Code
P1278
CHRYSLER
P — Powertrain
Accelerator Pedal Position Sensor 1 Circuit High Voltage
Views:
UK: 16
EN: 35
RU: 25
AI status
Completed
Completed
100%
Causes
- Short to battery voltage (power) on APP Sensor 1 signal circuit
- Faulty accelerator pedal position sensor (Sensor 1)
- Damaged wiring or chafed harness (open/short)
- Corroded/loose connector or poor pin contact at sensor or ECM
- Missing or poor ground or 5 V reference disruption
- Faulty ECM (rare)
Symptoms
- MIL/Check Engine lamp illuminated
- Reduced engine power / limp-home mode
- Poor or delayed throttle response, hesitation or surging
- Inconsistent or high idle (depending on throttle control strategy)
- Engine may not respond to accelerator pedal input
What to check
- Verify the DTC is current and record freeze-frame data and mileage
- Scan tool: view live APP sensor 1 and sensor 2 (if present) PIDs and compare behavior
- Visual inspection of pedal module, wiring harness, and connectors for damage, corrosion, or loose pins
- Back-probe APP sensor connector with key ON (engine OFF) and measure signal, 5 V reference and ground
- Perform wiggle test on harness/connectors while watching live data for intermittent changes
- Disconnect pedal connector: if high voltage disappears, suspect pedal/sensor; if persists, suspect wiring/ECM
Signal parameters
- 5 V reference from ECM to APP sensor (typical) — check for stable ~5.0 V reference
- Sensor 1 output at rest typically ~0.4–1.0 V and increases with pedal travel up to ~4.0–4.5 V (specs vary by model)
- High-voltage fault triggered when signal exceeds allowable threshold (near battery voltage or above expected max, e.g., >4.9–5.1 V)
- Sensor 1 and Sensor 2 (redundant) should correlate and track smoothly; large divergence indicates fault
Diagnostic algorithm
- Retrieve and record all stored codes and freeze-frame data. Clear codes and perform a key-cycle to see if code returns.
- With scan tool, monitor APP Sensor 1 and Sensor 2 PIDs at key ON engine OFF and during pedal travel. Note voltages at rest and full travel and whether they track each other.
- Visually inspect pedal assembly and harness; repair any physical damage.
- Back-probe connector at pedal: verify 5 V reference present, good ground, and measure signal voltage. If signal reads high (near battery) with pedal disconnected, suspect wiring short to B+ or ECM.
- If reference or ground missing or unstable, trace and repair harness to ECM and check related fuses/relays.
- Disconnect pedal connector; measure voltage at the harness side toward ECM. If harness still shows high voltage with pedal disconnected, suspect wiring short or ECM input stuck high — continue to check continuity back to ECM.
- Check continuity and resistance between APP signal pin and ECM input pin; inspect for short to battery or other circuits using wiring diagrams.
- If wiring and connectors are good and signal only high when pedal connected, replace accelerator pedal position sensor/module per OEM procedure.
- After repair, clear codes and road-test to confirm proper operation and that code does not return. If code persists after sensor and wiring proven good, consider ECM diagnosis/replacement.
Likely causes
- APP sensor internal failure causing high output voltage
- Signal wire shorted to B+ (after accessory relay/fuse)
- Corroded or bent terminal in pedal/module connector
- Faulty 5 V reference or sensor ground at harness/ECM
Fault status
Status
Accelerator Pedal Position Sensor 1 Circuit High Voltage — ECM detected an APP1 signal above allowable range; possible short to battery, sensor fault, wiring/connector issue, or ECM input failure.
Repair difficulty: Medium
Diagnostic time: 0.5-2.0 hours
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