P0122
- Powertrain
- Generic (SAE)
- Emissions
Get it looked at now
The vehicle will typically enter reduced power mode with little or no warning and very limited acceleration, and the fault can appear while the vehicle is moving. Merging, climbing grades and pulling into traffic all become hazardous. There is no emissions-only reading of this code.
- Safe to drive
- Treat reduced power as a get-it-home condition. Do not plan highway driving or towing until it is repaired.
Throttle/Pedal Position Sensor "A" Low Input
The throttle position signal voltage fell below the lowest value the computer will accept — lower than a fully closed throttle could ever produce. That is an electrical fault in the sensor, its wiring, or its power supply, not a mechanical throttle problem.
The sensor is fed a regulated 5 volt reference and a low reference (ground), and returns a signal proportional to throttle angle. Even at fully closed throttle the signal deliberately sits meaningfully above zero, precisely so the ECM can tell a closed throttle apart from a broken circuit. P0122 means the signal dropped below that designed floor. A signal wire shorted to ground, an open or collapsed 5 volt reference, an open signal wire, or a failed sensor element all produce it.
How the car detected itThe ECM compares the signal voltage against a fixed lower threshold — typically somewhere between 0.2 and 0.5 volts, with the exact value vehicle-specific — and sets the code when it stays below that for a calibrated time regardless of pedal position. Because the fault removes trustworthy throttle information, most vehicles immediately restrict torque and hold a default throttle angle rather than acting on an untrusted signal.
A trouble code records what a control module measured. It does not identify which part failed. Test before replacing anything.
- Safe to drive
- Treat reduced power as a get-it-home condition. Do not plan highway driving or towing until it is repaired.
- Urgency
- Get it looked at now
- Will it pass emissions
- No
- If ignored
- The vehicle stays in limp mode, may idle poorly or not at all, and may stall or refuse to start. Intermittent versions typically worsen as chafed insulation wears further through, turning an occasional power loss into a permanent one.
- Check engine light
- Solid
- Clearing the light
- A completed drive cycle is normally required before the monitor re-runs and the light can clear.
- Reduced throttle response, possible limp mode
- Hesitation or surging
- Idle too high or too low
Many vehicles show no symptom at all beyond the warning light. The absence of a symptom does not mean the fault is not real.
These are the causes this code can have. For the order to work through them in, see what to check first.
| # | Cause | How often | Component |
|---|---|---|---|
| 1 | Signal wire shorted to ground | Very common | — |
| 2 | TPS internally failed | Common | — |
| 3 | 5V reference voltage to TPS missing | Occasional | — |
Ordering reflects how often each cause is responsible in general, not a probability for your vehicle. Confirm by testing.
In order. Each of these is cheaper or faster than what follows it, and each one can make the rest unnecessary.
Whether other sensors set codes at the same time
A shorted sensor can drag down a shared 5 volt reference. If manifold pressure, throttle position and other sensors all fault together, chase the reference circuit rather than the throttle body.
The 5 volt reference at the sensor connector, key on
No reference means no signal and the sensor is innocent. It is the single fastest measurement that splits the whole diagnosis in two.
The connector for spread, corroded, or backed-out terminals
A signal terminal touching the ground terminal, or a broken reference pin, gives exactly this reading and costs nothing to find.
Harness routing near heat sources and moving parts
Chafe-through to a grounded surface is frequently the cause and is cheap to check, and it will return after any part is replaced if the rub point is not repaired.
What the scan tool reports with the sensor disconnected
It shows you how this vehicle's ECM behaves with a genuinely open circuit, giving you a baseline for every test that follows.
Record freeze frame and the full code set
Tools Scan tool
Note whether other sensors fed by the same 5 volt reference have also set codes, and whether the fault was present at key-on or appeared while driving.
ExpectedA clear picture of whether this is an isolated sensor circuit fault or a shared reference collapse.
Measure reference, ground and signal at the sensor connector
Tools Digital multimeter, back-probe leads, wiring diagram
Key on, engine off. Back-probe carefully so you do not spread the terminals.
ExpectedReference near 5 volts, ground under about 0.1 volt with respect to battery negative. The correct closed-throttle signal value is vehicle-specific, but it must be above zero.
Disconnect the sensor and watch the reported value
Tools Scan tool, service information
Compare what the scan tool shows with the sensor connected against what it shows disconnected. Establish from service information what this ECM is designed to report on an open signal circuit before you interpret the result.
