Code
P0122
ISUZU
P — Powertrain
Throttle Position Sensor Circuit Low
Views:
UK: 19
EN: 75
RU: 49
AI status
Completed
Completed
100%
Causes
- Faulty throttle position sensor (TPS)
- Short to ground in TPS signal circuit
- Poor reference voltage or sensor ground
- Corroded/loose connector or pins at the TPS
- Damaged wiring harness (chafing, broken conductor)
- Faulty ECM (less common)
Symptoms
- Check Engine Light (MIL) ON
- Poor or unstable idle
- Engine stalls at idle or when coming to a stop
- Hesitation or reduced throttle response
- Engine may enter limp mode/reduced power
- Surging or unexpected RPM changes during acceleration
What to check
- Retrieve freeze-frame and readiness data, confirm P0122 stored and active
- Visual inspection of TPS, connector, and wiring for damage/corrosion
- Backprobe TPS connector to measure signal, reference and ground voltages with key ON (engine off)
- Compare live TPS voltage/percentage to throttle position while manually moving throttle
- Check for wiring continuity and shorts to ground/power between TPS and ECM
- Wiggle test wiring while monitoring TPS signal for intermittent faults
Signal parameters
- TPS signal (closed/throttle at rest): typically ~0.2–0.8 V (varies by vehicle)
- TPS signal (wide open throttle): typically ~4.0–4.8 V
- Reference voltage to TPS: ~5.0 V (±0.25 V) from ECM
- Sensor ground: near 0 V; low resistance to chassis ground
- Signal should change smoothly and monotonically with throttle movement (no jumps or dropouts)
Diagnostic algorithm
- Read and record freeze-frame and all stored codes; note conditions when code set.
- Perform a visual inspection of TPS, throttle body, connector and wiring for damage, corrosion, loose pins or repairs.
- With ignition ON (engine OFF) backprobe TPS connector: verify reference ~5 V, sensor ground near 0 V, and signal voltage at closed throttle (should be within expected range).
- Slowly open throttle by hand while observing live TPS signal—voltage should rise smoothly to near wide-open values. Look for dropouts or stuck readings.
- If signal is low or absent, check continuity of signal wire between TPS and ECM and check for short to ground or power. Repair any shorted or open wiring.
- Perform wiggle test on wiring and connectors while watching the TPS signal to locate intermittent faults.
- If wiring and connector are good but signal remains incorrect, replace TPS with OEM or equivalent unit and retest.
- After repair clear codes, perform functional test and road test; verify code does not return and throttle response/idle are normal.
- If problem persists after replacing TPS and wiring checks, test/replace ECM or consult manufacturer-specific diagnostics.
Likely causes
- TPS element failure (internal short or open)
- Signal wire contact corrosion or bent pin at connector
- Wire chafing contacting chassis ground or engine ground
- Poor or high-resistance ground or 5V reference at sensor
- Connector pushed out, contamination, or broken latch after service
Fault status
Status
ECM detected TPS signal voltage below expected threshold on the throttle position sensor A circuit (low input). This indicates a low or missing TPS signal or circuit fault.
Repair difficulty: Medium
Diagnostic time: 0.5-2.0 hours
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