📌 What Does the Mercury P1290 Code Mean?
The Cylinder Head Temperature (CHT) sensor is installed in the cylinder head wall, without direct contact with the coolant.
It directly measures the metal temperature of the cylinder head and sends this data to the PCM (Powertrain Control Module).
🔎 In case of overheating:
- At 126°C (258°F): the PCM disables four injectors to reduce temperature (alternating them every 32 engine cycles)
- At 154°C (310°F): all injectors are disabled until the temperature decreases
The P1290 code activates when the PCM detects an abnormally low cylinder head temperature, indicating a problem in the engine’s thermal regulation.
🚨 Possible Symptoms
- Check Engine light on
- “Service Engine Soon” light activated
- Engine may run cold longer than expected
- Fuel consumption or engine performance issues
❗ Possible Causes
- Faulty thermostat
- Engine overheating
- Low coolant level
- Defective CHT sensor
- Open or short circuit in CHT sensor wiring
- Poor electrical connection of the CHT sensor
🛠️ How to Fix Mercury P1290 Code
- Check the possible causes listed above.
- Visually inspect:
- The wiring harness connected to the CHT sensor
- Connectors (look for broken, bent, pushed out, or corroded pins)
- Check:
- The proper operation of the thermostat
- The coolant level
- The condition and signal of the CHT sensor using a diagnostic tool
⏱️ When Is the P1290 Code Detected?
The code is recorded when the PCM detects a cylinder head temperature below expected values, which may indicate a sensor or cooling system problem.