P03B8 — Cylinder 4 Pressure Too Low

Diagnostic algorithm

1. Verify: confirm P03B8 with a full scan, record freeze frame and related codes, do not rely on single snapshot.
2. Visual & basic checks: inspect ignition coil, plug, injector, wiring and connectors for cylinder 4; repair/replace obvious faults and retest.
3. Swap test (if safe): swap coil and/or injector with another cylinder and see if fault follows (helps separate electrical vs mechanical).
4. Fuel & spark verification: confirm injector pulse and fuel pressure, and verify spark energy at cylinder 4 with a scope or probe.
5. Cylinder contribution / relative compression: use scan tool to run a contribution test or use a noid light/oscilloscope to verify injector/ignition timing and contribution.
6. Static compression test: measure compression on cylinder 4 vs others. Significant low reading indicates mechanical or sealing issue.
7. Leak‑down test: if compression is low, perform leak‑down to identify source (intake, exhaust, piston rings, head gasket).
8. Timing check: verify cam/crank timing (timing marks, cam phaser position, cam/crank correlation data from scan tool).
9. Cylinder pressure sensor check: if engine uses a pressure sensor, verify sensor signal with a scope, inspect wiring, and consider swapping/replace sensor.
10. Inspect head gasket and look for oil/coolant contamination or coolant loss; pressure test cooling system if suspected.
11. Repair: address verified mechanical, fuel, ignition, timing, or sensor faults. After repair, clear codes and perform road/scan-tool testing to confirm repair.
12. If intermittent or unclear, capture live waveform data during cranking and running and consider manufacturer-specific diagnostic procedures.