P03BA

What it means

P03BA is the generic SAE J2012 code for Cylinder 4 Pressure Variation Low. The engine control unit judged that the pressure developed in cylinder 4 was lower than expected, or varied less from cycle to cycle than it should. Depending on the engine, it works this out from an in-cylinder pressure sensor or from an estimate based on crankshaft speed. The exact monitoring method and thresholds vary by engine.

Symptoms

• Engine management light on, possibly flashing if a misfire is also present
• Rough idle or a noticeable shake from one cylinder
• Loss of power or hesitation under load
• Misfire codes stored alongside P03BA, especially for cylinder 4
• Poor fuel economy or a fuel smell from the exhaust
• Hard or uneven starting

Possible causes

• Ignition fault on cylinder 4 (worn spark plug, failing coil or poor coil connection) causing weak or missed combustionlikely
• Fuel delivery fault on cylinder 4 (blocked, leaking or sticking injector, or injector wiring fault)likely
• Loss of compression in cylinder 4 (worn piston rings, burnt or sticking valve, leaking head gasket)possible
• Valvetrain problem on cylinder 4 (worn cam lobe, collapsed lifter, or incorrect valve timing)possible
• Faulty cylinder pressure sensor, or damaged wiring or connector, on engines fitted with onepossible
• Intake air leak or restriction affecting cylinder 4 only, such as a leaking manifold gasket or carbon build-up on the inlet valverare
• Outdated or faulty control unit calibration, where the manufacturer has issued a known update for this coderare

How to diagnose

1. Read all stored codes and freeze frame data, noting engine speed, load and temperature when the code set, and look for cylinder 4 misfire codes
2. Check the live misfire counters and fuel trims to see whether cylinder 4 is the only one affected
3. Inspect the cylinder 4 coil, injector and pressure sensor wiring and connectors for damage, corrosion, oil or loose pins
4. Remove and inspect the cylinder 4 spark plug against the others, then swap the coil with another cylinder to see if the fault follows it
5. Test the cylinder 4 injector electrically and check its spray and flow against the others
6. Carry out a compression test and, if low, a cylinder leakdown test to find where the pressure is escaping
7. Inspect valve timing and valvetrain condition, using a borescope if needed to view the cylinder 4 piston and valves
8. Check the pressure sensor signal and supply against the manufacturer's data, if fitted; the procedure depends on the engine

How to fix

• Replace the faulty spark plug or ignition coil on cylinder 4 and repair any damaged ignition wiring
• Clean or replace the cylinder 4 injector, or repair its wiring or connector
• Repair the engine's internal mechanical fault, such as valve, piston ring or head gasket repairs, as the compression and leakdown results show
• Repair or replace worn valvetrain parts and correct the valve timing if it is out
• Replace the cylinder pressure sensor or repair its wiring if testing proves it faulty
• Fit any manufacturer-issued control unit calibration update only if one exists for this fault

Can I keep driving?

Nearby codes

• P03B9GenericCylinder 4 Pressure Too High
• P03BBGenericCylinder 4 Pressure Variation High