# P0308: Cylinder 8 Misfire Detected **Verdict:** Drive to a shop this week Ignition and misfire - Generic OBD-II code, applies to all gasoline cars and light trucks sold in the US since 1996. - **Typical cost:** $150-$3,500 - **Urgency:** This week - **Most likely:** Spark plug or ignition coil on cylinder 8 - **DIY?:** Steps 1-3 ## Quick answer P0308 means the engine computer has detected repeated misfires in cylinder 8, so that cylinder is not contributing its share of power. It appears on V8 and larger engines, where cylinder 8 is usually the last cylinder on one bank, tucked near the firewall. A worn spark plug or failing coil is the most likely cause, followed by injector, vacuum leak and compression problems. ## What does P0308 mean? Cylinder 8 only exists on engines with eight or more cylinders, so P0308 almost always means a V8, though V10 and V12 engines can set it too. On GM V8s, even-numbered cylinders run down the passenger side, making cylinder 8 the rear passenger-side cylinder. Chrysler Hemi V8s use the same numbering. Ford modular and Coyote V8s number 5 through 8 down the driver side, so cylinder 8 is at the rear of the driver side, often under the cowl and hard to reach. The engine computer detects misfires by watching for tiny slowdowns in crankshaft speed and matching each one to a cylinder through the firing order and camshaft signal. P0308 sets when cylinder 8 misfires often enough to raise emissions, usually confirmed on two drive cycles. If the rate is high enough to overheat the catalytic converter, the light flashes immediately. Because the code names one cylinder, the parts that serve only cylinder 8 come first. ## What are the symptoms of P0308? - Check engine light on, or flashing during hard acceleration - Rough or shaky idle - Stumble or loss of power when towing, climbing or passing - Worse fuel economy - Fuel or rotten-egg smell from the exhaust ## What causes P0308? Ranked from most to least likely. Check them in this order: the cheap, common causes first. 1. **Worn or fouled spark plug in cylinder 8** (most common). Spark plugs wear gradually, and rear plugs near the firewall are the hardest to reach, so they are the ones most likely to be skipped, cross-threaded or left loose. Oil from a leaking valve cover gasket can also pool in the plug well. If records show the plugs are overdue, replace the whole set before chasing anything else. 2. **Failing ignition coil or plug wire on cylinder 8** (common). Coils at the back of the engine sit in trapped heat, which speeds cracking. Plug wires can burn on exhaust manifolds or chafe on the firewall. On some trucks and SUVs, water from a clogged cowl drain collects in rear plug wells and shorts the coil boot. Swap the coil with a same-bank neighbor and see whether the misfire follows. 3. **Clogged or leaking cylinder 8 injector** (less common). A partly clogged injector starves cylinder 8 and misfires mainly at idle, while a leaking one floods it and can cause a rough, smoky start after the car sits. This becomes more likely with high mileage and is also seen on direct-injection V8s. Injector balance tests, a noid light check or an injector swap confirms it. 4. **Vacuum leak near the rear of the intake** (less common). On many V8s, PCV, brake booster or other vacuum hoses connect near the back of the intake, close to cylinder 8. A cracked hose or leaking intake gasket there can lean out that cylinder first, causing a misfire mostly at idle. High fuel trims on that bank are a clue, and a smoke test pinpoints the leak. 5. **Low compression in cylinder 8** (rare). A burnt or sticking valve, worn piston rings, a damaged lifter or a head gasket leak lowers compression in cylinder 8, so it misfires no matter how good the plug and coil are. Overheated, oil-burning or high-mileage engines are most at risk. A compression test shows a low reading, and a leak-down test shows where the air escapes. ## How do you diagnose P0308? 1. Read all codes and freeze-frame data to see the speed and load when the misfire happened. 