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P0128 engine code and no voltage from coolant harness

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Posted (edited)

I recently got a P0128 engine code on my RX300 (year 2000) and was trying to find the course of the issue. 

I began by testing the voltage and continuity of the coolant harness and got no voltage and no continuity - so it seems like replacing that be the first order of business. I tested the voltage with the key in the "on" position, but without starting the engine. I tested the continuity with the car fully off. I have not tested the sensor itself yet. 

Has anyone had a similar issue with the coolant harness readings or have any useful instructions on how to change out that harness and/or the wiring? 

Edited by burnse
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The P0128 code on your Lexus RX300 typically indicates that the engine coolant temperature is below the expected threshold, which is often due to a malfunctioning thermostat or a problem with the Engine Coolant Temperature (ECT) sensor circuit. Since you’ve found no voltage and no continuity in the coolant harness, it does suggest an issue with the wiring or the sensor itself.

Here’s a step-by-step guide to help you address the issue:

  1. Check the ECT Sensor: Before replacing the harness, it’s a good idea to test the ECT sensor itself to ensure it’s functioning properly. You can do this with a multimeter by measuring the resistance across the sensor terminals and comparing it to the manufacturer’s specifications.

  2. Inspect the Wiring: Look for any signs of damage, corrosion, or wear on the wiring and connectors that could cause a loss of continuity. Repair or replace any damaged sections.

  3. Replace the Coolant Harness: If the wiring and sensor are in good condition, proceed to replace the coolant harness. Here’s how you can do it:

    • Disconnect the battery to ensure safety.
    • Locate the coolant harness connection points on your vehicle.
    • Remove any components or covers that may be obstructing access to the harness.
    • Carefully disconnect the old harness from the sensor and any clips or retainers holding it in place.
    • Install the new harness by connecting it to the sensor and securing it with the original clips or retainers.
    • Reassemble any components or covers you removed.
    • Reconnect the battery and start the vehicle to check if the code has been resolved.
  4. Clear the Code: After the repair, clear the P0128 code with an OBD-II scanner and monitor the engine temperature to ensure the issue has been resolved.

Remember to always follow the specific repair manual for your Lexus RX300 year 2000 for detailed instructions and torque specifications. If you’re not comfortable performing these repairs yourself, it’s best to consult a professional mechanic. Safety is paramount, so take all necessary precautions when working on your vehicle. 


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