P0122 Code: Throttle/Pedal Position Sensor/Switch A Circuit Low

What the P0122 code means

The computer needs to know at every moment how much accelerator is being requested and how far the throttle has really opened. That is what the position sensors are for, usually potentiometers or Hall effect sensors, powered by a five volt reference.

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

The P0122 code is set when the signal falls below the lowest possible value, typically below 0.2 volts. That does not correspond to any pedal position. It means an open circuit, a short to ground or a sensor that stopped sending a signal.

Because the throttle is a safety system, the computer takes no chances. When it loses a reliable signal, it cuts power drastically so the vehicle can still be brought to a controlled stop. That is why this code, technically simple, has such dramatic consequences.

It is worth knowing how the redundancy in these systems works, because it explains how the vehicle behaves. An electronic throttle carries two independent signals that must move consistently with each other. If one circuit fails, the computer keeps the other but can no longer verify that it is correct. That is why the engine does not shut off, but power is drastically limited.

That same redundancy gives a useful diagnostic clue: if only one of the two signals has dropped out, the problem is in that particular circuit. If both fail at once, suspect the common supply or ground before the sensors.

P0122 code symptoms

• Limp mode with very limited power, often not getting past low rpm.
• Check engine light on.
• The engine does not respond to the accelerator, or responds abruptly.
• Rough idle, or an engine that stalls.
• Hard starting.
• Erratic shifting on automatic transmissions.
• The symptom may go away temporarily after you shut the engine off and restart.
• Only one of the two redundant signals shows as dropped in live data.

Likely causes of the P0122 code

1. Signal circuit open or shorted to ground.
2. Potentiometer track worn in the rest zone, because it is the most used position.
3. Connector with backed-out, corroded or wet pins.
4. Five volt reference missing, which also drags down other sensors.
5. Sensor ground with high resistance.
6. Electronic throttle body failed internally.
7. Faulty pedal sensor on electronic throttle systems.
8. Wiring rubbed along its route to the computer.
9. Damaged input stage in the computer. Uncommon, and checked last.
10. Open common ground on both position circuits, which takes down both signals at once.

How to diagnose the P0122 code step by step

1. Watch the signal in live data while you move the accelerator slowly. It should rise progressively, with no jumps or dead spots.
2. Check the five volt reference at the sensor connector. If it is missing, see whether other sensors share that reference and are also affected.
3. Measure the signal right at the connector with the key on. If it is correct there and the scan tool does not see it, the problem is in the wiring.
4. Verify the sensor ground by measuring its voltage drop.
5. Check the continuity of the signal wire to the computer and rule out a short to ground.
6. Wiggle the harness with the key on and watch the reading to catch internal breaks.
7. Inspect the connector for backed-out pins, green corrosion or moisture.
8. Check for a second position circuit, since these systems usually carry two redundant signals that must move consistently.
9. Check whether one or both redundant signals fail. A single one points to that circuit, and both point to the common supply or ground.

How to fix the P0122 code

• Repair the open circuit or the short with solder and heat-shrink tubing, following the original routing.
• Clean the connector with contact cleaner and apply dielectric grease.
• Replace the pedal position sensor if its signal shows dead spots or jumps.
• Replace the complete throttle body when the sensor is built in and not sold separately.
• Repair the sensor ground down to bare metal.
• Restore the five volt reference by finding the short that is loading it down.
• Repair the common ground of the position circuits if both signals show as dropped at the same time.

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: below 0.2 volts sustained.
• Reference voltage: 5 volts, usually shared with other sensors.
• Allowable ground voltage drop at the sensor: below 0.2 volts.
• Electronic throttle systems carry two redundant signals that must move consistently with each other.
• A throttle relearn procedure may be needed after the repair.
• An electronic throttle has two redundant signals: if both fail, suspect the common supply or ground.

Brand-specific notes

If the five volt reference is missing, first check whether other sensors are affected too. That reference is usually shared, and a single shorted sensor can pull it down for all of them, producing several codes that look like independent failures.

Wear on the potentiometer track concentrates in the rest zone, which is where it spends the most time. That is why the fault shows up with the pedal released and not at full throttle. Keep that in mind with the P0122 code.

Frequently asked questions about P0122