2. Find cylinder 8 for your engine in a service manual, since its location differs between GM, Ford and Chrysler. 3. Remove and inspect the cylinder 8 plug, and check its well for oil or water. 4. Swap the cylinder 8 coil or wire with a neighbor on the same bank, clear codes and drive to see if the misfire moves. 5. If the misfire stays, test the injector, smoke-test the intake and run a compression test. ## How do you fix P0308, and what does it cost? | Fix | Typical cost | What it involves | | --- | --- | --- | | Replace spark plugs (full V8 set) | $200-$600 | Engines where rear plugs need cowl or intake parts removed cost more. | | Replace cylinder 8 ignition coil or plug wire | $150-$400 | Clear any water from the plug well and fix the cowl drain if needed. | | Replace cylinder 8 fuel injector | $250-$750 | Higher on direct-injection engines. | | Repair intake vacuum leak | $100-$600 | From a cracked hose to an intake manifold gasket. | Full price breakdowns: [spark plug replacement cost](/cost/spark-plug-replacement), [ignition coil replacement cost](/cost/ignition-coil-replacement), [fuel injector replacement cost](/cost/fuel-injector-replacement). ## Is it safe to drive with P0308? > **Drive to a shop this week.** You can usually drive gently to a shop with a steady light, but avoid towing and hard acceleration. If the check engine light flashes, ease off and stop when it is safe, because a heavy misfire can overheat and ruin the catalytic converter. ## Will P0308 fail an emissions test? P0308 with the check engine light on fails an OBD-based emissions test in most states. A misfiring cylinder raises hydrocarbon emissions, and ongoing misfires can damage the converter on that bank, which can set P0420 or P0430 later. The misfire monitor rechecks soon after a repair, but the remaining monitors need several days of normal driving. ## After the repair: clearing P0308 Clear the code and check the cylinder 8 misfire counter on a scan tool at idle, steady cruise and under moderate load. Counts should stay at or near zero. Drive through the conditions in the freeze-frame data, such as towing or climbing, if that is when it misfired. A week of mixed driving with no returning light means the fix worked. ## How to prevent P0308 - Replace all spark plugs at the specified interval, including the hard-to-reach rear ones. - Keep cowl drains clear of leaves so water cannot reach the rear plug wells. - Fix valve cover gasket leaks before oil fills the spark plug tubes. - Keep plug wires and coil boots away from exhaust manifolds and the firewall. - Inspect vacuum hoses at the back of the intake whenever the engine is serviced. ## Codes often seen with P0308 - [P0300](/codes/p0300): Random/Multiple Cylinder Misfire Detected - [P0307](/codes/p0307): Cylinder 7 Misfire Detected - [P0304](/codes/p0304): Cylinder 4 Misfire Detected - [P0420](/codes/p0420): Catalyst System Efficiency Below Threshold (Bank 1) - [P0430](/codes/p0430): Catalyst System Efficiency Below Threshold (Bank 2) ## Frequently asked questions ### Can I drive with a P0308 code? Only gently, and only while the check engine light is steady. If it flashes, the misfire is bad enough to damage the catalytic converter, so stop and arrange a tow. With a steady light, drive straight to a shop and avoid towing, steep hills and hard acceleration. ### Where is cylinder 8? On GM and Chrysler Hemi V8s, cylinder 8 is the rear cylinder on the passenger side, near the firewall. On Ford modular and Coyote V8s it is the rear cylinder on the driver side. Other engines differ, so confirm the location in a service manual before replacing parts. ### How much does it cost to fix P0308? Most P0308 repairs cost $150 to $750 at an independent shop. A full set of V8 spark plugs runs $200 to $600, a coil or plug wire $150 to $400 and an injector $250 to $750. Compression problems such as valve or head gasket repairs can exceed $2,000. ### Why is cylinder 8 hard to work