P0308 – Cylinder 8 Misfire Detected

Last updated: April 4, 2026

DTC P0308 is an ISO/SAE controlled, powertrain, general fault that indicates the engine control module has detected a misfire event attributed to cylinder 8. A “misfire” means cylinder 8 is not producing the expected combustion torque during some firing events, which the module infers by monitoring small changes in crankshaft speed. When those speed variations repeatedly match the timing window for cylinder 8, the module stores P0308. You may notice a rough idle, hesitation, reduced power, or a check engine light that may flash when the misfire rate is high enough to threaten the catalytic converter. The code identifies the affected cylinder, not the exact failed component.

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P0308 Quick Answer

P0308 – Cylinder 8 Misfire Detected means the control module has identified misfire activity on cylinder 8 based on crankshaft speed changes. Confirm the misfire with scan-tool misfire counters and freeze-frame data, then test cylinder 8 ignition, fuel delivery, air/vacuum integrity, wiring, and engine mechanical condition before replacing parts.

What Does P0308 Mean?

P0308 – Cylinder 8 Misfire Detected means the powertrain control module has determined that cylinder 8 is not contributing normal power consistently. Instead of a smooth, repeatable crankshaft acceleration on cylinder 8’s power stroke, the module sees irregular acceleration that indicates incomplete or absent combustion in that cylinder.

This is a cylinder-specific diagnostic result, not a parts verdict. Cylinder 8 can misfire from ignition problems (weak or absent spark), fuel problems (incorrect injector operation or mixture), air issues (a localized vacuum leak affecting that cylinder), mechanical problems (low compression or valve sealing), or electrical issues that prevent the ignition or injector from operating correctly.

Theory of Operation

The misfire monitor works by comparing crankshaft speed changes to expected patterns. Each cylinder’s power stroke should create a predictable increase in crankshaft speed. The control module uses crankshaft position information (and engine synchronization strategy) to associate those speed changes with individual cylinders. When the crankshaft fails to accelerate as expected during cylinder 8’s firing window often enough, the module identifies that cylinder as misfiring and stores P0308.

For cylinder 8 to produce normal torque, it must receive the correct air charge, the correct fuel quantity, adequate cylinder sealing (compression), and a properly timed, sufficiently strong spark. A deficiency in any one of these areas can reduce cylinder 8 torque output and trigger the same detection logic. That is why accurate diagnosis requires testing, not guessing.

Symptoms

• Check engine light illuminated (may flash if the misfire rate is high)
• Rough or unstable idle
• Hesitation, stumble, or bucking during acceleration
• Reduced engine power and poor throttle response
• Engine vibration that changes with engine speed or load
• Increased fuel consumption due to inefficient combustion
• Possible exhaust odor of unburned fuel during active misfire

Common Causes

• Faulty, weak, or intermittent ignition output affecting cylinder 8 (coil/coil-on-plug unit, boot, or related ignition hardware)
• Worn, fouled, cracked, or incorrectly gapped spark plug on cylinder 8
• Injector for cylinder 8 not delivering correct fuel (electrical control issue or internal injector fault)
• Fuel pressure or fuel delivery problem that results in a lean condition that can trigger a cylinder-specific misfire
• Vacuum leak or unmetered air source that disproportionately affects cylinder 8 air/fuel mixture
• Low compression or poor cylinder sealing on cylinder 8 (valve, rings, head gasket, or related mechanical issue)
• Oil or coolant intrusion that contaminates the combustion process in cylinder 8
• Wiring, connector, terminal, or ground problem affecting cylinder 8 ignition or injector operation

Diagnosis Steps

Tools you’ll want: A scan tool capable of viewing cylinder misfire counters (and Mode $06 data if available), freeze-frame data, and fuel trim data; a DVOM for power/ground and voltage-drop checks; a spark tester or suitable ignition verification method; basic hand tools; and, when needed, a noid light or lab scope for injector control checks, plus a compression gauge and cylinder leak-down tester for mechanical confirmation.

