What does it mean?
P1290 is a Powertrain (engine, transmission, fuel system) fault code. It indicates: Cylinder Head Temperature Sensor Circuit Low .
Symptoms
- MIL (check engine lamp) on
- Temperature gauge reading high or pegged, cooling fans running constantly
- Rich running, black smoke and poor fuel economy because the module thinks the engine is hot
- No fast idle when cold and hard starting on a cold engine
- Overheat protection strategy may cut fuel to cylinders or limit power without cause
Likely causes
- CHT signal wire shorted to ground somewhere in the harness
- Internally shorted sensor with resistance far below specification
- Coolant or oil contamination inside the sensor connector
- Harness chafed against the cylinder head, a bracket or an exhaust component
- Corroded splice or high-resistance ground shifting the reading
- Module input circuit fault, considered only after the sensor and wiring are cleared
How to fix it
- On a fully cold engine, compare CHT with the coolant and intake air temperature values; all should sit close to ambient
- Unplug the sensor; if the displayed value jumps to the opposite extreme, the sensor or its connector is shorted
- Measure sensor resistance cold and again warm, comparing with the temperature/resistance table
- With the sensor unplugged, check the signal wire for a short to ground
- Inspect the connector for coolant, corrosion and spread terminals, and check the harness where it crosses the head
- Replace the sensor if it is out of specification, or repair the shorted harness section
- Clear the code and verify a normal warm-up curve through a full heat cycle