ExpectedHow the ECM reports an open signal circuit is design-dependent — some bias it high, some low — so establish the expected value from service information before drawing a conclusion. A reading that rises when the sensor is unplugged shows the sensor was pulling the circuit down. A reading that stays at zero does not yet clear the sensor; the decisive test is the next step, signal-to-ground resistance with both the sensor and the ECM disconnected.
Test the signal wire for a short to ground
Tools Digital multimeter, wiring diagram
Disconnect both the sensor and the ECM connector, then measure resistance from the signal wire to chassis ground. Leaving the ECM connected reads its internal pull-down and looks like a short that is not there.
ExpectedEffectively infinite resistance between the signal wire and ground with both ends disconnected.
Check end-to-end continuity with a wiggle test
Tools Digital multimeter, wiring diagram
Measure continuity on the signal, reference and ground circuits between the sensor connector and the ECM connector while flexing the harness along its full run.
ExpectedLow, stable resistance on each circuit that does not change while the harness is flexed.
If the reference is missing, isolate the cause
Tools Digital multimeter, wiring diagram
Trace the 5 volt reference back toward the ECM and identify every sensor sharing it. Disconnect those sensors one at a time and watch whether the reference recovers — a shorted sensor elsewhere can collapse the whole circuit.
ExpectedReference returns to about 5 volts when the faulted sensor or wiring section is disconnected, identifying the culprit.
Repair the fault and complete any required relearn
Tools Scan tool, service information
After the electrical repair, clear codes, cycle the ignition, and perform the throttle position or idle relearn if the vehicle requires one.
ExpectedNormal throttle position values through the full sweep and a stable idle with no reduced power indication.
Road test and re-check for pending codes
Tools Scan tool
Drive through varied load and throttle conditions, then re-scan. Chafed-wiring faults are intermittent before they become permanent.
ExpectedNo pending or confirmed codes after a full road test.
| Repair | Only after you confirm | Typical cost | Who |
|---|---|---|---|
| Repair a signal wire shorted to ground or a chafed harness section | Low resistance measured from the signal wire to ground with both the sensor and the ECM disconnected | $120–$450 | DIY |
| Repair an open or shorted 5 volt reference circuit | Reference voltage measures well below 5 volts at the sensor connector and returns to normal once the faulted section is isolated | $130–$500 | DIY |
| Replace the throttle position sensor or throttle body assembly | Reference, ground and signal wiring all test good — including effectively infinite signal-to-ground resistance with both the sensor and the ECM disconnected — while the signal still reads below threshold with the sensor connected | $200–$1,000 | DIY |
| Repair or replace a corroded connector or damaged terminal | Visible terminal damage or corrosion, with the circuit reading correctly when that terminal is temporarily bypassed | $100–$300 | DIY |
| Replace a different sensor on the shared 5 volt reference that is collapsing it | Reference voltage returns to 5 volts the moment one specific sensor on that circuit is disconnected | $120–$450 | DIY |
Wiring repairs dominate here and are priced by diagnostic time rather than parts. Expect an hour or more of labor to locate a chafe point. Replacing the throttle body first is the expensive mistake — verify the circuits before buying anything, and confirm whether the vehicle needs a scan tool relearn afterward.
- Replacing the sensor or the throttle body before measuring the 5 volt reference.
- Testing signal-to-ground resistance with the ECM still connected, which reads the module's internal pull-down and falsely looks like a short.
- Assuming an unplugged-sensor reading of zero clears the sensor. Many ECMs bias an open throttle position input low, so a good harness and a failed sensor look the same on that test.
- Back-probing carelessly and spreading a terminal, creating a second intermittent fault to chase.
- Clearing the code and assuming the repair held, without a road test.
- Skipping the throttle relearn after replacing the sensor or throttle body.
- Overlooking that one 5 volt reference commonly feeds several sensors, so the shorted component may be somewhere else entirely.
- Throttle Position Sensor
- Throttle Position Sensor Wiring
- Definition
- Generic code set (SAE J2012 / ISO 15031-6) via the OBDexopen database, dedicated to the public domain under CC0-1.0.
- Diagnostic guidance
- Written and reviewed in-house. See our editorial policy.
- Review status
- Reviewed
Found something wrong? Tell us — we publish corrections.