The accelerator pedal position (APP) sensor system consists of two independently scaled potentiometers (APP sensor 1 and APP sensor 2), that vary resistance according to the changes in the throttle position. The ECM measures the signal voltage from the two sensors, which are a reduced value of the ECM 5 volt reference. The voltage for APP sensor 1 and APP sensor 2 range from 0.4 - 3.8 (+/-.1) volts and 0.2 - 1.9 (+/-.1) volts, respectively. The APP sensor signal circuits are monitored continuously for an open, short to ground, or short to voltage condition. DTC P1278 will set if APP sensor 1 signal voltage at the ECM is above the calibrated voltage value.
DTC PARAMETERS
DTC P1278 will set if APP sensor 1 signal voltage at the ECM is above 4.82 volts when:
| • | Condition exists for longer than 140 milliseconds |
| • | Battery voltage is greater than 7.0 volts |
| • | A change in accelerator pedal position |
DTC P1278 diagnostic runs continuously with ignition On.
P1278 is a (type A) DTC
DIAGNOSTIC AIDS
To locate an intermittent problem, use the Scan tool to monitor APP SENSOR 1 with the ignition On, engine Off.
Wiggling wires while monitoring APP SENSOR 1 may locate the area where a short to voltage may exist.
If a circuit fault is detected with APP sensor 1, the ECM will use APP sensor 2 to determine accelerator pedal position for the entire ignition cycle. The engine will be slow to respond to accelerator position.
If there are circuit faults with both APP sensor 1 and APP sensor 2 in the same ignition cycle, no accelerator position can be determined.
