnose. If this is the case, the conditions which contributed 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 of your diagnosis will require that you search your vehicle information source for diagnostic flow-charts, connector pin-out charts, connector face views, and component testing procedures/specifications.

Step 3

Use the DVOM to test 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 system circuits. The voltage drop testing method works well for this task. Disconnect all controllers from the circuit prior to testing resistance with the DVOM.

Note: Test skip shift system fuses with the circuit loaded to avoid misdiagnosis

Recommended Parts

Below are some recommended auto parts to help you address the trouble code affecting your vehicle and get it running smoothly again:

Note: During the purchasing process, please check carefully whether the part you want to buy fits your car!

Reference Sources

P0804 1-4 Upshift (Skip Shift) Lamp Control Circuit Malfunction, OBD-Codes.