Code
P2123
HUMMER
P — Powertrain
Accelerator Pedal Position (APP) Sensor 1 Circuit High Voltage
AI status
Completed
Completed
100%
Causes
- Shorted APP1 signal wire to battery voltage or accessory power
- Faulty or failed APP sensor (sensor 1)
- Poor or corroded connector at the pedal sensor
- Damaged/chafed wiring harness between pedal and PCM
- Water intrusion in sensor or connector
- PCM/ECM internal fault or incorrect calibration/software
Symptoms
- Malfunction Indicator Lamp (MIL) on with P2123 stored
- Reduced engine power / limp mode or poor throttle response
- Unresponsive or erratic accelerator pedal behavior
- Inconsistent idle or hesitation
- Possible no-start if PCM enters failsafe
What to check
- Retrieve freeze frame and live data with a scan tool; observe APP1 and APP2 values and behavior
- Compare APP1 vs APP2 — they should track and follow expected relationship
- Visually inspect pedal sensor connector and harness for damage, corrosion, or water
- With key ON (engine OFF) measure reference voltage at pedal harness (expect ≈5V) and ground integrity
- Measure APP1 signal voltage at harness/connector at rest and while moving pedal; note if voltage is abnormally high (>4.8–4.9V)
- Disconnect pedal sensor and re-check voltage at harness end; if signal still high, suspect wiring/short or PCM
Signal parameters
- Reference (VREF) typical: ≈5.0 V (key ON, engine OFF) supplied by PCM to pedal sensor
- Signal voltage typical range: ≈0.5 V (idle/min) to ≈4.5 V (wide open) depending on pedal position
- High-voltage fault threshold: voltage approaching VREF or >4.8–4.9 V may set P2123
- APP sensors usually provide two independent sensor signals; they should correlate and meet expected scaling
Diagnostic algorithm
- Connect a scan tool; record freeze frame, pending codes, and live APP1/APP2 values.
- Visually inspect APP sensor connector and wiring for damage, corrosion, broken pins, or aftermarket splices.
- With the ignition ON (engine OFF) backprobe the VREF, ground, and APP1 signal pin at the pedal harness; confirm VREF ≈5V and good ground.
- Measure APP1 signal voltage at rest and while depressing pedal. Note values and compare to APP2; both should change smoothly and correlate.
- Disconnect the APP sensor connector and measure the signal line at the harness. If the signal line remains high with connector disconnected, suspect short to battery or PCM output fault.
- If signal drops to expected value when disconnected, reconnect and substitute a known-good APP sensor or perform bench testing per service manual.
- If wiring suspected, perform continuity checks from pedal connector to PCM for shorts to battery, power circuits, and ground. Repair any chafed or shorted wiring.
- After repairs or component replacement, clear codes and perform road test while monitoring APP1/APP2 to ensure correct operation and no return of the code.
- If wiring and sensor verify good but code persists, evaluate PCM for internal fault and consult manufacturer procedures for module testing or reprogramming.
Likely causes
- APP1 signal wire shorted to 12V at a pin or splice
- Failed APP sensor output stage driving high
- Connector pins pushed out, corroded, or contaminated causing high reading
- Aftermarket wiring or devices tied into harness creating backfeed
- PCM intermittent fault (less common)
Fault status
Status
Accelerator Pedal Position Sensor 1 circuit reading higher than expected voltage; possible short to battery or sensor failure. MIL set; vehicle may enter reduced power mode.
Repair difficulty: Medium
Diagnostic time: 1.0 - 2.0 hours
Repair manuals
Manual library for HUMMER
138
Browse 138 HUMMER manuals: repair procedures, diagnostics, wiring diagrams, component locations, service data and Labor Times by year, model and trim.
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