Code
P1129
LEXUS
P — Powertrain
Electronic Throttle Control System
Views:
UK: 28
EN: 57
RU: 35
AI status
Completed
Completed
100%
Causes
- Faulty throttle body / throttle actuator motor
- Faulty accelerator pedal position (APP) sensor
- Damaged wiring or poor connector connection between APP, throttle body, and ECM
- Low or unstable battery/charging system voltage
- Contaminated or mechanically sticking throttle plate
- ECM software or internal ECM fault
Symptoms
- Check Engine Light (MIL) illuminated
- Reduced engine power / limp mode
- Unexpected or delayed throttle response
- High or unstable idle, or engine stalling at idle
- Inability to maintain commanded engine speed under load
What to check
- Retrieve freeze frame and all stored DTCs with a capable scan tool; note PID values and status bits
- Check battery voltage and charging system; ensure >11.5 V during tests and >12.6 V at rest
- Visually inspect wiring harnesses and connectors at the throttle body, APP sensor, and ECM for damage or corrosion
- Check for technical service bulletins (TSBs) and ECM software updates for ETC issues
- Verify no mechanical interference with throttle plate (carbon buildup, foreign object)
Signal parameters
- Accelerator pedal sensor voltage: typically ~0.5–4.5 V per channel (two-channel redundancy)
- Throttle position sensor voltage: ~0.5–4.5 V depending on plate angle
- Throttle actuator motor current: nominal low current; abnormally high current indicates binding
- Battery voltage: 11.5–14.5 V (during tests)
- APP-to-throttle correlation: APP % vs throttle angle % should correlate smoothly without sudden jumps
Diagnostic algorithm
- Connect a diagnostic scan tool, record all DTCs and freeze-frame data; clear codes and attempt to reproduce under controlled conditions.
- Verify battery and charging system voltage; charge or replace battery if voltage low or unstable.
- Inspect connectors and wiring for corrosion, bent pins, chafing or damage at APP sensor, throttle body and ECM. Repair as needed.
- With ignition ON (engine off), monitor APP sensor voltages/channels on scan tool; verify both channels track smoothly and are within expected ranges.
- Command throttle open/close with scan tool (if supported) and observe throttle position PID and actuator current; look for lag, nonlinearity, or motor draw spikes.
- Physically inspect throttle plate for carbon build-up or binding; clean throttle body per manufacturer procedure if contamination present.
- Backprobe and measure voltages/grounds at sensors and throttle actuator; check sensor reference, signal, and ground continuity to ECM.
- If wiring and sensors check OK but fault persists, perform throttle relearn/calibration procedure as specified by manufacturer.
- If fault remains after above steps, consider replacing the throttle body assembly or APP sensor (replace only diagnosed failed component).
- After repair, clear codes and perform road test to confirm proper throttle response and that code does not return.
Likely causes
- Corroded or loose connector at throttle body
- Open/short in APP sensor wiring (signal or ground)
- Throttle actuator motor degraded or seized
- Low battery voltage during key-on or cranking
- Failed throttle position sensor or inconsistent APP-to-throttle correlation
Fault status
Status
Electronic Throttle Control fault detected. MIL on; engine may be in reduced-power (limp) mode until fault is resolved.
Repair difficulty: Medium
Diagnostic time: 1.0-3.0 hours
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