My temperature gauge consistently reads 0, and the engine cooling fans activate. The OBD2 code displayed is P0128. I've confirmed that there are 5 volts of power reaching the ECT (Engine Coolant Temperature) sensor, and the sensor's resistance reading is within normal range. However, the gauge remains at zero and the fan runs continuously. What should I check next to diagnose this issue?
I tried driving the vehicle with the cooling fans disconnected, and the temperature gauge dropped immediately. This strongly suggests there's a short circuit or intermittent connection in the wiring harness. I believe the problem lies somewhere along the sensor or its associated wires.
Replace the thermostat as the most likely cause of P0128 in this scenario. A stuck or malfunctioning thermostat can prevent proper temperature readings and trigger the code.
I experienced the exact same issue on a 2001 Pontiac Grand Am with a 3.4L V6 engine. The root cause was a broken wire — an intermittent connection that only appeared under certain conditions. Initially, when I tested the sensor and wiring, everything seemed fine. However, after driving, the problem reappeared. The vehicle's ECU began running the cooling fans continuously because it couldn't obtain a valid temperature reading. As a safety measure, it assumed the engine was overheating. I found that wiggling the wires — especially near the main harness at the top of the engine — caused intermittent behavior. The wire had become brittle and came loose inside the connector. I removed the damaged section and spliced the ends together; this resolved the issue. Another possible location is the thermostat housing plug, located on the driver's side of the engine, just in front of or beside the exhaust manifold. If wiring tests pass but symptoms persist, a faulty ECT sensor may be the culprit. Use an OBD2 scanner capable of reading live data while driving — the actual temperature should read around -39°F (249°F) or extremely high values. Such readings indicate either a short circuit or open circuit. Once this issue is fixed, engine idle and shift behavior will return to normal, and most stored codes will clear automatically. Please do not disconnect the fans — they are programmed to operate as designed for safety purposes.
The P0128 code typically indicates a faulty thermostat. If you're seeing this, start by checking the thermostat itself and ensure your coolant level is adequate. Since you have 5 volts to the ECT sensor and the fan is activating, it suggests power is reaching the fan — however, the fan may be running excessively. This could point to an intermittent wiring issue or a faulty connection.