P0312

Code

Generic P — Powertrain

Cylinder 12 Misfire Detected

Brand: Generic

Type: P — Powertrain

Views: UK: 44 EN: 47 RU: 38

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Causes

Faulty ignition component on cylinder 12 (coil, spark plug, plug wire)
Failed or clogged fuel injector for cylinder 12
Low compression in cylinder 12 (worn rings, valve issues, head gasket leak)
Open/shorted wiring or poor connector at ignition coil or injector
Fuel delivery problems (low rail pressure, restricted feed)
Air intake or vacuum leaks affecting cylinder 12

Symptoms

Check Engine Light (MIL) illuminated
Engine runs rough at idle or under load
Reduced engine power and poor acceleration
Poor fuel economy and elevated emissions
Occasional or continuous surging/stumbling
Possible backfiring, misfire codes saved for cylinder 12

What to check

Read freeze frame and stored data with a scan tool; confirm P0312 and any related codes
Monitor live misfire counters and per-cylinder misfire statistics
Compare short-term and long-term fuel trim values
Check fuel rail pressure and regulator operation
Inspect spark plug, coil, and connector at cylinder 12 for damage, fouling, or corrosion
Swap ignition coil between cylinder 12 and a known-good cylinder — see if misfire follows coil

Signal parameters

Misfire counter: incrementing on cylinder 12 in live data
Injector pulse width: expected pulse (varies with load) — compare cylinder 12 to others
Ignition primary/secondary waveform: visible abnormalities on oscilloscope for cylinder 12
Fuel rail pressure: within manufacturer spec (e.g., steady value; gasoline systems often ~30–70 psi depending on system)
Short-term fuel trim (STFT): localized correction may be visible; significant positive trim can indicate lean condition
Compression (static): typical healthy gasoline cylinder ≈120–200 psi (varies by engine); significantly lower on cylinder 12 indicates mechanical issue

Diagnostic algorithm

Connect a bi-directional scan tool, confirm P0312 and review freeze frame and live misfire data to verify recurrence.
Visually inspect spark plug, coil pack/ignition module, and connectors for cylinder 12; replace visibly damaged parts and clear code to retest.
Swap the ignition coil from cylinder 12 with a known-good cylinder. If misfire moves, replace the coil.
If ignition swap didn’t move the misfire, perform an injector test: listen for injector operation, run an injector balance/flow test or swap injectors between cylinders. If misfire follows the injector, replace it.
Measure fuel rail pressure to ensure proper fuel supply before further injector diagnostics.
Perform a static compression test and/or leak-down test on cylinder 12. Low results indicate mechanical repair (valve, piston, gasket) required.
Check wiring continuity/resistance and grounds for coil and injector circuits; repair any damaged wires or poor connections.
Verify cam/crank timing and inspect timing components if cam/crank correlation faults appear or if mechanical timing is suspected.
Check for intake leaks (smoke test) and exhaust leaks near cylinder 12 that could affect readings.
After repairs, clear codes and perform a road/idle test with live data to confirm misfire no longer appears and fuel trim/driveability are restored.
If all above pass and misfire persists, consider ECM/software updates or advanced diagnostics (oscilloscope of ignition/injector waveforms, bench testing components, factory service info).