Honda Civic P0123: Throttle/Pedal Position Sensor/Switch A Circuit High Input diagnosis
Code verification standard
P0123 is published with SAE J2012 / ISO 15031-6 code structure plus Honda Civic diagnostic context.
This page covers meaning, symptoms, first checks, live data, misdiagnosis risk, and repair verification.
What does this mean?
Code P0123 signifies that the Honda Engine Control Module (ECM) or Powertrain Control Module (PCM) has detected a voltage signal from the Throttle Position Sensor (TPS) or Accelerator Pedal Position (APP) Sensor 'A' circuit that is consistently higher than the specified maximum allowable threshold. This typically means the sensor is reporting a throttle opening that is either stuck wide open, or the circuit itself has a short to voltage, indicating an electrical issue rather than a mechanical one. This high voltage input can lead to erratic engine behavior, including high idle, lack of throttle response, or activation of limp mode.
How serious is this code?
Driving with a P0123 code can be hazardous. The engine's response to the accelerator pedal may be unpredictable, leading to sudden unintended acceleration, a high idle, or a complete loss of throttle response (limp mode). This can significantly impair your ability to control vehicle speed, especially in traffic or emergency situations. It is strongly recommended to diagnose and repair this issue immediately to prevent potential accidents.
Diagnostic system: Sensor circuit and live data
On a Honda Civic, the P0123 code should be diagnosed as part of the sensor circuit and live data group. Related codes and live data from this same system can change the repair priority, so avoid replacing parts from the code name alone.
Most Common Causes for Honda Civic
- Faulty throttle position sensor (TPS)
- Damaged, chafed, or broken wiring harness
- Corroded or loose electrical connector
- Internal PCM/ECM failure
- PCM/ECM software needs update (TSB)
Symptoms to Watch For
- Check Engine Light is illuminated
- Poor acceleration or sluggish response
- Abnormally high idle speed
- Vehicle enters reduced power / limp mode
- Engine surging or fluctuating RPM
- Decreased fuel economy
- Hesitation or stumble during acceleration
- Significant loss of engine power
Before replacing parts, check these first
- Inspect the connector for corrosion, broken locks, oil intrusion, or heat damage.
- Verify 5V reference, ground, and signal with a multimeter where applicable.
- Compare live sensor data to realistic engine temperature, pressure, airflow, or throttle values.
- Check for blown fuses, weak battery voltage, and damaged connectors before buying major parts.
Live data to compare
- 5V reference, ground, and signal voltage
- Sensor value compared with realistic engine conditions
- Connector wiggle-test result
How to Diagnose
Common Mistakes
Replacing the named sensor without testing power, ground, signal, and wiring continuity first. Always confirm the fault with freeze-frame data, live data, visual inspection, and circuit checks before spending money on parts.
Repair verification
After the repair, clear P0123, complete a drive cycle, and confirm the monitor runs without the code returning. Save the before/after scan report if the vehicle needs emissions inspection.
Frequently Asked Questions
Can I drive with the P0123 code?
If the check engine light is solid, you can usually drive to a safe location or mechanic. If the light is flashing, pull over immediately to prevent catastrophic engine or catalytic converter damage.
Will the P0123 code clear itself?
Once the underlying problem is fixed, the engine computer will run its self-checks. If it passes over several drive cycles, the light will turn off automatically. You can also clear it immediately using an OBD2 scanner.
Will this code fail an emissions test?
Yes. If the Check Engine Light is on due to the P0123 code, your Honda will automatically fail an OBD2 plug-in emissions or smog test.
What should I check first for P0123?
Inspect the connector for corrosion, broken locks, oil intrusion, or heat damage. Verify 5V reference, ground, and signal with a multimeter where applicable. Compare live sensor data to realistic engine temperature, pressure, airflow, or throttle values.
Join the Discussion
Repair Estimate
Fuse, connector, wiring, cleaning, or basic inspection
Major component replacement after live data and circuit tests confirm failure
Sensor replacement can be moderate cost, but wiring and connector repairs are common and cheaper.
DIY Difficulty
Can usually be performed at home with basic hand tools.