P03B3

What it means

P03B3 is the generic code for a fault in the cylinder 4 pressure sensor circuit. The engine control unit monitors the in-cylinder pressure sensor signal and has seen a voltage or signal that is out of range, missing or implausible. Fitment of in-cylinder pressure sensors is limited to certain engines, so the exact sensor type and fault thresholds depend on the model and engine.

Symptoms

• Engine warning light on
• Rough running or uneven idle
• Misfire or hesitation under load
• Reduced power or the car going into limp mode
• Poor fuel economy
• Increased exhaust smoke or emissions on some engines

Possible causes

• Damaged, chafed or corroded wiring to the cylinder 4 pressure sensor (open circuit, short to earth or short to supply)likely
• Poor connection at the sensor or control unit connector (loose pin, corrosion, water ingress)likely
• Faulty cylinder 4 pressure sensorlikely
• Fault with the sensor's supply or reference voltage or earth feedpossible
• Heat or oil contamination damage at the sensor, glow plug or injector integrated sensor areapossible
• Internal fault in the engine control unit or outdated software where a known issue is documented by the manufacturerrare

How to diagnose

1. Read all stored and pending codes and note freeze frame data, including engine speed, load and temperature when the fault set.
2. Check for related codes for other cylinder pressure sensors, sensor supply voltage or misfires, as they point to a shared supply or wiring fault.
3. Visually inspect the cylinder 4 sensor wiring and connector for chafing, heat damage, oil contamination, corrosion and bent or pushed-back pins.
4. With the connector disturbed gently (wiggle test) and live data monitored, see if the cylinder 4 pressure signal drops out or jumps.
5. Check the sensor supply, reference and earth at the connector against the manufacturer's wiring diagram.
6. Test wiring continuity and insulation between the sensor and control unit, with the battery disconnected as the procedure requires.
7. Compare the cylinder 4 live signal with the other cylinders at idle to judge whether the sensor reads plausibly.
8. Check for manufacturer service bulletins or software updates relevant to the engine, as procedures vary by model.

How to fix

• Repair or replace damaged wiring and fix any short or open circuit found.
• Clean, repair or replace corroded or loose connectors and terminals.
• Replace the cylinder 4 pressure sensor if it fails testing, following the manufacturer's procedure and torque figures.
• Restore correct supply or earth if a fault is found in the shared feed.
• Update the control unit software only if a documented fault requires it, then clear the code and road test to confirm.

Can I keep driving?

Nearby codes

• P03B2GenericCylinder 3 Combustion Performance
• P03B4GenericCylinder 4 Pressure Sensor Circuit Range/Performance