P0122 Code: Throttle Position Sensor Low — Causes & How to Fix

P0122 means the engine computer is reading a signal from position sensor “A” that is lower than a working circuit should produce. On most vehicles, sensor A is the first throttle position sensor, and a reading stuck low usually means the sensor has lost its 5-volt reference supply, its signal wire is shorted to ground, or the signal wire is broken. The detail that saves the most money is this: the reference supply is usually shared with other sensors, so a fault in a completely different sensor, or its wiring, can drag this one’s signal down and set P0122. This article covers what sets P0122, the six real causes in order of likelihood, how to find a fault on a shared reference circuit, and what each repair typically costs.

Start with the table, then read the triage note under it.

Cause	Telltale Sign	Typical Cost
Lost or dragged-down 5-volt reference	Several sensor codes at once; reference well below 5 volts at the connector	$50 to $400
Signal wire shorted to ground	Signal near zero at every throttle position; chafed or crushed harness	$50 to $300
Open signal wire or poor connection	Code comes and goes; signal drops out when the harness is moved	$20 to $300
Failed position sensor	Wiring and reference test good; signal drops low at a particular position	$100 to $800 (often a throttle body or pedal)
Known throttle body or harness defect	Your vehicle is listed in a manufacturer bulletin	$0 to $800
Engine computer fault	Reference output low with every sensor on it unplugged	$300 to $1,000

Drive only as far as you need to if the car is in reduced-power mode, the engine stalls or idles erratically, or the throttle does not respond properly. On electronic throttle systems the computer limits power when it cannot trust a position sensor. With a reference fault, several sensors can drop out at once, which can make the engine stall. If P0122 is stored but the car drives normally, it can be driven carefully to a diagnosis.

What P0122 Actually Means

A throttle or pedal position sensor receives a 5-volt reference and a ground from the engine computer and returns a signal voltage that changes with position. A working sensor’s signal never reaches zero, so the computer treats a signal below the lowest legitimate value as a circuit fault and sets P0122. Its opposite, a signal above the highest legitimate value, sets P0123.

Electrically, three faults drive the signal low. If the reference supply is missing, the sensor has nothing to divide and outputs nothing. If the signal wire touches ground, the signal is pulled to zero. And on many engine computers, a broken signal wire also reads low, because the computer’s input is designed to fall to zero when nothing is connected. Mazda’s service bulletin 01-017/13 for the 2011 to 2013 CX-9 describes its throttle sensor “circuit low input” code in exactly these terms: “a low voltage or open circuit”. It lists the checks as a connector or terminal malfunction, a short to ground, an open circuit between the throttle body and the powertrain computer, and the sensor itself.

One caution before testing. The code’s official name is “throttle/pedal position sensor/switch A circuit low”, and on some vehicles sensor A is the accelerator pedal. Volkswagen’s 2014 Beetle quick reference specification book lists P0122 on the Beetle’s gasoline engines as an accelerator pedal position sensor circuit low input, set below 0.20 volts. On the 2.0L TDI in the same book, P0122 sets on a signal above 4.85 volts. Check what P0122 means for your exact engine first. The P0123 guide covers these definition differences in more detail.

The Shared Reference Trap

This is the point that stops the wrong part being bought. Engine computers rarely give each sensor its own 5-volt supply. More often one reference circuit feeds several sensors, commonly including the throttle position sensors, the accelerator pedal sensors, and sensors such as manifold pressure, fuel tank pressure or camshaft position, depending on the engine. The computer protects that supply, so if any sensor on it shorts internally, or any wire on it chafes to ground, the whole reference voltage sags. Every sensor on that circuit then reads low at once.

The signature is P0122 arriving together with other “low” or reference codes, for example the second throttle sensor’s P0222, pedal sensor codes, a pressure sensor code, or a reference voltage code such as P0641. Replacing the throttle body in that situation changes nothing, because the throttle position sensor was never the problem. To find the culprit:

1. With the ignition on, measure the reference at the throttle sensor connector. It should be close to 5 volts.
2. If it is low, use the wiring diagram to find every sensor that shares that reference.
3. Unplug those sensors one at a time, watching the reference voltage. When it jumps back to 5 volts, the sensor you just unplugged, or its wiring, is shorting the circuit.
4. If the reference stays low with every sensor unplugged, the short is in the harness itself or, rarely, inside the computer.

Symptoms You May Notice Alongside the Code

On electronic throttle systems, expect a reduced-power mode, with a “reduced engine power” message, an EPC light or a wrench light, and sluggish or no throttle response. On older cable-throttle engines, the computer believes the throttle is closed or barely open, so symptoms include hesitation or stumbling when accelerating, a low or unstable idle, stalling as the car comes to a stop, and harsh or late shifts on automatics that use throttle position. With a shared reference fault, expect several warnings at once. The site’s articles on the symptoms of a bad throttle position sensor and a check engine light that comes on and off when accelerating cover these patterns.

