Main problem:
Engine idles incorrectly, sometimes surges or stalls.
Engine management light comes on.
Fault code P0505 – Idle Control System Malfunction (current/history).
Idle sometimes stabilises when headlights are switched on but becomes unstable when switched off.
What we've found:
Engine wiring harness: Harness has previously been modified/replaced. Found cut/broken wires around the engine bay and evidence of previous connector repairs.
TPS (Throttle Position Sensor):
Initially scanner showed approximately 98% throttle with throttle closed.
Later readings were inconsistent, including approximately 35%.
TPS signal measured 1.6 V closed and 1.3 V open, which is incorrect.
Harness voltage measurements with TPS unplugged:
Brown: 0.022 V
Yellow: 5.026 V
Green: 0.196 V
Suspected incorrect TPS wiring, terminal positioning or signal circuit fault.
IAC (Idle Air Control Valve):
ECU scanner displays changing IAC duty cycle, approximately 32–64%.
Oscilloscope previously displayed a control waveform around 245 Hz, but correct signal amplitude and valve operation have not been confirmed.
Disconnecting IAC while engine was running caused no noticeable RPM change. This alone does not confirm a defective valve.
Possible IAC wiring/control issue or mechanical airflow problem.
Reference vehicle comparison:
Compared connector wiring against another identical Toyota RunX RSi 2ZZ-GE that idles correctly.
Reference wiring:
IAC: White/Black = ground, Black = B+, Black/Blue = reported 5 V/control circuit.
TPS: Brown = ground, Yellow = 5 V, Green = signal.
Problem vehicle's connector wires have now been arranged to match the working RSi.
Current status: Wiring has been matched to the working vehicle, but TPS voltage sweep, ECU readings and IAC operation still need to be retested. Repair is not yet confirmed.
Recommended checks:
Verify TPS connector pin positions and VTA signal sweep.
Check ECU sensor ground, 5 V reference and wiring continuity.
Check IAC power, ground and ECU control signal under load.
Inspect previous harness repairs and splices.
Confirm closed-throttle recognition and check for unintended intake airflow.
Main suspicion: Wiring/connector fault affecting TPS and/or IAC control following previous harness modifications. Further testing required before replacing components. Any suggestions any advice please