OBD-II code · P0301
On this page
- What the code actually means
- Symptoms
- Is it safe to drive?
- What causes it, most common first
- How to diagnose it, in order
- 1. Swap parts with cylinder 2 — the 5-minute diagnostic
- 2. Pull and inspect the cylinder 1 spark plug
- 3. Check injector pulse and resistance
- 4. Test for vacuum leaks at cylinder 1 intake runner
- 5. Compression test if all electrical tests pass
- Fixes, cheapest first
- How to reset the code after a repair
- What to do if it comes back
- Related OBD-II codes
P0301 Code: Cylinder 1 Misfire
What the code actually means
SAE J2012 defines P0301 as "Cylinder 1 Misfire Detected". The PCM
monitors crankshaft acceleration after each cylinder fires. A healthy
combustion event spins the crank slightly faster; a misfire results in
a small dip in acceleration the PCM can measure. When dips associated
with one specific cylinder exceed an emissions threshold (typically 2%
of firing events) within 200 revolutions, that cylinder's misfire
counter trips and P0301 sets.
Cylinder #1 is the cylinder at one end of the engine block, by
SAE convention, the timing-belt or front end. On inline four engines
(Civic, Corolla, Camry 4-cyl, Focus, Cruze), cylinder 1 is at the
accessory-belt side of the block. On most V6 engines, cylinder 1 is at
the front of the passenger side (Bank 1, forward-most position). The
P0302, P0303, and
P0304 codes cover cylinders 2, 3, and 4.
P0301 is a one-trip code with a flashing MIL trigger at higher
misfire rates. A solid CEL means the misfire is intermittent or low
rate; a flashing CEL means active and catalyst-damaging.
Symptoms
- Check engine light, solid or flashing.
- Rough idle that improves above 2,000 rpm.
- Hesitation or stumble during light acceleration.
- A faint shake felt through the steering wheel at idle.
P0420catalyst code arriving days or weeks later.- Occasionally raw fuel smell at the tailpipe.
Is it safe to drive?
Solid light: short distances only. Flashing light: tow it. A sustained misfire dumps raw fuel into the catalyst, overheats the substrate, and can destroy a $1,500 part within an hour of highway driving. Diagnose within a few days.
A misfire that vanishes at warm idle and only appears in cold morning starts is less urgent; that pattern is usually a marginal coil or spark plug that still has weeks of life. Replace it at the next opportunity.
What causes it, most common first
Single-cylinder misfires are far easier to diagnose than P0300
because the root cause must be confined to that one cylinder's
combustion chain.
1. Failing ignition coil or plug wire (~40%). Coil-on-plug systems
(post-2000 vehicles) are the dominant ignition fault. Clue: misfire
gets worse in damp weather or under load; swap the cylinder 1 coil
with cylinder 2's and watch the code change to P0302.
2. Worn or fouled spark plug (~30%). Plugs gap-erode past about 60,000 miles. A fouled plug looks black-sooted or wet. Clue: mileage since last plug replacement is high; pulled plug shows obvious contamination versus a clean one from another cylinder.
3. Leaking, dirty, or open fuel injector (~10%). A clogged injector under-delivers; a leaking one over-delivers. Clue: injector noid-light test shows missing pulse, or sound test shows missing click; cylinder balance test fails this cylinder; injector resistance differs from the others by more than 1 ohm.
4. Vacuum leak local to cylinder 1 (~7%). Cracked intake gasket, broken PCV hose, brittle vacuum line: any leak between the throttle body and that cylinder's intake runner causes a lean misfire. Clue: misfire improves at higher rpm; spraying carb cleaner around the cylinder 1 intake area changes idle.
5. Low compression on cylinder 1 (~7%). Burned exhaust valve, worn ring, head-gasket leak. Clue: compression test shows cylinder 1 below 80% of the highest cylinder; wet test (adding oil) does or does not recover the reading — distinguishes rings from valves.
