Engine misfire: how to find which cylinder and what is causing it
An engine misfire means a cylinder skipped a combustion event. Here is how to read the codes, find the cylinder, and fix the common causes.
A misfire means a cylinder failed to ignite its air-fuel charge on a given cycle. The crankshaft position sensor catches the missing pulse, the PCM logs a code, and you usually feel it as a stumble, a vibration at idle, or a check engine light that flashes when it gets serious. Find which cylinder first, then chase the cause.
Read the code first
A scan tool will give you one or both of these:
- P0300: random or multiple cylinder misfire.
- P0301 through P0312: misfire on the cylinder matching the last digit. P0303 is cylinder 3, P0306 is cylinder 6.
If you have P0301 alongside P0300, cylinder 1 is the worst offender. Knowing the cylinder cuts the diagnosis in half.
A flashing check engine light is different
Solid CEL plus a misfire code is a problem you can drive home with. Flashing CEL plus a misfire code is “do not keep driving.” A flashing light means raw fuel is reaching the catalytic converter and the cat is overheating. Twenty minutes of driving like that can ruin a $1,500 catalytic converter.
The likely causes, ranked
For a single-cylinder misfire (P0303, P0304, etc.):
| Cause | How to test |
|---|---|
| Bad ignition coil | Swap coil to a different cylinder. If the misfire follows, coil is bad. |
| Worn or fouled spark plug | Pull and inspect. Black sooty, gas fouled. Oily, valve seal failing. |
| Bad fuel injector | Listen with a stethoscope or use a noid light. |
| Low compression | Compression or leakdown test. Below 70 percent of factory spec is a problem. |
| Vacuum leak feeding only one cylinder | Smoke test or carb cleaner spray on intake gasket area. |
For random / multi-cylinder misfires (P0300 alone):
| Cause | Test or fix |
|---|---|
| Bad fuel pump or fuel pressure regulator | Test fuel pressure at the rail |
| Clogged fuel filter | Replace if overdue |
| MAF or MAP sensor reading wrong | Live data, compare to expected values |
| Vacuum leak | Smoke test |
| Bad coolant temp sensor (lying about temp) | Live data, compare to actual engine temp |
| Stretched timing chain | Check cam-crank correlation on scan tool |
| Bad gas | Add Top Tier brand fuel, drive a tank through |
The quick coil swap test
On any coil-on-plug engine (most cars from 2003 onward), pull the misfiring cylinder’s coil and swap it with a neighbor’s. Clear codes, drive 10 minutes, scan again.
- Misfire moves to the new cylinder: bad coil.
- Misfire stays where it was: not the coil. Check the spark plug next.
This 10-minute test will save you a lot of guessing.
Spark plug condition cheat sheet
| Plug looks like | Means |
|---|---|
| Tan or light brown deposits | Normal |
| Black, dry, sooty | Rich mixture |
| Black, wet, oily | Oil leaking past valve seals or rings |
| White or chalky | Lean mixture or overheating |
| Melted electrode | Severe detonation, look at fuel quality and timing |
| Cracked porcelain | Replace, that one is failed |
Plug replacement intervals run 30,000 miles (older copper plugs) to 100,000 miles (iridium and platinum). Cheap plugs in a coil-on-plug engine are a false economy.
Costs to plan for
| Fix | DIY part | Shop total |
|---|---|---|
| Single spark plug | $5 to $30 | $80 to $200 |
| Full plug set | $30 to $200 | $200 to $500 |
| Single ignition coil | $40 to $120 | $150 to $300 |
| All ignition coils | $200 to $800 | $400 to $1,200 |
| Fuel injector (one) | $50 to $250 | $200 to $600 |
| Cleaning fuel injectors | $20 (additive) | $80 to $300 (service) |
| Compression test only | $35 (gauge) | $80 to $180 |
When to take it in
If you have replaced the obvious wear items (plugs, coil on the offending cylinder) and the misfire is still there, the next steps get expensive: leakdown test, fuel injector flow test, timing chain inspection. Those are shop jobs unless you have the right tools and a service manual. Random misfire with a healthy fuel and spark system often points at a head gasket leak, a burnt valve, or a stretched timing chain, and those decide whether the engine is worth keeping.
Quick tank-of-fuel sanity check
If a misfire appears soon after a fill-up and is intermittent across multiple cylinders, the gas is suspect. Top up at a Top Tier station (Shell V-Power, Chevron with Techron, Costco Kirkland gas), run a bottle of Techron or Red Line SI-1 through the tank, and see if it clears. A cheap test that solves it more often than you would expect.