P0397

What it means

P0397 means the engine control unit has seen the signal from the cylinder 1 pressure sensor (an in-cylinder combustion pressure sensor) sit below its expected electrical range. This points to a circuit fault such as a short to earth, an open circuit, a poor connection or a failed sensor or signal conditioning module. Cylinder pressure sensors are only fitted to certain engines, so the exact layout and test values depend on the model and engine.

Symptoms

• Engine warning light on
• Rough running or misfire on cylinder 1
• Uneven idle or hesitation under load
• Reduced power or the engine running in a limited-performance mode
• Poor fuel economy or harsher combustion noise

Possible causes

• Damaged, chafed or corroded wiring to the cylinder 1 pressure sensor, causing a short to earth or open circuitlikely
• Poor connection at the sensor or control unit connector (water ingress, corrosion, loose or spread terminals)likely
• Faulty cylinder 1 pressure sensorpossible
• Faulty sensor reference supply or earth shared with other sensorspossible
• Fault in a separate signal conditioning or interface module, where fittedrare
• Internal fault in the engine control unit or outdated softwarerare

How to diagnose

1. Read all stored and pending codes and freeze frame data, noting engine load, temperature and speed when the fault set, and look for other cylinder pressure, misfire or sensor supply codes.
2. Check the manufacturer's information for how the cylinder pressure sensor is fitted on this engine, as layout and test values vary by model.
3. Visually inspect the sensor, its wiring and connectors for heat damage, chafing, oil or coolant contamination, corrosion and bent or pushed-back pins.
4. With the ignition on, view the live cylinder 1 pressure sensor signal and see whether it is stuck low, and whether wiggling the harness changes it.
5. Check the sensor's supply voltage and earth at the connector against the manufacturer's specification.
6. Test the signal wire for continuity and for a short to earth, with the connectors disconnected, following the wiring diagram.
7. Compare the sensor's resistance or output with specification, or swap with a known good unit only where the manufacturer allows it.
8. Clear the code, run the engine and recheck to confirm the fault does not return.

How to fix

• Repair or replace damaged wiring and ensure good insulation and routing away from heat and sharp edges.
• Clean, repair or replace corroded or loose connector terminals and ensure seals are fitted correctly.
• Replace the cylinder 1 pressure sensor if it fails testing, using the correct torque and sealing as specified.
• Repair the supply or earth fault affecting the sensor.
• Replace a faulty signal conditioning module or, once all else is ruled out, the control unit, and update the software if the manufacturer lists an update for this fault.

Can I keep driving?

Nearby codes

• P0396GenericCylinder 1 Pressure Sensor Circuit Range/Performance
• P0398GenericCylinder 1 Pressure Sensor Circuit High