6. PCM driver circuit or harness (~3%). Open or shorted coil primary wire. Clue: coil swap does not move the misfire to another cylinder; voltage drop test on the coil control wire shows fault.
7. Carbon buildup on direct-injection intake valves (~3%). Common on VW/Audi, BMW N-series, GDI Hyundai/Kia, Mazda Skyactiv direct injection. Clue: vehicle over 60k miles with direct injection; borescope inspection shows heavy carbon on intake valves.
How to diagnose it, in order
1. Swap parts with cylinder 2 — the 5-minute diagnostic
This is the single most powerful test for any single-cylinder misfire. Engine off and cool:
- Swap the cylinder 1 ignition coil with cylinder 2's coil.
- Clear the code, start the engine, drive 10 miles.
- If the code becomes
P0302, the failed part moved with the coil — coil is the issue. Replace it. - If the code stays
P0301, the failure is in cylinder 1 itself — plug, injector, compression, or harness.
Then repeat with the spark plug if the coil swap didn't isolate it.
2. Pull and inspect the cylinder 1 spark plug
Compare against plugs from other cylinders.
- Light tan or gray: plug is healthy; problem is elsewhere.
- Black soot, dry: rich condition or weak ignition. Replace plug and continue diagnosing.
- Black wet: flooded with fuel — injector stuck open or low compression. Compression test next.
- Oil-wet: rings or valve seals leaking oil into combustion. Major repair likely.
- White or burned electrode: running too lean or pre-ignition; check for vacuum leaks.
3. Check injector pulse and resistance
With a noid light or a digital multimeter on the injector connector, verify pulse at idle. No pulse = wiring or PCM driver problem. With the connector unplugged, measure injector resistance and compare against the others; a healthy injector reads 11–17 ohms (port injection) or 1–3 ohms (GDI). Outliers fail.
4. Test for vacuum leaks at cylinder 1 intake runner
Engine running at idle, briefly spray short bursts of carb cleaner or propane around the intake manifold gasket, the PCV hose, and any vacuum lines feeding cylinder 1. A momentary rpm rise or change in misfire confirms a leak in that area.
5. Compression test if all electrical tests pass
Pull all plugs, disable fuel and ignition, crank the engine 5–7 revolutions per cylinder with a compression gauge. Cylinder 1 below 80% of the highest reading confirms a mechanical issue. Wet test recovers compression = rings; no recovery = valve or head gasket.
Fixes, cheapest first
| Fix | Cost (USD) | When it applies |
|---|---|---|
| Replace one spark plug | $4–$25 | Plug worn, fouled, or gap drifted |
| Replace one ignition coil | $30–$120 | Coil swap moved the misfire code |
| Clean or replace one fuel injector | $40–$300 | Cylinder balance test failed; pulse missing |
| Fix vacuum leak (gasket, hose, PCV) | $15–$200 | Carb cleaner test isolated a leak |
| Walnut-blast intake valves (GDI) | $400–$900 | Direct injection, heavy carbon confirmed by borescope |
| Repair head-gasket or burned valve | $1,000–$2,500 | Compression test confirmed mechanical failure |
A whole set of plugs and a single coil, the most common real-world
P0301 repair, runs $80–$200 at a shop and under $80 DIY.
How to reset the code after a repair
Clear with a scan tool, then drive a mix of city and highway for one warm-up cycle. The misfire monitor runs continuously, so the code either clears within the first 5 miles or returns to confirm the fault is still there.
Without a scan tool, the code self-clears after 3 successful drive cycles without a misfire. A flashing-CEL misfire that has since been repaired should clear within a day of normal driving.
What to do if it comes back
- Same plug and coil already replaced: test the injector. A stuck injector can look like ignition until you pull the plug and see it fuel-soaked.
- Cycles between cylinder 1 and cylinder 2: intermittent harness fault or PCM driver. Probe the coil control wire with a scope or DVOM while wiggling the harness.
- Returns after compression test was normal: carbon on the intake valves (direct injection only) or a sticking exhaust valve at cold start. Walnut blast or induction service.