P0123 Code: Throttle Position Sensor High — Causes & How to Fix

P0123 means the engine computer is reading a signal from position sensor “A” that is higher than a working circuit should produce. On most vehicles, sensor A is the first throttle position sensor, and a reading pinned high usually means the sensor has lost its ground or its signal wire is shorted to a voltage source. But the code’s official name is “throttle/pedal position sensor/switch A circuit high”, and on some vehicles sensor A is the accelerator pedal, not the throttle. On at least one engine the “high” code even sets on a very low voltage. So before testing anything, confirm which sensor your vehicle means. This article covers that, the six real causes in order of likelihood, the tests that find them, and what each repair typically costs.

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

Cause	Telltale Sign	Typical Cost
Lost sensor ground or open ground circuit	Signal pinned near the reference voltage; other sensors’ codes on the same ground	$50 to $300
Signal wire shorted to the 5-volt reference or another voltage	Chafed harness; reading high at every throttle position	$50 to $300
Connector or terminal fault	Code comes and goes; corrosion or spread terminals at the sensor	$20 to $200
Failed position sensor	Wiring tests good; signal jumps or sits high on its own	$100 to $800 (often a throttle body or pedal)
Known harness defect covered by a bulletin	Your vehicle is listed in a manufacturer bulletin for P0123	$0 to $300
Engine computer fault	Sensor, wiring and reference all test good	$300 to $1,000

Drive only as far as you need to if the car is in reduced-power mode, the throttle response is erratic, or the engine surges on its own. On electronic throttle systems, the computer limits power when it cannot trust a position sensor, and an erratic throttle signal can make the engine surge or hesitate unexpectedly. If P0123 is stored but the car drives normally, it can be driven carefully to a diagnosis.

What P0123 Actually Means

A position sensor of this type is fed a 5-volt reference and a ground by the engine computer, and returns a signal voltage that changes with position. Throttle sensors typically read low with the throttle closed and rise as it opens, though some are designed the other way round. A working sensor never reaches the full reference voltage or zero, and the computer uses that fact to spot circuit faults. A signal above the highest value the sensor can legitimately produce sets P0123. A signal below the lowest sets P0122.

The reason a high reading usually means a lost ground is simple electrics. If the sensor’s ground connection breaks, nothing pulls the signal down, so it rises to the full reference voltage. The same happens if the signal wire touches the reference wire, or any other voltage. That is why the ground circuit is the first thing to test for P0123.

Which Sensor Is “A”, and What Does “High” Mean?

This is the point that stops the wrong part being bought. Volkswagen’s 2014 Beetle quick reference specification book shows how much the definition can vary within a single model. On the Beetle’s gasoline engines, P0123 is an accelerator pedal position sensor fault, “Accelerator Pedal Position Sensor 1/Accelerator Pedal Position Sensor 2 Circuit High Input” on the 1.8L and 2.0L, set on a signal voltage above 4.81 volts. On the 2.0L TDI diesel in the same book, P0123 is “Throttle/Pedal Position Sensor/Switch ‘A’ Circuit High”, and it sets on a signal voltage below 150 millivolts, while the “low” code P0122 sets above 4.85 volts. The manufacturer’s labels are the reverse of the raw voltage on that engine.

So look up the definition for your exact engine before testing. Replacing a throttle body for a pedal sensor fault, or testing for a high voltage when the code actually means a low one, wastes both money and time. The site’s article on the symptoms of a bad accelerator pedal position sensor covers the pedal side.

Reading the Three Wires

Most position sensors have three connections, and one meter reading on each, with the sensor plugged in, the ignition on and the engine off, tells you most of what you need. Back-probe the connector rather than piercing the wires.

• Reference: should read close to 5 volts. Present and steady means the computer is supplying the sensor.
• Ground: should read very close to 0 volts measured against a good engine ground. Anything more than a small fraction of a volt means resistance in the ground path, which drags the signal upward, the classic P0123 fault.
• Signal: should sit within the sensor’s normal range and move smoothly as the throttle or pedal moves. A signal that reads the same as the reference voltage at every position means the signal is either shorted to the reference or has lost its ground.

On electronic throttles and pedals there are usually two sensors in one housing, each with its own signal wire, and often with a shared reference and ground. Compare the two signals as you sweep. If both jump together, suspect the shared wiring. If only sensor A misbehaves, suspect its own signal circuit or its track inside the housing. Record the freeze frame data too: the throttle position and engine load when P0123 set show whether it happened at a particular throttle angle, which points to the sensor, or at random, which points to wiring.

Symptoms You May Notice Alongside the Code

On electronic throttle systems, expect a reduced-power mode, often with a “reduced engine power” message, a wrench light or an EPC light, and a throttle that responds sluggishly or not at all. Mazda’s bulletin for the CX-9 describes exactly that combination with P0123. On older cable-throttle engines, where the sensor only informs fuelling and idle control, symptoms are milder: hesitation, surging, a high or unstable idle, and shift problems on automatics that use throttle position. The site’s articles on an engine that surges while driving and the symptoms of a bad throttle position sensor cover these in more detail.

