P2123 Code: Accelerator Pedal Position Sensor D Circuit High
The main gas pedal sensor reads near its full 5-volt supply or higher, even with your foot off the pedal.
At a glance
Severity: Moderate
- Meaning
- Accelerator Pedal Position Sensor D Circuit High
- System
- Speed & Idle Control · Powertrain
- Safe to drive?
- With caution
- Typical repair cost
- $0–$450
- DIY difficulty
- Moderate
At the floor, pedal sensor D normally tops out around 4 to 4.5 V, never at the full 5-volt reference. P2123 sets when the D signal rises above the PCM's upper limit, typically about 4.7–4.9 V. The two classic reasons are a lost sensor ground, which lets the output float up to the reference voltage, and a signal wire touching a voltage source: the 5-volt reference itself, or worse, a 12-volt circuit in the dash harness. Aftermarket throttle response controllers plugged in between pedal and harness are another frequent trigger, as is moisture bridging pins inside a corroded connector. A quick unplug test tells you whether the high voltage comes from the pedal side or from the harness. Because a PCM would otherwise read this as a floored pedal, it ignores D, relies on E and restricts power.
Can I keep driving? Drive gently to a shop; throttle response will be limited. If the signal wire turns out to be shorted to 12 volts, avoid driving until it's repaired, as that can damage the PCM input.
How to read P2123
- P Powertrain
- 2 Generic (SAE)
- 1 Fuel & air metering
- 23 Specific fault
Symptoms
- Check engine light with limp or reduced-power mode
- Pedal D reads 4.8–5 V in live data even with the pedal released
- Engine responds only gently to the pedal, no matter how far it's pressed
- Idle normal, cruise control unavailable
- Code appeared right after installing a throttle response controller or other under-dash accessory
Possible causes
Ranked by how often each cause is behind this code (editorial estimate).
- Open sensor D ground at the pedal connector or a ground splice
- Pedal sensor D failed with its output stuck high
- Aftermarket throttle response controller or pedal booster fault
- Moisture or corrosion bridging the reference and signal pins
- D signal wire shorted to the 5-volt reference or a 12-volt circuit in the harness
- PCM input fault
Diagnosis step by step
Confirm D is high and check E
Key on, engine off, read both pedal sensors with the pedal released. D near 5 V while E shows its normal rest value confirms a D-only fault. Remove any add-on throttle controller at this point and recheck with the factory connector plugged straight into the pedal.
Tools: OBD-II scanner
Unplug the pedal and watch the reading
With the key on and scan tool connected, unplug the pedal. The D reading should fall toward 0 V because the PCM pulls the input low. If it drops, the high voltage came from the pedal or its ground. If it stays high, the signal wire is touching a voltage source in the harness.
Tools: OBD-II scanner
Voltage-drop test the D ground
Plug the pedal back in, key on, and measure from the D ground pin (back-probed) to battery negative. More than about 0.1 V means a poor or open ground. Follow the ground wire to any splice or ground point it shares and check those joints.
Tools: Multimeter
Measure the signal wire for foreign voltage
With the pedal unplugged and key on, measure the D signal terminal in the harness to ground. Near 0 V is correct. About 5 V indicates a short to a reference wire; battery voltage means a short to a 12-volt feed, which must be repaired before a new pedal or PCM is fitted.
Tools: Multimeter
Clean or repair the connector
Inspect the connector for green deposits or moisture between pins, which can leak reference voltage onto the signal. Clean with electrical contact cleaner, let it dry fully and apply dielectric grease to the seal only, or replace the connector if terminals are damaged.
Tools: Flashlight, Screwdriver
Replace the pedal if ground and wiring pass
If the ground is good, the harness shows no foreign voltage and D still reads high with the pedal connected, the sensor has failed. Fit a new pedal module, clear the codes and confirm D rests at its normal low value.
Tools: Screwdriver, OBD-II scanner
Where the parts are
- 1Ignition coil
- 2Spark plug
- 3Fuel injector
- 4Camshaft sensor
- 5Crankshaft sensor
- 6Engine computer (ECM)
- 7Intake manifold
Repair options & costs
| Repair | DIY? | Estimated cost |
|---|---|---|
| Remove aftermarket throttle controller | Yes | $0–$50 |
| Repair open sensor ground or splice | No | $100–$300 |
| Clean or replace corroded pedal connector | Yes | $60–$220 |
| Replace accelerator pedal module | Yes | $150–$450 |
| Repair signal wire shorted to voltage | No | $150–$400 |
Rough US estimates for parts and labor; shop rates vary by region. Not a quote.
Emissions / smog test impact
Will fail
With P2123 stored, the check engine light is on and the car fails an OBD-II emissions inspection. Clearing the code without a repair won't help because it resets at the next key-on.
Frequently asked questions
Why does a missing ground make the signal read high?
Inside the pedal, the sensor output sits between the 5-volt reference and ground. Take away the ground and nothing pulls the output down anymore, so it rises to almost the full reference voltage.
I installed a throttle response controller. Could that be it?
Very possibly. These boxes intercept and rescale the pedal signals, and a faulty unit or loose plug can send an out-of-range D signal. Refit the factory connection before diagnosing anything else.
Could P2123 make the car accelerate on its own?
Not on a correctly working system. The PCM compares D against E, sees that they disagree and ignores the high reading, then limits power. That cross-check is exactly why the pedal has two sensors.
Is a new pedal always the fix?
No. An open ground or a shorted wire gives the same code, and a new pedal won't cure either. The unplug test and a ground voltage-drop check take a few minutes and tell you which side is at fault.