on? It usually sits at the back of the engine, close to the firewall, where hands and tools have little room. On some trucks, the cowl or other parts must come off to reach the plug and coil. That extra labor is why rear-cylinder repairs can cost more than front ones. ### Can I fix P0308 myself? Often yes, if you can reach the rear of the engine. Checking the plug, swapping the coil with a neighbor and replacing a wire are reasonable home jobs with basic tools. Work on a cold engine, and use a torque wrench on plugs. Injector and compression work belong at a shop. ### Can water in the spark plug well cause P0308? Yes, water in a plug well can short the coil boot and cause a misfire. On some trucks and SUVs, a blocked cowl drain lets rainwater run into the rear plug wells. Blow out the well, dry the boot, replace damaged parts and clear the drain to stop it happening again. ### What is the difference between P0308 and P0307? They are the same type of fault on different cylinders: P0308 is cylinder 8 and P0307 is cylinder 7. On GM and Hemi V8s they are on opposite banks, while on Ford V8s they sit side by side. Both together on one bank can point to a shared cause. ### Will P0308 damage my engine? A light misfire is unlikely to damage the engine itself, but ignoring it is risky. Unburned fuel can overheat the catalytic converter, and a leaking injector can wash oil from the cylinder wall. A misfire caused by low compression usually means a mechanical problem that will get worse. ### Will P0308 pass an emissions test? No, a car with P0308 and the check engine light on fails an OBD-based emissions test in most states. After the repair, clear the code and drive for several days so the readiness monitors complete. Check that they read ready on a scan tool before you return for the test. ### Should I replace all coils if one fails? Not usually. Coil-on-plug engines typically lose one coil at a time, and replacing only the failed one is normal. Spark plugs are different, since they are normally replaced as a full set. If several coils have failed over a short time, have the charging system and wiring checked. ## Sources - [SAE J2012: Diagnostic Trouble Code Definitions](https://www.sae.org/standards/content/j2012_201812/) - [EPA: On-Board Diagnostics (OBD)](https://www.epa.gov/state-and-local-transportation/vehicle-emissions-board-diagnostics-obd) ## Other ignition and misfire codes - [P0300 Random/Multiple Cylinder Misfire Detected](https://diagxcar.com/codes/p0300) - [P0301 Cylinder 1 Misfire Detected](https://diagxcar.com/codes/p0301) - [P0302 Cylinder 2 Misfire Detected](https://diagxcar.com/codes/p0302) - [P0303 Cylinder 3 Misfire Detected](https://diagxcar.com/codes/p0303) - [P0304 Cylinder 4 Misfire Detected](https://diagxcar.com/codes/p0304) - [P0305 Cylinder 5 Misfire Detected](https://diagxcar.com/codes/p0305) - [P0306 Cylinder 6 Misfire Detected](https://diagxcar.com/codes/p0306) - [P0307 Cylinder 7 Misfire Detected](https://diagxcar.com/codes/p0307) - [P0316 Engine Misfire Detected on Startup (First 1000 Revolutions)](https://diagxcar.com/codes/p0316) - [P0320 Ignition/Distributor Engine Speed Input Circuit](https://diagxcar.com/codes/p0320) ## Symptoms - [Why is my car idling rough?](https://diagxcar.com/symptoms/rough-idle) - [Why does my car shake at idle?](https://diagxcar.com/symptoms/car-shakes-at-idle) - [Why does my car jerk when I accelerate?](https://diagxcar.com/symptoms/car-jerks-when-accelerating) - [Why is my car getting bad gas mileage?](https://diagxcar.com/symptoms/poor-gas-mileage) ## Repair costs - [Spark plug replacement cost](https://diagxcar.com/cost/spark-plug-replacement): $120-$450 - [Ignition coil replacement cost](https://diagxcar.com/cost/ignition-coil-replacement): $150-$400 - [Fuel injector replacement cost](https://diagxcar.com/cost/fuel-injector-replacement): $300-$1,500 --- Source page: https://diagxcar.com/codes/p0308 Updated: 2026-11-03 General information, not a substitute for a qualified mechanic.