1. Lost or Dragged-Down 5-Volt Reference

The most important cause to rule out, because it can make a good sensor look dead. Either the reference wire to this sensor is broken, in which case only this sensor reads low, or another component on the shared circuit is pulling the whole reference down, in which case several sensors do. Use the unplug-one-at-a-time method above. Reference faults are also why P0122 sometimes appears after unrelated work, such as a sensor replaced elsewhere on the engine with a harness left pinched or chafing. Reference shorts can also be intermittent. A harness that only touches metal when the engine rocks under load can set P0122 briefly and then test perfectly on the bench, so read the freeze frame data for engine load and speed when the code set, and wiggle-test the harness with the engine running if it is safe to do so.

2. Signal Wire Shorted to Ground

If the signal wire rubs through its insulation against the engine, a bracket or the intake, it is pulled to zero whatever the throttle does. Throttle body harnesses sit on top of the engine and can be crushed under an air intake duct or chafed by engine movement. With the sensor and computer both disconnected, check the signal wire for continuity to ground. Any continuity means a short. Repair the wire properly and re-secure the harness.

3. Open Signal Wire or Poor Connection

A broken signal wire, a terminal that has lost its grip, or corrosion in the connector can all read as a low signal, often intermittently. Watch the signal in live data while wiggling the connector and the harness near it. A reading that drops out as the wiring moves points to the connection. Unplug the connector and inspect for corrosion and spread or pushed-back terminals, then check the signal wire’s continuity end to end.

4. Failed Position Sensor

Once the reference, ground and signal wiring are proven good, the sensor is the suspect. Sweep the throttle or pedal slowly through its range with the signal graphed. A worn contact-type sensor often shows a dead spot, a drop toward zero at one particular position, usually where the throttle spends most of its time. On most electronic throttles the sensor is built into the throttle body, and on most electronic pedals into the pedal assembly, so the repair is the whole unit. On older engines it is a separate, inexpensive part. After replacement, most vehicles need a throttle or idle relearn procedure.

5. Known Throttle Body or Harness Defect

Check for bulletins before buying parts. Mazda’s service bulletin 01-007/13 for 2009 to 2010 Tribute models built between June 22 and October 15, 2009 describes P2135 with P2111 and/or P0122, and symptoms including intermittent rough idle, low idle speed and reduced power. It attributes these to the throttle body assembly hardware, and the repair is a new throttle body. For the CX-9, Mazda’s fix for its throttle sensor circuit codes was a modified emissions harness if no other fault was found. The right answer depends on the vehicle, which is exactly why the bulletin check comes first.

6. Engine Computer Fault

Rare, and the last conclusion. If the reference output stays low at the computer’s own connector with every sensor on that circuit unplugged and the harness proven good, the computer’s reference supply may have failed. Before replacing it, make sure the short that may have damaged it has been found, and check for software updates.

Diagnostic Order

1. Look up P0122 for your exact engine: which sensor is A, and what “low” means.
2. Record every stored code. Several low or reference codes at once point to the shared reference.
3. Check for service bulletins.
4. Measure the 5-volt reference at the sensor connector, and if it is low, unplug the sensors that share it one at a time.
5. Check the signal wire for a short to ground and for continuity, and do a wiggle test in live data.
6. Sweep the sensor and look for dead spots.
7. Replace the throttle body, pedal or sensor only once the tests point at it, and perform the relearn.

Repair Cost

Typical costs, labelled as typical because parts vary widely between vehicles:

• Connector or terminal repair: roughly $20 to $200.
• Wiring or reference circuit repair: roughly $50 to $300, more if the fault is hard to reach.
• Another sensor found shorting the shared reference: roughly $50 to $400, depending on the sensor.
• Separate throttle position sensor (older engines): roughly $100 to $250 installed.
• Electronic throttle body: roughly $250 to $800 installed, including the relearn.
• Accelerator pedal assembly: roughly $150 to $450 installed.

Can You Keep Driving With a P0122 Code?

Carefully, and not for long. On electronic throttle systems, reduced power can leave you unable to accelerate when you need to. On older systems, the engine may stumble or stall as you come to a stop. A shared reference fault is the most disruptive version, because several sensors can drop out together. If the car is already in reduced power, stalling or behaving erratically, drive only as far as needed to get it somewhere safe.

Related Codes

• P0123: the same sensor’s circuit reading high, the opposite fault, usually a lost ground or a short to voltage. See the P0123 guide.
• P0121: sensor A range or performance. The signal is electrically possible but does not make sense. See the P0121 guide.
• P0222: throttle position sensor B circuit low. P0122 and P0222 together point to the shared reference or the throttle body connector.
• P0641 and similar sensor reference voltage codes: the 5-volt reference itself is out of range. With P0122, suspect the reference first.
• P2135: the two throttle position sensors disagreeing. It often sets alongside a circuit code, as Mazda’s Tribute bulletin shows. See the P2135 guide.
• Throttle body faults: carbon build-up affects idle and airflow, not the sensor circuit. See the symptoms of a bad throttle body.

Frequently Asked Questions

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