Accelerator pedal position sensor 1 internally failed and outputting a high voltage; Signal circuit between the pedal sensor and ECM shorted to a 5-volt reference or battery voltage; Corroded, pushed-back, or water-intruded terminals at the pedal connector or ECM connector; Chafed harness wiring rubbing through near the pedal bracket or under the dash; Poor ground or reference voltage shared with sensor 2 causing a skewed comparison; ECM internal fault pulling the signal line high, though this is the least common cause

DTC codes / GM codes / P1278

P1278 · GMAccelerator Pedal Position Sensor 1 Circuit High Voltage

Q: What does code P1278 mean?

Accelerator Pedal Position Sensor 1 Circuit High Voltage. P1278 on a GM vehicle means the engine control module (ECM) has detected an abnormally high voltage on the signal circuit from accelerator pedal position sensor 1, which is one half of the redundant pedal assembly. The ECM expects this sensor's output to stay within a calibrated low-voltage range that rises smoothly as the pedal is pressed, and a sustained high reading tells the module the circuit is shorted to voltage or the sensor has failed internally.

P1278 on a GM vehicle means the engine control module (ECM) has detected an abnormally high voltage on the signal circuit from accelerator pedal position sensor 1, which is one half of the redundant pedal assembly. The ECM expects this sensor's output to stay within a calibrated low-voltage range that rises smoothly as the pedal is pressed, and a sustained high reading tells the module the circuit is shorted to voltage or the sensor has failed internally.

SystemPowertrain

Code typeGM specific

SeverityMedium

Fix difficultyIntermediate

AreaFuel and air metering (injector circuits)

Symptoms you may notice

Reduced engine power message or limp-home mode with the throttle responding only weakly
The accelerator pedal feels dead or unresponsive for the first part of its travel
A stored P1278 with the check engine light or reduced power light illuminated
Rough or erratic idle if the ECM substitutes a default throttle position
Often no symptoms besides the warning light when the fault is intermittent
Cruise control refusing to engage or dropping out on its own

Common causes, most likely first

Accelerator pedal position sensor 1 internally failed and outputting a high voltage
Signal circuit between the pedal sensor and ECM shorted to a 5-volt reference or battery voltage
Corroded, pushed-back, or water-intruded terminals at the pedal connector or ECM connector
Chafed harness wiring rubbing through near the pedal bracket or under the dash
Poor ground or reference voltage shared with sensor 2 causing a skewed comparison
ECM internal fault pulling the signal line high, though this is the least common cause

How to diagnose and fix it

Scan all modules and record freeze-frame data for P1278, then clear and road test to see if it returns
Inspect the accelerator pedal connector and harness for corrosion, water intrusion, and chafed wires
Backprobe sensor 1 signal at the pedal and compare voltage to the scan tool pedal position PID through the full sweep
Check the 5-volt reference and ground circuits at the pedal connector with a multimeter under load
Wiggle-test the harness while watching live data to expose an intermittent short to voltage
If voltages are out of range with the connector secure, replace the accelerator pedal assembly and perform the relearn
Clear codes and verify the repair with a full pedal sweep and a road test before returning the vehicle

What a high pedal voltage really tells you

A high signal voltage almost always means the circuit is being fed voltage from somewhere other than the sensor, so inspect for a short to the 5-volt reference or battery feed before condemning the pedal.
Sensor 1 and sensor 2 are redundant and their voltages should track in opposite directions; comparing both PIDs on the scan tool quickly separates a bad pedal from a wiring fault.
Water intrusion at the pedal connector from a leaking windshield or cowl is a common hidden cause on older GM trucks and SUVs, and the corrosion is often on the back side of the terminals.
An intermittent P1278 that only sets on cold mornings or over bumps points to a harness chafe rather than a failed sensor, so do not replace the pedal until a wiggle test is done.
A weak or shared ground can pull the reference voltage off and make sensor 1 read high relative to sensor 2, so check grounds before condemning the ECM.

Can I keep driving with P1278?

The vehicle is usually safe to drive to a shop, but expect reduced power and a throttle that may not respond normally, so avoid highway merging and heavy traffic until it is repaired. If the pedal goes completely unresponsive, stop driving and have the vehicle towed.

Tools that help

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Guidance is general and written for GM vehicles. Exact tests and values vary by model, so confirm the fault before replacing parts.