P0308 Code: Causes, Symptoms, Diagnosis & How to Fix It
The P0308 code means Cylinder 8 Misfire Detected.
This generic OBD2 trouble code is triggered when the engine computer detects that cylinder 8 is not firing properly. P0308 identifies the affected cylinder, but it does not tell you which component caused the misfire. Possible causes include ignition problems, fuel-delivery issues, intake leaks, low compression, wiring faults, or engine timing problems.
Cylinder numbering varies by engine design, so use the manufacturer's service information to correctly locate cylinder 8 before testing or replacing components.
P0308 Code Symptoms
Common symptoms include:
Check Engine Light
Rough or shaking engine
Rough idle
Hesitation during acceleration
Engine stumbling
Reduced power
Poor fuel economy
Hard starting
Engine stalling
Unusual exhaust odor
Flashing Check Engine Light
A severe misfire can cause noticeable jerking or stumbling, particularly under acceleration. A flashing Check Engine Light can indicate a severe active misfire with a risk of catalytic-converter damage.
Common Causes of P0308
1. Faulty Spark Plug
A worn, fouled, damaged, or incorrectly gapped spark plug can prevent cylinder 8 from producing consistent combustion.
2. Bad Ignition Coil
A weak or failed ignition coil can prevent cylinder 8 from receiving a strong spark. If the vehicle uses individual coils, a coil-swap test can help determine whether the misfire follows the coil.
3. Faulty Fuel Injector
A clogged, restricted, leaking, or electrically defective injector can cause incorrect fuel delivery to cylinder 8.
4. Vacuum or Intake Leak
Unmetered air entering the intake system can disturb the air-fuel mixture and contribute to a cylinder-specific misfire.
5. Low Fuel Pressure
A weak fuel pump or another fuel-delivery problem can reduce fuel supply and cause misfires, especially under load.
6. Low Engine Compression
A burned valve, worn piston rings, damaged piston, or head-gasket problem can reduce compression and create a persistent misfire.
7. Wiring or Connector Problems
Damaged wiring, loose connectors, corrosion, or poor electrical connections can interfere with the ignition coil or fuel injector.
8. Engine Timing Problems
Incorrect valve or ignition timing can prevent proper combustion and contribute to a cylinder 8 misfire.
How to Diagnose P0308
1. Scan for Additional Codes
Connect an OBD2 scanner and check for stored and pending codes.
Record freeze-frame information before clearing the codes. Additional misfire, fuel-system, lean-condition, sensor, or ignition codes can provide important diagnostic clues.
2. Inspect the Spark Plug
Remove the cylinder 8 spark plug and check for:
Carbon buildup
Oil or fuel fouling
Worn electrodes
Incorrect gap
Cracks or physical damage
Spark-plug condition can help indicate whether the problem is related to ignition or another engine condition.
3. Test the Ignition Coil
Inspect the cylinder 8 ignition coil, boot, connector, and wiring.
If the vehicle allows coil swapping, move the suspected coil to another cylinder. If the misfire moves with the coil, the coil becomes a strong suspect.
4. Check the Fuel Injector
Inspect the cylinder 8 injector and electrical connector.
Depending on the vehicle, testing may include injector resistance, electrical operation, fuel delivery, or injector-balance testing.
5. Check for Intake or Vacuum Leaks
Inspect vacuum hoses, intake components, and manifold areas around the affected cylinder.
A smoke test can help locate leaks that are difficult to identify through visual inspection.
6. Perform a Compression Test
If the ignition and fuel systems appear normal, test engine compression.
Low compression can point to an internal mechanical problem involving a valve, piston, piston rings, or head gasket.
7. Check Engine Timing and Sensor Data
If ignition, fuel, air intake, and compression checks do not reveal the problem, inspect camshaft/crankshaft synchronization and relevant engine-management data according to the manufacturer's diagnostic procedure.
How to Fix P0308
The correct repair depends on the confirmed cause. Possible repairs include:
Replace the faulty spark plug
Replace a defective ignition coil
Repair ignition wiring or connectors
Replace or repair the fuel injector
Repair an intake or vacuum leak
Correct low fuel pressure
Repair engine timing problems
Repair an internal engine problem
Replace a failed sensor when testing confirms it
After the repair, clear the trouble code and perform a test drive while monitoring misfire data.
Do not automatically replace the ignition coil because P0308 is present. The code identifies the misfiring cylinder, not necessarily the failed component.
Is P0308 Serious?
P0308 should be diagnosed promptly. A persistent engine misfire can reduce performance and fuel economy and may damage the catalytic converter.
If the Check Engine Light is flashing, the engine is shaking severely, the vehicle has major power loss, or it is stalling, avoid unnecessary driving and have the vehicle inspected promptly.
If the Check Engine Light is steady and symptoms are mild, arrange diagnosis as soon as practical.
P0308 vs. P0307
Code | Meaning |
P0307 | Cylinder 7 Misfire Detected |
P0308 | Cylinder 8 Misfire Detected |
Both are cylinder-specific misfire codes. The difference is the cylinder identified by the engine computer.
P0308 vs. P0300
P0300 indicates a random or multiple-cylinder misfire without identifying one specific cylinder.
P0308 specifically identifies cylinder 8 as the cylinder experiencing the misfire.
Frequently Asked Questions
Can a bad spark plug cause P0308?
Yes. A worn, fouled, damaged, or incorrectly gapped spark plug can cause cylinder 8 to misfire.
Can a bad ignition coil cause P0308?
Yes. A defective ignition coil can prevent cylinder 8 from receiving a reliable spark.
Can I drive with P0308?
Avoid continued driving if the Check Engine Light is flashing, the engine is shaking severely, or there is major power loss or stalling. A severe misfire can increase the risk of catalytic-converter damage.
Does P0308 mean cylinder 8 is damaged?
No. P0308 identifies a misfire in cylinder 8. The underlying problem could be the spark plug, coil, injector, intake system, compression, wiring, or timing.
Can low compression cause P0308?
Yes. Low compression caused by an internal engine problem can result in a persistent cylinder 8 misfire.
