P0308 Code: Cylinder 8 Misfire Detected
Cylinder 8 is misfiring. It's a rear V8 cylinder against the firewall, so its plug is often the one skipped.
At a glance
Severity: Moderate
- Meaning
- Cylinder 8 Misfire Detected
- System
- Ignition & Misfire · Powertrain
- Safe to drive?
- With caution
- Typical repair cost
- $40–$4,000
- DIY difficulty
- Moderate
P0308 means the engine computer is counting repeated missed power strokes on cylinder 8, the last cylinder of a V8 (or a middle one on a V10). On GM small-blocks and the Chrysler Hemi it is the rear cylinder on the passenger side; on Ford V8s it is the rear cylinder on the driver's side, jammed between the valve cover, the brake booster and the cowl. That position shapes the diagnosis. The plug is the hardest one to reach, so it is the plug most likely to have been skipped or damaged at the last tune-up, and the coil and wiring there see the most heat and water from the cowl. Cylinder 8 is not one of the deactivated cylinders on GM or Hemi engines, so a lifter is less likely here than on cylinders 1, 4, 6 and 7, but a vacuum leak at the rear of the intake, where the brake booster hose often connects, deserves a look.
Can I keep driving? With a steady light, drive gently to a shop without towing. If the light flashes, pull over, because the catalytic converter on that bank is being flooded with fuel. A misfire together with a soft brake pedal or a hiss when braking points to a booster leak that also affects your brakes, so get it checked right away.
How to read P0308
- P Powertrain
- 0 Generic (SAE)
- 3 Ignition / misfire
- 08 Specific fault
Symptoms
- Rough idle that is worse with the brakes applied (booster hose leak)
- Misfire that got worse right after a spark plug job
- Check-engine light flashing under load, often with a lean code for the same bank
- Ticking from the rear of the engine that isn't the valvetrain (exhaust manifold leak)
- Water or rust visible around the rear coil after rain
Possible causes
Ranked by how often each cause is behind this code (editorial estimate).
- Worn plug 8 left in place because it is hard to reach, or a plug cross-threaded or broken during a previous tune-up
- Failing coil 8 or plug wire, cooked by heat at the firewall or by a leaking exhaust manifold
- Vacuum leak at the rear of the intake: brake booster hose, PCV hose or rear intake gasket
- Clogged or leaking cylinder 8 injector
- Burned exhaust valve or poor valve seal on the hot rear cylinder
- Water damage or chafing on the rear coil and injector connectors from a blocked cowl drain
Diagnosis step by step
Check plug 8's history
Ask when the plugs were last changed and whether all eight were done. If the misfire started right after a tune-up, suspect plug 8 itself: a cracked insulator, a boot not fully seated, or a damaged thread. On Ford 3-valve V8s, check for a plug that broke during removal and was left partly in place.
Tools: Flashlight
Listen for the brake booster and rear intake
At idle, press the brake pedal and listen for a hiss near the booster, and watch whether the idle drops. Then smoke-test the intake and watch the rear runner, the PCV hose and the booster hose connection. A leak here leans out the rear cylinders first.
Tools: Smoke machine
Inspect the rear coil, wire and connector
Look at coil 8, its plug wire boot and its connector for heat cracks, water and rust. On GM trucks, look for a broken exhaust manifold bolt at the rear passenger corner: escaping exhaust gas can burn the nearby plug wire boot. Then swap coil 8 with coil 6 (GM, Hemi) or coil 7 (Ford) and see whether the misfire moves.
Tools: Flashlight, Screwdriver, OBD-II scanner
Injector balance test
Use a bidirectional scan tool to shut off each injector in turn and compare the rpm drop, or have a shop do a pressure-drop balance test. Cylinder 8 dropping much less rpm than the others while ignition is good points to a restricted injector.
Tools: OBD-II scanner, Fuel pressure gauge
Compression and leak-down on cylinder 8
If the plug, coil, air and fuel all check out, compare cylinder 8's compression with cylinder 6 or 7. On a leak-down test, air hissing from the tailpipe means a burned exhaust valve, from the intake means an intake valve, and from the oil filler means rings.
Tools: Compression tester
Where the parts are
- 1Ignition coil
- 2Spark plug
- 3Fuel injector
- 4Camshaft sensor
- 5Crankshaft sensor
- 6Engine computer (ECM)
- 7Intake manifold
Repair options & costs
| Repair | DIY? | Estimated cost |
|---|---|---|
| Replace spark plug 8 (plus full set if overdue) | Yes | $60–$550 |
| Replace coil 8 and plug wire | Yes | $80–$260 |
| Replace brake booster hose or check valve, or PCV hose | Yes | $40–$180 |
| Repair broken exhaust manifold bolts and gasket (GM truck) | No | $300–$900 |
| Cylinder head repair for a burned valve | No | $1,800–$4,000 |
Rough US estimates for parts and labor; shop rates vary by region. Not a quote.
Notes by brand
Ford
On Ford's 4.6L and 5.4L V8s, cylinder 8 is at the rear of the driver's bank, under the cowl and next to the brake booster, and it's the plug most often skipped or rushed. On 3-valve versions the two-piece plugs can break off in the head, so the correct removal procedure matters more here than anywhere else.
Chevrolet
GM truck V8s are known for broken exhaust manifold bolts, often at the rear corners. At the rear passenger corner the escaping exhaust can heat the cylinder 8 plug wire boot until it arcs to ground, causing a P0308 that returns after a new coil.
Ram
The 5.7L Hemi has two plugs per cylinder. On earlier versions each coil also fires the neighboring cylinder's second plug through a short wire, so a bad coil or wire on the passenger side can affect more than one cylinder. Check both plugs and both leads for cylinder 8.
Field data
| GMC | 3 | Sierra 1500 |
|---|
Source: NHTSA Complaints API (US government open data), analyzed by DTC mention.
Emissions / smog test impact
Will fail
A P0308 with the check-engine light on fails an OBD emissions test. Also check the lean-code status for that bank, since a vacuum leak behind the misfire will fail the retest if only the plug is changed.
Frequently asked questions
The misfire started right after my spark plugs were changed. Why cylinder 8?
Cylinder 8 is the hardest plug to reach on most V8s, so it's the one most likely to be cross-threaded, have a boot not fully seated, or get a cracked insulator from a slipping socket. Pull it and check before doing anything else.
Could a brake booster hose really cause P0308?
Yes. On many V8s the booster hose connects near the rear of the intake. A cracked hose or leaking check valve lets unmetered air into the rear runners, leaning out cylinder 8 at idle. A hiss when you press the brake is the giveaway.
Is cylinder 8 a cylinder-deactivation cylinder on my Silverado?
No. On GM V8s the deactivated cylinders are 1, 4, 6 and 7. That makes a lifter less likely on cylinder 8, so focus first on the plug, coil, wire, vacuum leaks and injector.
How can I reach plug 8 on a truck?
It depends on the model. On some trucks it's easier from above with a long extension and swivel; on others, pulling back the inner fender liner gives better access. Let the engine cool first, since the exhaust manifold is right there.