P2138 Code: Throttle/Pedal Position Sensor/Switch D/E Voltage Correlation

What the P2138 code means

The gas pedal on a modern vehicle carries not one sensor but two, and that is not decorative redundancy. It is the only way the computer can verify that what it reads is true.

Those two signals move in step but are not identical. Usually one climbs at twice the slope of the other, or one rises while the other falls. What matters is that their relationship is always the same, because that is how one gives the other away if it drifts.

The P2138 code is stored when that relationship breaks. Both signals exist and are inside electrically valid ranges, but they no longer agree, and the computer has no way to know which one to believe.

The system response is the same as for any doubt about the accelerator: ignore both, close the throttle to a minimal opening and limit the engine. It is worth understanding because the symptom looks out of proportion, a vehicle that barely moves, when the cause is often a loose connector or a rusty ground.

A note on verification: after the repair, it pays to sweep the pedal very slowly from top to bottom while watching both signals at once. What you are looking for is not that both rise, but that their relationship stays constant along the whole travel. Two signals that both rise but whose proportion changes in some stretch will set the code again as soon as the driver passes through that zone.

P2138 code symptoms

• Limp mode with very limited power, often with the engine not getting past a low rpm.
• Check engine light on and the electronic throttle warning symbol.
• The engine does not respond to the pedal.
• Rough idle, or an engine that stalls.
• The symptom appears suddenly while driving.
• It goes away for a while after you shut the engine off and restart it.
• Erratic shifting on automatic transmissions.
• The relationship between the two signals changes in one specific stretch of the pedal travel.

Likely causes of the P2138 code

1. Pedal connector with backed-out, dirty or wet pins. The most frequent cause.
2. Shared ground of both sensors with high resistance.
3. Pedal sensor with one of its tracks worn, usually in the rest zone.
4. Chafed wiring between the pedal and the computer, with one of the two circuits affected.
5. Five volt reference voltage pulled down by another shorted sensor.
6. Liquid that got into the pedal, from spills or wet floor mats.
7. Weak battery, whose voltage drop throws the readings off.
8. Failed control module. The last possibility.

How to diagnose the P2138 code step by step

1. Watch both signals at the same time in live data while you press the pedal slowly. They should move in step across the whole travel, with no jumps or flat spots.
2. Identify which of the two behaves abnormally. That is the one with the problem in its circuit.
3. Check the shared ground by measuring its voltage drop. A ground with resistance affects the two signals differently and breaks the correlation.
4. Inspect the pedal connector by unplugging it and seating it firmly again, looking for backed-out pins or moisture.
5. Verify the five volt reference at the connector.
6. Wiggle the harness with the key on while watching the readings.
7. Check for spills around the pedal area.
8. Check the battery condition.
9. Verify that the relationship between the two signals stays constant along the whole travel, not just that both rise.

How to fix the P2138 code

• Clean the pedal connector with contact cleaner and apply dielectric grease.
• Replace terminals with backed-out pins instead of forcing them.
• Repair the shared ground down to bare metal.
• Replace the pedal sensor if one of its signals has dead spots or jumps.
• Repair damaged wiring with solder and heat-shrink tubing.
• Dry the area if liquid got in, and check the condition of the floor mats.
• Replace the battery if its voltage falls below the acceptable level.

After the repair, run the throttle relearn if the vehicle requires it and check that both signals travel their range in step with each other.

• Repeat the full pedal sweep after the repair, checking the proportion between the two signals at every stretch.

Technical reference data

• Sensor D signal at rest: between 0.5 and 0.9 volts. Wide open: between 3.8 and 4.7 volts.
• Sensor E signal: proportional to the first, usually half of its value or inverted.
• Tolerated deviation between the two before the code is stored: around 10 percent.
• Allowable voltage drop on the shared ground: below 0.2 volts.
• Reference voltage: 5 volts, shared with other sensors.
• What has to stay constant is the relationship between the two signals, not just that both advance.

Brand-specific notes

The shared ground is the subtlest cause of this code. Because it affects both sensors but not by the same amount, it breaks the correlation without either signal looking clearly bad on its own. Measuring its voltage drop before you buy anything keeps you from replacing a good pedal.

The pedal area is vulnerable to spills and wet floor mats. If the code appeared after rain, after washing the interior or after a spill, dry and clean the connector before anything else.

Frequently asked questions about P2138