What does it mean?
P1278 is a Powertrain (engine, transmission, fuel system) fault code. It indicates: APP Sensor 1 High Voltage .
Symptoms
- The MIL is on and a reduced power warning is displayed
- The engine runs only at idle and ignores the pedal completely
- Limp mode, sometimes with the idle raised above normal
- Loss of acceleration while driving, forcing a stop
- Cruise control disabled
- Live data shows sensor 1 pinned at maximum with the pedal released
Likely causes
- Open circuit in the sensor ground so the signal is pulled up toward reference voltage
- Signal wire shorted to the 5 volt reference or to battery voltage
- Corroded ground terminal in the pedal connector adding resistance
- Internal sensor fault leaving the output at the top of its range
- Chafed loom allowing the signal to touch a powered circuit
- Water damage bridging the signal and reference pins
- Module input fault, considered after the harness is proven
How to fix it
- Check live data with the ignition on; sensor 1 at full scale with the pedal at rest indicates a high signal fault
- Unplug the pedal connector and check whether the reading drops, which points to the sensor rather than the harness
- Measure the sensor ground circuit for continuity and voltage drop back to the module
- Test the signal wire for a short to the 5 volt reference and for a short to battery voltage
- Inspect the connector for water, corrosion and terminals touching one another
- Follow the loom for chafe damage where it passes the bulkhead or moves with the pedal
- Repair the ground or the short, or replace the pedal sensor, then run the learn procedure and verify the signal returns to its rest value