1. Lost Sensor Ground or Open Ground Circuit

The most likely electrical cause of a high reading. With the sensor connected and the ignition on, back-probe the sensor’s ground terminal and measure voltage to a good engine ground. It should be very close to zero, typically well under a tenth of a volt. A higher reading means resistance or a break in the ground path. Many engines share one sensor ground between several sensors, so P0123 alongside other “high” codes, such as intake air temperature or coolant temperature circuit high, points strongly to that shared ground.

2. Signal Wire Shorted to the 5-Volt Reference or Another Voltage

If the signal wire touches the reference wire, or any powered circuit, the signal is pulled high whatever the throttle does. Mazda’s service bulletin 01-017/13 for the 2011 to 2013 CX-9 lists the checks for P0123 as: a connector or terminal malfunction, a “short to constant voltage supply circuit” between the throttle body and the powertrain computer, the throttle position “sensor signal circuits are shorting each other”, and a sensor malfunction. With the sensor and computer both disconnected, check the signal wire for continuity to the reference wire, to the other sensor’s signal and to battery voltage.

3. Connector or Terminal Fault

A ground terminal that has lost its grip, or corrosion that adds resistance on the ground side, produces exactly the high-reading symptom, often intermittently. Unplug the sensor connector and inspect both halves for green corrosion, spread or pushed-back terminals and a broken locking tab. Then watch the signal in live data while gently wiggling the connector and harness. A reading that jumps as the wiring moves points to the connection, not the sensor.

4. Failed Position Sensor

Once the reference, ground and signal wiring are proven good, the sensor itself is the suspect. Sweep the throttle or pedal slowly through its range, graphing the signal. A healthy sensor produces a smooth line within its normal range, while a failing one jumps high at particular positions or sits high regardless. 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 that whole unit. On older engines the sensor is a separate, inexpensive part. The site’s guide to P0121 covers the related range fault.

5. Known Harness Defect Covered by a Bulletin

Some vehicles have a documented P0123 problem that no amount of sensor replacement will fix. Mazda’s service bulletin 01-008/13 for the 2011 to 2013 Mazda6 describes P0123 and P0222 (sensor 2 circuit low) set immediately “due to an abnormal electrical noise within the engine harness”, corrected by installing a throttle body harness with a diode. For the CX-9, the fix was an improved emissions harness. Always check for bulletins before buying a throttle body.

6. Engine Computer Fault

Rare, and the last conclusion. If the reference, ground and signal all test correct at the sensor connector and the computer still reports a high reading, the fault may be in the computer’s input. Check for a software update first, and confirm the computer’s own grounds before condemning it.

Diagnostic Order

1. Look up P0123 for your exact engine: which sensor is A, and what “high” means.
2. Record every stored code. Other “high” codes point to a shared ground; P2135 or P0222 point to the throttle body circuit.
3. Check for service bulletins.
4. Inspect the connector and do a wiggle test while watching live data.
5. Measure the ground, then the 5-volt reference, then the signal at the connector.
6. Check the signal wire for shorts to the reference and to voltage with both ends disconnected.
7. Replace the sensor, throttle body or pedal only once the tests point at it, and perform any relearn procedure.

Repair Cost

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

• Connector or terminal repair: roughly $20 to $200.
• Ground or wiring repair: roughly $50 to $300.
• Separate throttle position sensor (older engines): roughly $100 to $250 installed.
• Electronic throttle body: roughly $250 to $800 installed.
• Accelerator pedal assembly: roughly $150 to $450 installed.
• Harness kit under a bulletin: often free under warranty.

Can You Keep Driving With a P0123 Code?

Carefully, and not for long. On electronic throttle systems, the computer’s protective response is to limit power, which can leave you unable to accelerate when you need to. On older systems, a false high throttle reading can cause surging, a high idle or harsh shifting. If the car is already in reduced power or behaving erratically, drive only as far as needed to get it somewhere safe.

Related Codes

• P0122: the same sensor’s circuit reading low, the opposite fault. It usually points to a lost 5-volt reference or a short to ground. See the P0122 guide.
• P0121: sensor A range or performance. The signal is electrically possible but does not make sense. See the P0121 guide.
• P0222 and P0223: throttle position sensor B circuit low and high, for the second sensor. P0123 with P0222 together points at the shared circuit, as in the Mazda bulletins.
• P2135: the two throttle position sensors disagreeing with each other. It often sets alongside a circuit code. See the P2135 guide.
• P2138: the accelerator pedal’s own correlation code. See the P2138 guide.
• Throttle body faults: carbon build-up causes idle and airflow problems, not circuit codes. See the symptoms of a bad throttle body.

Frequently Asked Questions

As an Amazon Associate we earn from qualifying purchases.