Code
P1278
HUMMER
P — Powertrain
Accelerator Pedal Position Sensor 1 Circuit High Voltage
AI status
Completed
Completed
100%
Causes
- APP sensor 1 internal fault
- Signal wire shorted to battery voltage (VB) or other high-voltage source
- Damaged/shorted harness or chafed wiring
- Poor or corroded connector or pin making intermittent contact
- Poor or missing sensor ground or Vref (reference) circuit issues
- PCM/ECM internal fault (rare)
Symptoms
- Malfunction Indicator Lamp (MIL) or DTC lamp illuminated
- Reduced engine power or limp-home/throttle reduced mode
- Unresponsive or inconsistent accelerator pedal behavior
- Hard or erratic idle when pedal is pressed or released
- Multiple pedal/throttle-related DTCs may be present
What to check
- Read and record DTC(s) and freeze frame with a compatible scan tool
- Check live APP sensor 1 values while slowly moving the pedal
- Visually inspect APP sensor connector and wiring for damage, corrosion, or pin intrusion
- Backprobe connector with key ON (engine OFF) and measure reference, signal, and ground voltages
- Unplug sensor to see if DTC clears or changes (note: follow manufacturer safety procedures)
- Check continuity and for short to battery voltage between signal wire and VB
Signal parameters
- Reference voltage (Vref): ≈5.0 V (typically 5 V ±0.25 V) with key ON
- APP sensor 1 signal voltage expected range: about 0.5 V (closed) to ~4.5 V (wide open), proportional to pedal position
- High-voltage threshold (approx): sensor signal > ~4.6–4.8 V may set P1278 (manufacturer-specific thresholds vary)
- Signal wiring should show no continuity to battery positive (VB) with key OFF
Diagnostic algorithm
- Retrieve all stored DTCs and freeze-frame data. Note any related throttle/pedal codes.
- With a scan tool, observe APP sensor 1 live data while slowly depressing and releasing the pedal. Confirm signal is stable and how it behaves relative to pedal movement.
- Turn key OFF. Visually inspect the pedal sensor connector and wiring harness for damage, chafing, pin push-out, moisture, or corrosion.
- With key ON (engine OFF) backprobe the connector: verify Vref ≈5 V, sensor ground present, and measure APP1 signal voltage. If signal is above the expected range, proceed to wiring checks.
- Check for a short to battery: with key OFF, measure resistance/continuity between APP1 signal wire and battery positive (VB). Infinite or very high resistance is expected; low resistance indicates a short.
- Wiggle test harness and connectors while monitoring live data to detect intermittent shorts or opens.
- If wiring and connector check good, disconnect the APP sensor and observe whether the PCM reports open/changes. DO NOT operate vehicle with pedal unplugged unless following safe manufacturer procedures.
- Measure sensor output against pedal travel with a DVOM or oscilloscope; compare to expected linear change. Replace sensor if it outputs a constant high or out-of-range voltage.
- If wiring harness and sensor test good, test/replace PCM only after confirming all wiring, connectors, and sensors are good and other causes are ruled out.
- After repairs, clear codes and perform a road test or relearn procedure if required. Re-scan to confirm code does not return.
Likely causes
- Signal conductor pin pushed out or contacting a VB source at the connector
- Water intrusion/corrosion at sensor connector causing high resistance paths
- Sensor failed and producing an out-of-range high output
- Short to battery voltage in the harness near the pedal or along the loom
Fault status
Status
APP Sensor 1 Circuit High Voltage — PCM detected the accelerator pedal position sensor 1 signal above the allowed voltage threshold. Possible causes include a short to battery voltage, sensor failure, damaged wiring, or poor connector/ground. Further diagnostics required.
Repair difficulty: Medium
Diagnostic time: 0.5-2.0 hours
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