What does P0308 mean?
The engine control module detected repeated misfires specifically in cylinder 8.
Quick Decision Guide
- Worn or fouled spark plug in cylinder 8
- Failing ignition coil on cylinder 8
- Clogged or leaking fuel injector on cylinder 8
When to stop driving
If the check-engine light is flashing, stop driving immediately — active misfires are destroying the catalytic converter in real time. A solid light is less urgent but should be diagnosed within days.
Reviewed by ratchit Automotive Solutions for clarity and inspection-first safety framing. Not a substitute for professional diagnosis.
Repair Decision & Cost Guide
Use this guide to decide what to inspect first, what it may cost, and when to move from DIY checks to professional diagnosis.
P0308 points to a misfire condition, not a guaranteed failed coil or plug. Isolate the failing cylinder before buying parts.
DIY vs. Shop
Can you keep driving?
Best first DIY check
Don't replace this first
When to seek a shop
Recommended next action
Estimated Repair Cost
Inspection cost
$0–$25 DIY scanner
DIY parts cost
$5–$130
Inspect first before replacing parts.
If the vehicle is unsafe to drive, stop driving and seek professional help.
Shop diagnostic cost
$80–$150 (typical shop diagnostic fee)
Shop repair cost
$60–$550
Diagnostic uncertainty: This code points to a system or condition, not a guaranteed failed part. Inspect before replacing.
Cost variation: Costs vary by vehicle, location, part quality, labor rate, and the confirmed root cause.
Inspect Before Replacing
These checks are cheap, fast, and rule out the most common causes before spending money on parts.
- 1Swap the ignition coil from cylinder 8 with another cylinder, clear the code, and drive a full cycle to see if the misfire follows.
- 2Read the plug: fouling, oil, or cracked insulator point to different causes than a worn electrode.
Track this misfire issue
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ratchit AI Insight
· P0308What this usually means:P0308 narrows the misfire to a specific cylinder, but the cause still needs to be confirmed before replacing parts. Misfire codes do not automatically mean spark plugs or coils are bad — air/fuel, vacuum, fuel delivery, and mechanical issues can all trigger the same code.
Common mistake: Many drivers replace all the spark plugs and coils first. That sometimes fixes it, but if the real cause is a vacuum leak, an injector problem, or low compression, the misfire returns and the parts money is wasted.
Inspect before replacing parts
- 1Check for a flashing check-engine light — a flashing CEL during a misfire is a high-risk condition for the catalytic converter.
- 2Swap the suspect coil/plug to a known-good cylinder and re-scan to see if the misfire follows.
- 3Inspect for obvious vacuum leaks, cracked intake boots, and disconnected PCV hoses.
- 4Review fuel trims and injector behavior before assuming an ignition part is bad.
Confirming the cylinder and the system (ignition vs. air/fuel vs. mechanical) before buying parts usually keeps the repair in the $20–$200 range instead of replacing a full ignition set.
Safety threshold: If the check-engine light is flashing, stop driving as soon as it is safe — continued driving in this state can damage the catalytic converter.
AI-assisted guidance reviewed against ratchit's inspection-first safety rules. Codes are clues, not confirmed diagnoses — always verify before replacing parts.
Understanding P0308 in detail
P0308 means the engine control module has isolated a repeating misfire to cylinder 8. Cylinder 8 only exists on eight-cylinder engines, and firing order, coil layout, and fueling strategy vary between manufacturers — so instead of prescribing a fixed replacement sequence, the useful angle here is fuel-side confirmation. Ignition (plug and coil) gets almost all of the attention on misfire codes because those parts are cheap and easy to swap, but a leaking, dribbling, or slow-opening injector on cylinder 8 will pass a resistance check and still cause the same code. If a plug read and coil swap both come back clean on P0308, the next test is an injector balance or drop test: with the engine running at idle, disable each injector one cylinder at a time and watch RPM drop. A cylinder whose disabled injector produces little or no RPM drop is the misfiring cylinder. Only after ignition and fuel isolation should compression or leakdown testing be sequenced, because those are the most invasive and time-consuming checks. Note: some manufacturer service bulletins address injector supersessions, revised control strategies, or software updates on specific year/make/model vehicles — worth a search before parts replacement.
Symptoms of P0308
- Check engine light on or flashing
- Rough idle with noticeable vibration
- Loss of power during acceleration
- Engine stumbling or hesitation
- Fuel smell from exhaust
Common causes
- Worn or fouled spark plug in cylinder 8
- Failing ignition coil on cylinder 8
- Clogged or leaking fuel injector on cylinder 8
- Degraded spark plug wire (on wire-to-plug systems)
- Low compression in cylinder 8 (valve or ring issue)
How to diagnose P0308
Steps ordered by urgency — tackle Now items first.
- NowSwap the ignition coil from cylinder 8 with another cylinder, clear the code, and drive a full cycle to see if the misfire follows.
- NowRead the plug: fouling, oil, or cracked insulator point to different causes than a worn electrode.
- SoonRun an injector balance/drop test — disable each injector at idle and watch RPM drop; a weak drop identifies a fuel-side cause on that cylinder.
- LaterIf ignition and fuel both check out, perform a compression or leakdown test on cylinder 8.
How to fix P0308
Fixes listed in order of likelihood and cost-efficiency.
- 1Replace spark plug in cylinder 8
- 2Replace ignition coil on cylinder 8
- 3Replace spark plug wire (if applicable)
- 4Replace or clean fuel injector
- 5Address low compression (valve job or piston rings)
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Recommended tools & parts
These recommendations appear because this issue commonly requires inspection in this system area. ratchit does not recommend replacing parts until the likely cause is confirmed.
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Estimated Repair Cost — P0308
Ranges vary by vehicle, region, and shop. Use as a ballpark — always get a quote.
DIY estimate
$5 – $25
Shop estimate
$60 – $200
DIY estimate
$25 – $80
Shop estimate
$120 – $350
DIY estimate
$40 – $130
Shop estimate
$200 – $550
Cost data is estimated and may not reflect your specific vehicle or location.
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Frequently Asked Questions
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