P0125 and P1135 Trouble Codes - 1999 Lexus RX300 Engine Diagnosis
Re: 1999 Lexus RX300 3.0L engine - experiencing two persistent trouble codes: P0125 and P1135. The check engine light is currently on. The Mass Air Flow (MAF) sensor was recently replaced, and knock sensors were also installed to resolve transmission shift issues. Despite these repairs, the issue persists. P0125 - Engine Coolant Temperature (ECT) Insufficient for Closed Loop Fuel Control: This code typically indicates that the engine is not reaching a sufficient operating temperature to enter closed-loop fuel control. Possible causes include a stuck-open thermostat preventing full warm-up, faulty ECT sensor readings, or poor electrical connections leading to erratic mode switching. P1135 - Heated Oxygen Sensor (HO2S) Lean Condition Bank 1 Sensor 1: This code is triggered when the ECM heater output to the oxygen sensor exceeds 8 amps or falls below 0.25 amps during two consecutive driving sessions with active fault detection. Common root causes include a short circuit in the power wire to the front O2 sensor (Bank 1 = cylinder side with #1), excessive voltage output from the ECM, or poor connections at the sensor or wiring harness. It may also stem from a dirty, loose, or corroded connection. Recommended troubleshooting steps: Test the resistance of the heated oxygen sensor to ground, verify all electrical connections for proper ECM signal output, and inspect the wiring harness for damage or corrosion. In some cases, replacing the ECM may be necessary if internal faults are detected. This issue is common in 1999 Lexus RX300 models with high mileage (over 150,000 miles), especially when combined with aging sensor components and electrical degradation.
P0125 - Engine Coolant Temperature (ECT) Insufficient for Closed Loop Fuel Control. A stuck-open thermostat may prevent the engine from fully warming up, which is required for continuous closed-loop fuel operation. Alternatively, a faulty ECT sensor or poor electrical connections could cause erratic switching between open and closed loop modes. P1135 - Heated Oxygen Sensor (HO2S) Lean Condition Bank 1 Sensor - This code typically activates when the ECM heater output to the O2 sensor exceeds 8 amps or drops below 0.25 amps, detected during two consecutive driving sessions with active faults. Neighboring P113x codes indicate specific voltage thresholds and diagnostic procedures. Common causes include a short in the power wire to Bank 1 (cylinder #1 side), ECM output outside specifications, or degraded connections at the sensor or wiring harness. Recommended actions: Test sensor resistance to ground, verify all ECM-to-O2 sensor connections for proper signal output. A dirty, loose, or corroded connection is also a likely culprit. This diagnosis applies specifically to 1999 Lexus RX300 models with high mileage and should be evaluated in conjunction with other engine performance symptoms.