What's the difference between P0122 and P0123?

Both are hard electrical TPS faults — the sensor signal isn't a believable real-world value. P0122 means the signal is pegged near zero volts (shorted to ground, open signal wire, or internally-grounded sensor). P0123 means the signal is pegged near 5V (open ground, short to reference voltage, or internally-shorted-to-reference sensor). The driver experience is nearly identical — both trigger limp mode — but the wiring tests you run during diagnosis are different. P0122 makes you check signal continuity and reference voltage. P0123 makes you check ground continuity and look for a short to the 5V reference. The repair, when it's the sensor, is the same in both cases (usually throttle body replacement on modern vehicles).

Can I keep driving with P0123?

Short distance home or to a shop — yes. Continuing to drive normally is unsafe because the PCM has the throttle locked at a low ceiling. You can't accelerate to merge, can't pass, can't climb steep hills at normal speed. On freeways or rural highways, that's a real safety issue. P0123 also doesn't tend to clear by itself because the failure is electrical and persistent. Get it diagnosed within days.

Could P0123 have been caused by a recent repair?

Yes, frequently. Under-hood work in the area of the throttle body — intake manifold service, air filter housing replacement, EGR work, even battery service — can disturb the throttle body connector or pinch the harness. If P0123 appeared right after a service visit, pull the throttle body connector and check for: pushed-back pins, missing or damaged locking tab, harness routing against a sharp edge or hot component, or visible wire damage. This is the leading cause of post-repair P0123 codes. Fixing the wiring issue clears the code without needing a throttle body replacement.

How do I test for P0123 at home?

You need a multimeter and the wiring pinout. Start with the ground test because open ground is the most common P0123 cause: with the sensor connector disconnected, measure continuity between the harness-side ground pin and chassis ground. Should be near zero ohms. If it's open, repair the ground wire — that's likely your fix. If ground is good, check reference voltage (5V at the reference pin with key on). If reference is correct and ground is good but the sensor reads pegged-high when connected, the throttle body needs replacement. If reference is missing or pegged-high before you connect the sensor, you have a wiring short between the signal and reference wires somewhere upstream.

OBD-II trouble code

P0123: Throttle/Pedal Position Sensor 'A' Circuit High Input

The throttle position sensor signal is pegged near the 5V reference voltage no matter where the pedal is. Mirror of P0122 — usually a short to reference voltage, an open ground at the sensor, or a failed-internal-shorted sensor.

Quick facts

System: Powertrain
Category: Throttle / Idle
Severity: High severity
Drivable: Usually safe to drive short-term
Repair cost range: $100 – $700
DIY difficulty: Intermediate DIY

What does P0123 mean?

P0123 is the high-side electrical mirror of P0122. The TPS signal sits pegged near the 5V reference voltage — typically above 4.7-4.8V — regardless of where the throttle plate or pedal is. On a healthy sensor, signal voltage at closed throttle should be around 0.5V; pegged at 4.8V means the sensor or its wiring is broken in a way that drags the signal up to reference.

The PCM treats P0123 as an unbelievable reading. Even at wide-open throttle, a working TPS won't peg at the reference voltage — it'll come close (around 4.5V) but stop short. When the signal stays at or above the upper threshold for long enough, the PCM concludes the circuit is shorted to reference voltage or the ground is open, ignores the TPS, and falls back to a reduced-power limp strategy.

What the driver experiences is almost identical to P0122 — 'Reduced Engine Power' warning, capped RPM, unresponsive throttle. The internal mechanism is different but the safety response is the same. The PCM can't drive the throttle based on an untrustworthy sensor, so it locks the throttle near idle until the issue is resolved.

Three main causes account for most P0123 codes. Open ground at the TPS connector is number one — without a ground reference, the signal floats up toward the supply voltage. Short between the signal wire and the 5V reference wire (somewhere in the harness) sends full reference voltage straight to the PCM input. Finally, a failed-internal TPS that shorts to reference voltage produces the same symptom. The diagnostic split between these is whether the ground continuity test passes at the connector — if ground is open, the issue is wiring; if ground is good, the sensor is shorted internally.

Common causes

Open or broken ground wire at the TPS — the most common cause
Short between the TPS signal wire and the 5V reference wire (harness damage)
Failed TPS element internally shorted to reference voltage
Water intrusion in the throttle body connector creating an unintended path to 5V
Corroded or pushed-back ground pin in the throttle body connector
Damaged wiring near the throttle body — chafing against a sharp edge or hot exhaust component
Rodent damage to the harness
Pinched or partially-disconnected wire from a recent under-hood repair
Failed PCM driver circuit (rare)

Symptoms

Check Engine Light on, often immediately at key-on
'Reduced Engine Power' warning on the dash
Hard limp mode — RPM capped, throttle unresponsive past idle range
Engine starts but won't accelerate beyond a low ceiling
Stalling when coming to a stop
Rough or unstable idle
Some platforms refuse to start until the code is addressed

Diagnostic steps

1.Pull all codes. P0123 with companion codes (P0121, P2135) tells a different story than P0123 alone.
2.Inspect the throttle body connector — disconnect, look for water, corrosion, pushed-back ground pin, or damaged locking tab. Many P0123 codes resolve at the connector.
3.With the sensor disconnected and the key on, verify reference voltage at the harness side (5V between reference pin and chassis ground). Confirms the PCM is supplying the reference correctly.
4.Check ground continuity from the harness-side ground pin to chassis ground — should be near zero ohms. An open here is the most common P0123 cause and is often what looks like a 'failed sensor.'
5.Reconnect the sensor. With engine off, key on, watch TPS signal voltage on a scan tool. Pegged at 4.5V or higher confirms the failure pattern.
6.If wiring tests pass and the signal stays pegged-high when connected, the sensor (or throttle body) is the failure point.
7.After repair, perform the platform-specific throttle relearn procedure.

Repair cost

$100 – $700

Low end is a ground wire repair or connector cleanup — under $150 once diagnosis is done. Mid-range $300-500 is throttle body replacement on mainstream platforms where the TPS is integrated. Upper end $500-700 covers luxury platform throttle body replacement. Harness repairs add diagnostic time because finding the exact location of a short to reference voltage can take a couple hours of bench work. If wiring is the cause and a wiring repair is sloppy, the code may recur within months — sometimes it's cheaper to replace a damaged section of harness with a new pigtail.

Estimate your repair

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Open the Repair Cost Estimator with throttle body replacement preselected. Adjust labor rate and vehicle category to fit your situation.

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DIY vs shop

This is an intermediate DIY job. It usually involves diagnostic steps, specialty parts, and some careful work in tight spaces. If you have the tools and a service manual or trustworthy video for your specific vehicle, it is achievable in a weekend. Otherwise, a competent independent shop will be faster.