1. Verify the code and capture evidence. Confirm P0308 is present (stored, pending, or history). Record freeze-frame data (RPM, load, coolant temperature, fuel trims, and operating mode) so you know the conditions under which cylinder 8 misfire was detected.
2. Check for additional diagnostic trouble codes. Look for codes that change the diagnostic direction (for example, injector circuit faults, lean-condition codes, crank/cam correlation faults, or catalyst-related codes). Addressing related codes may be necessary before cylinder-specific conclusions are made.
3. Review misfire data for pattern and severity. Use live misfire counters and, if available, Mode $06 results to confirm cylinder 8 is the primary contributor. Compare cylinder 8 counts to other cylinders at idle and at a steady elevated RPM.
4. Perform a targeted visual inspection. Inspect cylinder 8 ignition and injector connectors for broken locks, corrosion, oil intrusion, damaged terminals, or evidence of heat damage. Inspect harness routing for rub-through and contact with hot or moving components.
5. Confirm proper power and ground integrity. Check the power supply circuits that feed ignition and injection components, and perform voltage-drop testing on grounds while the engine is running (and under load if possible). Electrical supply issues can mimic component failure.
6. Test ignition first with a controlled swap (when applicable). If the engine design allows it, swap the cylinder 8 ignition coil (or equivalent ignition component) with another cylinder and recheck misfire counters. If the misfire moves with the swapped part, focus on that component and its connection.
7. Inspect the cylinder 8 spark plug condition. Remove and evaluate the plug for fouling, cracks, abnormal wear, incorrect gap, or signs of oil/coolant contamination. Compare it to another cylinder’s plug for context. If contamination is present, identify the source before completing repairs.
8. Check injector operation on cylinder 8. Verify injector feed voltage and control (with a noid light or scope where appropriate). Listen for consistent injector operation and consider a balance/contribution approach if available to compare cylinder 8 fueling to other cylinders.
9. Check for air leaks that can affect cylinder 8. Inspect intake sealing surfaces, runner areas, and vacuum plumbing for leaks. If available, use smoke testing to find small leaks that may not produce an obvious fuel-trim change but can still create a cylinder-specific lean misfire.
10. Verify mechanical integrity if ignition and fuel checks do not resolve the misfire. Perform a compression test and compare cylinder 8 to other cylinders. Follow up with a leak-down test to identify whether leakage is through intake valves, exhaust valves, rings, or the head gasket.

Professional tip: Use freeze-frame conditions to reproduce the misfire. If P0308 is pending or intermittent, misfire counters and Mode $06 results (when supported) can provide a clearer confirmation than relying on how the engine “feels,” especially if the misfire occurs only at a specific load, temperature, or RPM range.

Possible Fixes

• Repair wiring, connector, or terminal issues affecting cylinder 8 ignition or injector circuits
• Restore proper power supply or ground integrity confirmed by testing (including voltage-drop correction)
• Replace or service the spark plug on cylinder 8 if inspection confirms wear, damage, incorrect gap, or fouling (and address any underlying contamination cause)
• Replace or service the cylinder 8 ignition component found to be faulty through testing (including swap confirmation where applicable)
• Repair injector circuit faults or replace/service the cylinder 8 injector if electrical and/or functional tests confirm improper operation
• Repair intake air leaks or vacuum leaks that affect cylinder 8 mixture
• Correct mechanical engine issues confirmed by compression/leak-down testing (valve sealing, rings, or head gasket concerns)

Can I Still Drive With P0308?

Driving with P0308 depends on whether the misfire is active and severe. If the check engine light is flashing, the engine is shaking noticeably, power is reduced, or you detect strong signs of unburned fuel, continued driving can quickly overheat and damage the catalytic converter and may create a safety risk due to poor acceleration. If the vehicle seems to run nearly normal and the light is steady, limit driving to what is necessary to reach a repair location and avoid heavy throttle and high load until the underlying cause of the cylinder 8 misfire is diagnosed and repaired.

How Serious Is This Code?

P0308 can range from moderate to urgent. A minor, intermittent cylinder 8 misfire may only cause a slight roughness and increased emissions, but a higher misfire rate can cause a flashing check engine light and can damage the catalytic converter due to unburned fuel entering the exhaust. Because misfires also reduce engine output and can worsen under load, the severity should be judged by misfire intensity, drivability impact, and whether the light flashes. Prompt diagnosis is recommended to prevent secondary damage and to restore reliable engine operation.

Common Misdiagnoses

A frequent misdiagnosis is replacing ignition parts immediately based only on the cylinder number without confirming the failure with testing (such as misfire counters, swap testing, or spark verification). Another is overlooking electrical supply issues—poor power or ground can cause weak ignition or injector performance that mimics a bad component. It is also easy to miss a localized air leak near the intake path affecting cylinder 8, or to skip mechanical tests after ignition and fuel checks appear normal. Finally, clearing codes before recording freeze-frame data can remove valuable information needed to reproduce and confirm the fault.

Most Likely Fix

The most likely fix is the one proven by testing that restores normal combustion in cylinder 8. In many diagnostic workflows, this starts by verifying cylinder 8 ignition performance (including spark plug condition and the ignition component’s ability to produce consistent spark under load), then validating injector operation and mixture integrity, and finally confirming cylinder sealing with compression and leak-down tests if needed. The correct repair is the repair supported by evidence that the misfire no longer occurs in cylinder 8 under the same conditions that originally set P0308.

Repair Costs

Repair cost depends on whether the confirmed root cause is wiring, connector condition, a sensor, a module, or the labor needed to diagnose the fault correctly.