P128A MERCURY - Cylinder Head Temperature Sensor Circuit Intermittent/Erratic
- Wrong thermostat
- Not at operating range
- Engine overheated
- Low engine coolant level
- Faulty Cylinder Head Temperature (CHT) sensor
- Cylinder Head Temperature (CHT) sensor harness is open or shorted
- Cylinder Head Temperature (CHT) sensor circuit poor electrical connection
- Engine Light ON (or Service Engine Soon Warning Light)
P128a Mercury DescriptionThe Cylinder Head Temperature (CHT) Sensor is mounted into the wall of the cylinder head and is not connected to any coolant passages. The CHT sends a signal to the Powertrain Control Module (PCM) indicating the cylinder head temperature. If the temperature exceeds 126°C (-258°F), the PCM disables four fuel injectors at a time. The PCM will alternate which four injectors are disabled every 32 engine cycles. The four cylinders that are not being fuel injected act as air pumps to aid in cooling the engine. If the temperature exceeds 154 °C (310°F), the PCM disables all of the fuel injectors until the engine temperature drops below 154°C (310°F).
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