P03B5

What it means

P03B5 is a generic code meaning the engine control unit has detected a low signal voltage in the circuit of the cylinder 4 pressure sensor (in-cylinder combustion pressure sensor). The control unit saw the sensor signal sit below the expected range, which usually points to a short to ground, an open circuit or a faulty sensor. Fitment of these sensors is limited to certain engines, so exact circuit details depend on the model and engine.

Symptoms

• Engine warning light on
• Rough running or uneven idle, particularly on cylinder 4
• Reduced power or the engine running in a limited mode
• Poor fuel economy
• Misfire or hesitation under load
• Possible rough combustion noise or knocking if the engine relies on this sensor for combustion control

Possible causes

• Faulty cylinder 4 pressure sensorlikely
• Signal wire shorted to ground, with chafed or damaged wiring between the sensor and control unitlikely
• Corroded, loose or damaged connector at the sensor or control unitlikely
• Open circuit or high resistance in the sensor supply or signal wiringpossible
• Sensor reference supply or ground faultpossible
• Fault inside the engine control unit or its sensor input circuitrare

How to diagnose

1. Read all stored and pending codes and record the freeze frame data, noting engine load, speed and temperature when the fault set
2. Check for related codes on other cylinder pressure sensors, sensor supply voltage or misfires, which may point to a shared supply or ground fault
3. Inspect the cylinder 4 sensor wiring and connector for chafing, heat damage, water ingress, corrosion and bent or pushed-back pins
4. Clear the code and see if it returns immediately or only when hot or under vibration; wiggle the harness while watching live data
5. Check the sensor supply voltage, reference and ground at the connector using the manufacturer wiring diagram for that engine
6. Check the signal wire for a short to ground and for continuity back to the control unit with the connectors disconnected
7. Compare live cylinder 4 sensor readings with the other cylinders at idle where the engine has them
8. Check for any applicable technical bulletins or control unit software updates for the model before replacing the control unit

How to fix

• Repair or replace damaged wiring that is shorted to ground or open circuit
• Clean, repair or replace corroded or damaged connectors and terminals
• Replace the cylinder 4 pressure sensor if its signal is out of range with good supply and wiring, following the manufacturer procedure and torque figures
• Restore the sensor supply or ground connection if that circuit is faulty
• Replace or repair the control unit only after all wiring and sensor faults are ruled out, or apply a manufacturer software update if one is known to address the fault

Can I keep driving?

Nearby codes

• P03B4GenericCylinder 4 Pressure Sensor Circuit Range/Performance
• P03B6GenericCylinder 4 Pressure Sensor Circuit High