Engine coolant temperature (ECT) sensor codes

How its codes differ

• P0128 usually means a thermostat stuck open, but a sensor that reads lower than the real temperature can also set it; comparing the sensor reading with the gauge or a separate thermometer separates the two.
• P0118 is the circuit version: the signal is at the high-voltage end, which the computer reads as extremely cold, pointing to an open circuit or failed sensor.
• P0125 means the engine took too long to reach the temperature needed for closed-loop fuel control; like P0128 it can be a thermostat or the sensor.
• A sensor that reads cold makes the computer add fuel, so a rich code (P0172) can follow it.
• P0175 (bank 2 rich) can follow a sensor that reads too cold, for the same reason as P0172.

Codes

• P0128

  Engine coolant stays below thermostat temperature

  Severity: Monitor

• P0118

  Engine coolant temperature sensor circuit high

  Severity: Severity not verified

• P0125

  Coolant too cold for closed-loop fuel control

  Severity: Monitor

• P0172

  Engine computer has reached its limit removing fuel on bank 1: the mixture is rich

  Severity: Monitor

• P0175

  Engine computer has reached its limit removing fuel on bank 2: the mixture is rich

  Severity: Monitor

Signs it's failing

• Check engine light with P0128, P0118 or P0125
• Higher idle speed
• Higher fuel use
• Poor starting

How to test it

1. On a cold engine, compare the scan-tool coolant temperature with outside air; after a drive, compare it with the dash gauge. A reading that is far off or jumps around points to the sensor or wiring. (Low risk)

2. With the engine off, check the sensor connector for corrosion or damage. (Use caution)

   Warning: Only on a cold engine: hot coolant can scald, and never open the radiator or reservoir cap while the engine is hot.

3. Resistance tests at temperature, and sensor replacement (which can mean losing coolant), are best done by a shop. (Best left to a shop)

   Warning: Hot, pressurised coolant can cause severe burns. Let the engine cool completely before any cooling system work.

After the repair

• Check that the coolant temperature reading rises normally and matches the dash gauge.
• Top up and bleed the cooling system if coolant was lost.
• Clear codes only after the repair, then drive normally over several days with both city and highway driving so the readiness monitors can complete; clearing codes or disconnecting the battery resets them to not ready, which fails an emissions test.