1-4 Upshift (Skip Shift) Lamp Control Circuit Malfunction
The P0804 code indicates a malfunction in the control circuit of the 1-4 upshift (skip shift) lamp. This system is designed to improve fuel economy by forcing the transmission to skip from 1st to 4th gear under certain light-load conditions. The lamp illuminates to inform the driver that the skip shift is active. A circuit malfunction means the PCM has detected an issue with the lamp circuit, such as an open, short, or incorrect voltage.
1. Symptoms You Will Notice
- Check Engine Light illuminated
- Skip shift lamp does not illuminate when expected
- Skip shift lamp stays on continuously
- Transmission may not skip shift as designed
2. Most Common Causes
- Faulty skip shift lamp bulb or LED
- Open or shorted wiring in the lamp circuit
- Damaged connector or poor electrical connection
- Failed PCM (rare)
3. Step-by-Step Diagnosis
Step 1: Use a scan tool to retrieve all stored codes and freeze frame data.
Step 2: Visually inspect the skip shift lamp bulb and socket for damage or corrosion.
Step 3: Check wiring and connectors between the PCM and the lamp for opens, shorts, or poor connections.
Step 4: Test the lamp circuit voltage and ground with a multimeter to verify proper operation.
Step 5: If all checks pass, suspect a faulty PCM and perform further diagnostics.
4. Frequently Asked Questions
Is it safe to drive with this code?▼
Yes, it is generally safe to drive, but the skip shift feature may not work correctly, potentially affecting fuel economy. The check engine light may be on.
How much does it cost to fix?▼
The cost typically ranges from $50 to $150, depending on the cause. Replacing a bulb or repairing wiring is inexpensive, while PCM replacement can be more costly.
5. Commonly Affected Makes
This is a generic OBD2 code, potentially applicable to any manufacturer using this diagnostic standard.