What does it mean?
P1798 is a Powertrain (engine, transmission, fuel system) fault code. It indicates: Engine Coolant Temperature (ECT) sensor Malfunction .
Symptoms
- MIL (check engine lamp) is on and the temperature gauge reads high, low or does not move
- Hard starting when cold and rough running until the engine warms
- Rich fuel smell or black smoke from the exhaust
- Cooling fans run all the time from a cold start, or never come on
- Fuel consumption noticeably higher than normal
- Late, harsh shifts and no torque converter lockup
Likely causes
- Sensor thermistor drifted with age, so it reads a temperature that is consistently wrong
- Sensor failed open or shorted internally
- Corroded, cracked or loose connector at the sensor, made worse by heat cycling
- Open, shorted or high-resistance signal or ground wire
- Low coolant level or an air pocket, leaving the sensor tip out of the coolant
- Thermostat stuck open, so the engine never reaches the temperature the module expects
- Wrong replacement sensor with a different resistance characteristic
How to fix it
- Leave the vehicle to cool completely, then compare the sensor reading with ambient temperature
- Watch the reading climb smoothly during warm-up and check it against a surface temperature reading of the housing
- Measure the sensor resistance at several temperatures and compare with the specified table
- Back-probe the signal wire and ground at the connector with the ignition on
- Check the coolant level, bleed trapped air and check thermostat operation before condemning the sensor
- Fit the correct sensor, refill and bleed the cooling system, then verify a smooth warm-up curve in live data