P0123 Code: Throttle/Pedal Position Sensor/Switch A Circuit High

What the P0123 code means

Throttle position sensors deliver a signal that moves within a limited range: at rest a low value but never zero, and at full throttle a high value but never the full five volts. Those margins exist so a real position can be told apart from an electrical fault.

The P0123 code is set when the signal exceeds the highest possible value, typically more than 4.8 volts sustained. No pedal position produces that value.

The two electrical causes that explain it are a signal wire shorted to a voltage line, or an open sensor ground. The second is less obvious: without a ground, the sensor internal circuit can no longer pull the voltage down and the signal goes to the maximum even though the sensor is intact.

Because the throttle is a safety system, the computer responds the same way it does to any doubt about that signal: it closes the throttle to a minimum opening and limits the engine to low rpm. That is why a simple electrical fault leaves the vehicle almost immobile.

The most useful clue with this code is the reading with the pedal released. If the scan tool shows the accelerator at maximum with your foot off, the fault is electrical for certain, because no real position produces that value at rest. And if the reading does not change when you press the pedal, the circuit is shorted directly to a voltage line.

P0123 code symptoms

• Limp mode with very limited power.
• Check engine light on.
• The engine does not respond to the pedal, or responds abruptly.
• High idle, or an engine that stalls.
• Hard starting.
• Erratic shifting on automatic transmissions.
• The symptom goes away temporarily when you shut the engine off and restart.
• Accelerator reading at maximum on the scan tool with the pedal released.
• The reading does not change when you press the pedal, which confirms a direct short to voltage.

Likely causes of the P0123 code

1. Signal wire shorted to a voltage line.
2. Sensor ground open or with high resistance, which drives the signal to the maximum without the sensor being bad.
3. Connector with moisture or conductive residue between pins.
4. Failed pedal sensor.
5. Electronic throttle body with a damaged sensor.
6. Wiring rubbed between the pedal and the computer.
7. Backed-out pins in the connector, which cause a loose connection.
8. Damaged input stage in the computer. Uncommon.
9. Signal wire in direct contact with a power line from insulation rubbed through.

How to diagnose the P0123 code step by step

1. Watch the reading with the pedal released. A maximum value at rest confirms the electrical fault.
2. Check the sensor ground before anything else. An open ground drives the signal to the maximum with the sensor intact, and it is a cause that gets overlooked often.
3. Unplug the sensor and watch the reading. If the high voltage persists, the problem is in the wiring.
4. Look for shorts between the signal wire and the power lines.
5. Inspect the connector for moisture and conductive residue between pins.
6. Wiggle the harness with the key on and watch the reading.
7. Check the two redundant signals. If only one is affected, the problem is in that circuit. If both are, suspect the common ground or supply.
8. Press the pedal while watching the reading. If it does not change, the circuit is shorted to a voltage line and the sensor has nothing to do with the fault.

How to fix the P0123 code

• Repair the sensor ground down to bare metal, checking that its voltage drop is below 0.2 volts.
• Fix the short on the signal wire and protect the repaired section.
• Dry and clean the connector, and protect it with dielectric grease.
• Replace the pedal sensor if its signal stays at the maximum with the circuit good.
• Replace the throttle body when the sensor is built in and not sold separately.
• Replace terminals with backed-out pins instead of forcing them.
• Find and repair the spot where the signal wire rubs against a power line, and protect that section.

After the repair, run the throttle relearn procedure if the vehicle requires it and check that the signal sweeps its whole range with no jumps.

Technical reference data

• Typical signal at rest: between 0.5 and 0.9 volts. At full throttle: between 3.8 and 4.7 volts.
• Threshold that sets the code: above 4.8 volts sustained.
• An open ground drives the signal to the maximum even if the sensor is intact.
• Allowable ground voltage drop: below 0.2 volts.
• Electronic throttle systems carry two redundant signals.
• P0122 is the opposite code: voltage too low.
• A maximum reading with the pedal at rest is a certain electrical fault, because no real position produces that value.

Brand-specific notes

An open ground is the cause that gets the most sensors replaced for no reason with the P0123 code. Before you buy anything, measure the voltage drop on the sensor ground. If it is high, the sensor is perfectly fine and the problem is in the wiring.

Check also whether the vehicle has more sensor codes. Ground and supply are usually shared, and a single corroded point produces several codes that look like independent failures.

Frequently asked questions about P0123