Engine

P03C9 — Cylinder 6 Pressure Sensor Circuit Low

Severity: medium Drive with care, to a shop

What P03C9 means

The engine control module monitors the cylinder 6 pressure sensor to evaluate combustion and fine-tune engine operation. P03C9 sets when the signal voltage in this circuit stays below the expected range. The cause can be the sensor itself, an open or shorted-to-ground wire, or sometimes a mechanical problem in the cylinder.

Symptoms

  • Check engine light on
  • Rough idle and vibration
  • Misfire on cylinder 6
  • Reduced engine power
  • Worse fuel economy

Possible causes

  1. Failed cylinder 6 pressure sensor
  2. Open, poor contact or short to ground in the sensor wiring
  3. Corroded or damaged sensor connector
  4. Mechanical problem in cylinder 6 lowering the signal
  5. Rarely, a faulty ECM

How to diagnose

  1. Scan all codes and freeze frame data, and note any misfire or other cylinder-related codes
  2. Inspect the cylinder 6 sensor harness and connector for damage, moisture, corrosion and chafing
  3. Check sensor power, reference voltage and ground at the connector per the factory wiring diagram
  4. Test the signal wire for opens and shorts to ground with a multimeter
  5. Compare live sensor data on cylinder 6 with the other cylinders
  6. Check the spark plug, coil and injector on cylinder 6, and run a compression test if needed
  7. If the wiring tests good, replace the sensor and see whether the code returns

Common fixes

  • Replace the cylinder 6 pressure sensor
  • Repair the damaged wiring or replace the connector
  • Fix any mechanical fault found in the cylinder
  • Update software or replace the ECM if it is confirmed faulty

Questions and answers

What does code P03C9 mean?

It means the signal from the cylinder 6 pressure sensor circuit is too low. The ECM is not getting a normal voltage, which points to the sensor or its wiring.

Can I drive with P03C9?

Briefly and carefully if the engine runs smoothly. If it misfires or the check engine light flashes, do not drive, since the catalytic converter can be damaged.

How do I fix P03C9?

Start by inspecting the sensor wiring and connector and testing the signal with a multimeter. If the wiring is fine, the sensor is usually replaced.

This is general information about the code. The meaning can differ between makes and models: a scan tool and the repair manual for your car will show the exact cause.