A8 · ENGINE PERFORMANCE · BAY REFERENCE
MISFIRE PATTERN DECODER
A misfire code (P0300–P0312) tells you the SYMPTOM. The PATTERN — which cylinders, under what conditions, with what fuel trim — tells you the CAUSE. This reference maps every common misfire pattern to the most likely root cause and the test that confirms it.
SINGLE CYLINDER PATTERNS
| Pattern | Conditions | Most Likely Cause | Confirm With |
|---|---|---|---|
| P030X — one cyl, constant | All RPM, all load | Coil, plug, injector on that cylinder | Swap test: move coil → does misfire follow? |
| P030X — one cyl, idle only | Goes away ≥ 1500 RPM | Lean miss — vacuum leak on that runner | Propane enrichment near intake runner |
| P030X — one cyl, under load | Cruise/WOT only | Weak spark under high cyl pressure — plug gap, coil weak | Swap coil + check secondary scope under load |
| P030X — one cyl, cold start only | First 2–5 min | Cold-fouled plug, low compression on cold engine | Pull plug — wet/fouled? Then compression cold |
| P030X — one cyl, w/ low compression | Comp test < 75% of others | Burnt valve, worn rings, head gasket | Wet compression, leak-down test |
| P030X — one cyl, w/ +15% LTFT bank | Fuel trim only on that bank | Lean miss from injector or vacuum leak | Injector flow + balance test |
MULTI-CYLINDER / RANDOM PATTERNS
| Pattern | Conditions | Most Likely Cause | Confirm With |
|---|---|---|---|
| P0300 — random, all cyls | All conditions | Common-cause: fuel pressure low, large vacuum leak, timing chain stretch, bad MAF | Fuel pressure under load + smoke test intake |
| P0300 — random, idle only | Idle, cold or warm | Vacuum leak, IAC sticking, idle air bypass | Propane around intake gasket + IAC test |
| P0300 — random, under load | WOT or cruise only | Fuel pump weak, restricted filter, MAF underreporting | Fuel volume + MAF at load (PID scan) |
| P0301 + P0304 (companions) | 4-cyl, paired cyls | Wasted-spark coil dying (drives 1+4 together) | Swap to coil for 2+3 — misfire moves? |
| Same-bank misfires | Both cyls on one bank | Bank-side issue: vacuum, fuel rail, head gasket | Compare LTFT, smoke that bank only |
| Misfires after fueling | Right after fill-up | Bad fuel batch or overfilled tank flooding canister | Drain + refill with known-good fuel |
FUEL TRIM CROSS-REFERENCE
| Misfire + LTFT | Means | First Look At |
|---|---|---|
| Cyl misfire + LTFT > +10% | Lean misfire on that bank | Vacuum leak, injector, fuel pressure |
| Cyl misfire + LTFT < -10% | Rich misfire — too much fuel | Leaking injector, stuck open EVAP purge |
| Cyl misfire + LTFT ±5% | Mechanical or ignition | Compression, coil, plug, wire |
| Random misfire + LTFT both +15% | Big vacuum leak or fuel starvation | Smoke test + fuel pressure under load |
| Random misfire + LTFT both -8% | Excess fuel — regulator stuck closed, leaking injector(s) | Fuel pressure (low side), injector leak test |
SWAP TEST PROTOCOL
- 1.Identify the misfiring cylinder from misfire counters or scan PIDs.
- 2.Swap the coil with an adjacent known-good cylinder. Clear codes. Retest.
- 3.If misfire follows the coil → coil is the cause. Replace it.
- 4.If misfire stays on the original cyl → swap the plug next. Same logic.
- 5.If plug also clears → swap injectors. If still won\'t move, the fault is in the cylinder itself (compression, valve, rings).
- 6.Total time: ~15 minutes saves $300 of parts cannon and a comeback.
PRO MOVE
CATALYST-DAMAGING vs EMISSIONS-FAILING THRESHOLDS
Mode $06 tracks two misfire counts. The 1,000-revolution "catalyst-damaging" threshold is the higher one — when crossed, the MIL FLASHES (not steady) to warn the driver to stop NOW. The "emissions-failing" threshold is lower; when crossed it sets a steady MIL.
If a customer arrives with a flashing MIL — do not road test. Move the car, kill the engine. The cat is being actively damaged on every drive. This is the call to make BEFORE you start diagnostics.