Do not drive. Limp-home only. Repair immediately. P0122 means the throttle position sensor A (VTA1 signal circuit) voltage has dropped to 0.2 V or below for two or more seconds -- indicating a shorted signal wire, open reference circuit, or a failed throttle body sensor.
What P0122 means
Electronic throttle bodies integrate a non-contact Hall-effect throttle position sensor with two independent signal circuits: VTA1 (the primary angle signal feeding engine control calculations) and VTA2 (a secondary monitoring signal used for cross-check). The ECM applies a 5 V reference through the VC pin; both VTA1 and VTA2 outputs vary proportionally from roughly 0.5 V at idle to 4.5-4.8 V at wide-open throttle. P0122 is a 1-trip code set when VTA1 measures 0.2 V or less for two continuous seconds, indicating either the signal wire has shorted to ground, the 5 V VC reference supply is broken, or the sensor element itself has failed. The ECM immediately activates fail-safe mode: the throttle actuator is de-energised and the valve returns to a partial-open rest position (approximately 7 degrees on Toyota systems), severely limiting engine output.
Symptoms
- Check engine light ON -- typically the first symptom
- Severe loss of power -- engine enters limp-home mode with throttle fixed near idle position
- Hesitation or stumble on acceleration, vehicle may feel unresponsive to throttle input
- Throttle unresponsive or stuck in a low-power range (fail-safe mode holds throttle at ~7 degrees on Toyota)
- Possible stall when coming to a stop if throttle valve rest angle is set too low
Common causes
- Failed throttle position sensor element inside the throttle body -- most common cause on higher-mileage vehicles
- Short to ground on the VTA1 signal wire between the throttle body connector and ECM
- Open circuit on the VC (5 V reference) wire, which causes both VTA1 and VTA2 to read 0-0.2 V simultaneously
- Corroded or backed-out pins at the throttle body connector
- Throttle body connector moisture intrusion causing signal-to-ground short
- ECM internal fault (rare) -- damaged VTA1 input or VC reference driver
Severity & driving advice
Severity: High — Fail-safe mode severely limits engine power. Do not drive on a highway. Repair before the next journey.
Can I drive? Do not drive. Limp-home only. Repair immediately.
Diagnostic approach
- Read live scan tool data to identify which circuit is at fault — Connect a scan tool and read Throttle Position No. 1 (VTA1) and Throttle Position No. 2 (VTA2) simultaneously. If both read near 0 V at all pedal positions, the VC 5 V reference supply is open. If VTA1 reads 0-0.2 V while VTA2 reads a normal 2.1-3.1 V at idle, VTA1 is shorted to ground or open. The pattern of which signal is low isolates whether the fault is in the shared reference, VTA1 alone, or the sensor element.
- Inspect the throttle body connector and harness for physical damage — Disconnect the throttle body connector and inspect pin terminals for corrosion, spread pins, or burnt insulation. On Toyota engines the 6-pin throttle body connector is mounted to the intake side of the engine where heat cycles can crack the connector housing. A melted or corroded pin on the VTA1 or VC circuit is a common finding.
- Measure harness continuity and shorts with both connectors disconnected — With the throttle body connector and ECM connector both unplugged, measure resistance between the VC pin at the throttle body and the VCTA pin at the ECM -- should be below 1 ohm. Measure VTA1 pin at the throttle body to VTA1 at ECM -- below 1 ohm. Measure VTA1 pin to body ground -- should be 10 kohm or more. A VTA1 pin reading near zero ohms to ground confirms the signal wire is shorted.
- Verify ECM supplies the 5 V reference voltage (VC check) — With the throttle body connector unplugged, turn the ignition on and measure voltage from the VC pin at the throttle body harness side to the E2 (sensor ground) pin. Should read 4.5-5.5 V. If it reads 0 V, the ECM VC driver is faulty or the reference wire is shorted to ground. Trace the VC wire for pinch damage before replacing the ECM.
- Replace the throttle body assembly if circuit checks pass — If harness continuity and ECM reference voltage are within spec, the internal sensor element is faulty. On most modern vehicles the TP sensor is not serviceable separately from the throttle body. After replacement, perform the throttle position initialisation procedure: ignition on 2+ seconds, ignition off 10+ seconds, start engine and verify VTA1 at 0.5-1.1 V at idle. Toyota and many other manufacturers require this step.
Make & model notes
Toyota: Toyota 4Runner, FJ Cruiser, and Tacoma with 4.0L 1GR-FE engines (2003-2015) experience P0122 most often from a corroded throttle body connector rather than a failed sensor. The connector sits close to the intake manifold and accumulates heat and moisture. Inspect the 6-pin connector and VC/VTA1 terminal condition before ordering a throttle body. The fail-safe throttle rest angle on these vehicles is approximately 7 degrees.
Chevrolet/GMC: GM Silverado, Tahoe, and Sierra with LS or LM7 V8s occasionally develop P0122 from the drive-by-wire throttle body sensor at high mileage. GM recommends running the throttle relearn procedure (idle the engine for 3 minutes without touching the accelerator) after any throttle body replacement to allow the ECM to establish its closed-throttle position calibration.
FAQ
Can P0122 be caused by a loose throttle body connector?
Yes. A partially connected or corroded throttle body connector is one of the most common causes. The VTA1 signal line can intermittently lose contact, dropping the reading below 0.2 V and triggering the code. Wiggling the connector while watching live data on a scan tool can reveal intermittent drop-outs before condemning the throttle body.
Is the throttle position sensor on the throttle body replaceable separately?
On most modern vehicles (post-2002) the TP sensor is permanently integrated into the throttle body unit and cannot be replaced independently. The entire throttle body assembly must be replaced. On older cable-actuated throttle designs, the sensor was bolted on separately and could be swapped, but this is uncommon on vehicles built after 2005.
What is the throttle initialisation procedure after replacement?
Most manufacturers require a relearn after throttle body replacement to calibrate the closed-throttle position. On Toyota vehicles the procedure involves: turn ignition on for at least 2 seconds, then off for 10 seconds, then start the engine and allow it to idle without touching the accelerator for 30-60 seconds. Some Chrysler and GM vehicles require a scan tool to command the throttle relearn.
Will P0122 cause the engine to stall?
P0122 activates fail-safe mode, which returns the throttle plate to a fixed partial-open position (around 7 degrees on Toyota systems). In most cases this prevents stalling at idle but severely limits top-end power. However, if the throttle valve mechanically sticks fully closed rather than returning to the rest position, the engine can stall. Inspect the throttle plate for carbon build-up blocking movement.