P0122 Code: Throttle/Pedal Position Sensor A Circuit Low
The throttle position signal fell to nearly 0 volts, lower than a closed throttle could ever produce.
At a glance
Severity: Moderate
- Meaning
- Throttle/Pedal Position Sensor A Circuit Low
- System
- Fuel & Air Metering · Powertrain
- Safe to drive?
- With caution
- Typical repair cost
- $0–$850
- DIY difficulty
- Moderate
A closed throttle normally produces about 0.4 to 0.9 volts from sensor A, never zero. P0122 sets when the signal stays below the minimum the PCM accepts, often somewhere around 0.15 to 0.25 volts. Because the signal input is held low by a pull-down resistor inside the PCM, anything that takes the sensor out of the circuit drives the reading toward zero: an unplugged connector, a broken 5-volt reference wire, a failed sensor, or a signal wire rubbed through to ground. Loss of the 5-volt reference is worth checking early, because the same supply often feeds the MAP sensor, the accelerator pedal sensor and others, and a short on any of them can pull the whole reference down. On drive-by-wire engines the PCM responds by parking the throttle at a spring-held fast idle and limiting power.
Can I keep driving? Drive only to get home or to a shop. On electronic throttles the car will barely accelerate and may not keep up with traffic; on older cable-throttle engines it usually drives but can stall at stops and shift poorly.
How to read P0122
- P Powertrain
- 0 Generic (SAE)
- 1 Fuel & air metering
- 22 Specific fault
Symptoms
- Check engine light with reduced-power or limp mode on drive-by-wire cars
- Throttle percentage stuck at 0% in live data no matter how far the pedal is pressed
- Engine revs only to a limited speed and accelerates very slowly
- Stalling or dipping idle when coming to a stop on cable-throttle engines
- Automatic upshifts very early and lugs the engine at low rpm
- Several other sensor codes at the same time if the 5-volt reference has dropped out
Possible causes
Ranked by how often each cause is behind this code (editorial estimate).
- Open 5-volt reference wire or loose reference terminal at the sensor connector
- Sensor failed internally (open track at the reference end or dead Hall-effect element)
- Connector unplugged, not latched or with a pin pushed back, often after throttle body cleaning
- Signal wire chafed and shorted to ground
- 5-volt reference pulled down by a short in another sensor sharing the supply
- PCM reference driver or input fault
Diagnosis step by step
Look at throttle voltage and the other 5-volt sensors
Key on, engine off, read throttle position voltage or percentage. A steady reading near 0 V that doesn't respond to the pedal confirms the fault is present. Check MAP and pedal position at the same time: if they also read zero or show codes, the shared 5-volt reference is down rather than the throttle sensor itself.
Tools: OBD-II scanner
Measure reference and ground at the harness
Unplug the sensor and measure the reference terminal to battery negative with the key on; expect 4.9 to 5.1 volts. Then check the ground terminal for continuity to engine ground. No reference with the sensor unplugged means an open reference wire or a supply pulled down elsewhere.
Tools: Multimeter
Hunt down a loaded reference
If the reference is low, unplug the other sensors on that circuit one at a time, such as MAP, pedal, A/C pressure or fuel rail pressure, while watching the voltage. The one that brings the reference back to 5 volts when disconnected has an internal short or a shorted harness.
Tools: Multimeter
Prove the signal wire with a jumper
With the sensor unplugged and reference present, briefly connect the harness reference terminal to the signal terminal using a fused jumper. The scan tool should jump to about 5 volts or 100%. If it does, the signal wire and PCM input are good and the sensor is at fault; if it stays at zero, the signal wire is open or shorted to ground.
Tools: Pliers, OBD-II scanner
Check the sensor itself
On a potentiometer-type sensor, resistance between reference and ground pins should be a steady few thousand ohms, and signal-to-ground should change smoothly as the throttle opens. An open reading means a failed track. Hall-effect sensors can't be checked with an ohmmeter; judge them by voltage with the connector plugged in and back-probed.
Tools: Multimeter
Repair and run the throttle relearn
Repair the wire or connector, or replace the sensor or complete throttle body, then clear the codes. Drive-by-wire engines typically need a throttle or idle relearn afterwards, and the limp mode often only clears after a key cycle.
Tools: OBD-II scanner, Screwdriver
Where the parts are
- 1Ignition coil
- 2Spark plug
- 3Fuel injector
- 4Camshaft sensor
- 5Crankshaft sensor
- 6Engine computer (ECM)
- 7Intake manifold
Repair options & costs
| Repair | DIY? | Estimated cost |
|---|---|---|
| Reconnect and secure throttle sensor connector | Yes | $0–$50 |
| Repair 5-volt reference or signal wiring | No | $100–$350 |
| Replace throttle position sensor (cable throttle) | Yes | $90–$260 |
| Replace electronic throttle body | No | $300–$850 |
| Replace shorted sensor that was loading the reference circuit | Yes | $80–$400 |
Rough US estimates for parts and labor; shop rates vary by region. Not a quote.
Notes by brand
Field data
| Ford | 5 | Transit Connect, Focus Bev, Freestyle | |
|---|---|---|---|
| Chevrolet | 2 | Camaro, Silverado 2500 |
Source: NHTSA Complaints API (US government open data), analyzed by DTC mention.
Emissions / smog test impact
Will fail
P0122 lights the check engine light and fails an OBD-II emissions inspection. If the 5-volt reference was involved, confirm all related sensor codes are gone before retesting.
Frequently asked questions
Why does unplugging the TPS give a low code and not a high one?
The PCM holds the signal input low through a pull-down resistor, so with no sensor connected the signal falls toward 0 volts. A coolant temperature sensor works the other way around, which is why an unplugged coolant sensor reads high and an unplugged TPS reads low.
I have P0122 and a MAP sensor code at the same time. Two bad sensors?
Unlikely. Both sensors usually share one 5-volt reference from the PCM, and a single short or an open reference feed knocks out both readings. Measure the reference voltage before buying anything.
Why won't my car rev above a fast idle?
Without a trustworthy throttle position, a drive-by-wire PCM cuts power to the throttle motor and lets springs hold the plate slightly open. That gives just enough air to move the car slowly, which protects you from unintended acceleration.
I cleaned my throttle body and now have P0122. What happened?
The connector may not be fully latched, a terminal may have been pushed back, or cleaner may have gotten into the sensor. Check the connector first. On some throttle bodies, forcing the plate open by hand while it's powered can also damage the gears or sensor.