Fuel Delivery
P0308
Cylinder 8 Misfire Detected
P0308 — Cylinder 8 Misfire Detected — is a Misfire Detection and Combustion DTC. Diagnosis should compare cylinder 8 misfire counters, ignition energy, injector command/delivery, compression/leakage, relative compression, fuel trim, crankshaft speed variation, and operating-condition dependence. Preserve freeze-frame, reproduce the failure, prove…
Diagnostic Snapshot
Atlas Academy
Learn the System
Before diagnosing this code, it helps to understand the system behind it.
Read Understanding Fuel Delivery SystemsTechnical Summary
P0308 indicates the PCM detected a repeated misfire on cylinder 8. The code identifies the affected cylinder, but it does not identify whether the failure is caused by ignition, fuel delivery, wiring, compression, valve-train operation, or another mechanical condition.
Final QA emphasis: P0308 requires correlation of cylinder 8 misfire counters, ignition energy, injector command/delivery, compression/leakage, relative compression, fuel trim, crankshaft speed variation, and operating-condition dependence. The code should be diagnosed by reproducing the enable condition, proving loaded electrical integrity, and independently confirming the physical system. OEM thresholds and system design take priority over universal values.
What You'll Learn
- Understand what a cylinder 8 misfire means
- Use freeze-frame and Mode $06 data to confirm when the misfire occurs
- Separate ignition, injector, wiring, and compression faults
- Use controlled coil and injector tests without creating new problems
- Know when compression, leakdown, or valve-train testing is required
- Avoid replacing parts before the evidence supports the repair
Think Like a Technician
Treat P0308 as a location clue, not a parts recommendation. Start by confirming the operating conditions that produce the misfire. If a controlled coil swap moves the misfire to another cylinder, the coil becomes the leading suspect. If the misfire remains on cylinder 8, continue with spark-plug inspection, injector command and balance, wiring checks, compression, leakdown, and valve-train diagnosis.
For P0308, ask three questions in order: what did the controller command or observe, can the circuit carry and report that condition correctly, and did the physical system actually respond? Use cylinder 8 misfire counters, ignition energy, injector command/delivery, compression/leakage, relative compression, fuel trim, crankshaft speed variation, and operating-condition dependence to identify which layer fails first.
What This Code Means
Cylinder 8 is not contributing normal engine power because combustion is incomplete, weak, or missing. A cylinder-specific code narrows the location of the problem, but further testing is required to identify the failed system or component.
System Overview
The PCM detects misfire by monitoring very small changes in crankshaft rotational speed as each cylinder contributes torque. P0308 identifies cylinder 8 as the cylinder whose combustion events are not producing the expected crankshaft acceleration. The monitor can be affected by ignition, fuel delivery, injector control, intake leakage, compression, valvetrain, coolant or oil intrusion, and less commonly crankshaft-signal or controller problems. A cylinder-specific misfire code identifies the affected combustion event; it does not identify which component failed.
Why This Code Sets
The PCM monitors crankshaft speed after each combustion event. When cylinder 8 repeatedly produces less acceleration than expected, the PCM counts those events as misfires. Once the misfire rate exceeds the manufacturer's threshold, P0308 is stored.
Common Symptoms
- Check Engine Light
- Flashing Check Engine Light during an active severe misfire
- Rough idle
- Hesitation or jerking under acceleration
- Reduced power
- Poor fuel economy
- Fuel odor from unburned fuel
- Hard starting in some cases
Most Likely Causes
- 1. ignition fault on the affected cylinder
- 2. fuel injector delivery/control problem
- 3. compression, valve, piston/ring, or valvetrain fault
- 4. vacuum/intake leak affecting that runner or cylinder
- 5. coolant/oil intrusion or deposit-related combustion problem
- 6. PCM/driver or crank-signal interpretation fault after basic combustion causes are proven
Freeze Frame Clues
Idle
A misfire that is strongest at idle may point toward a local vacuum leak, injector imbalance, weak compression, or a valve-sealing problem.
Under Load
A misfire that increases during acceleration commonly points toward ignition breakdown, excessive spark-plug gap, or inadequate fuel delivery.
Cold Start
A cold-start-only misfire may indicate injector leakage, coolant intrusion, poor sealing, or a spark plug that is fouled after the vehicle sits.
Hot Restart
A hot-soak misfire may point toward an injector leaking after shutdown, heat-sensitive wiring, or a coil that fails as temperature rises.
Live Data Expectations
Review cylinder 8 misfire counters, Mode $06 data, short-term and long-term fuel trims, injector pulse width, and cylinder-contribution data when available. Compare cylinder 8 to the other cylinders under the same operating conditions. After repair, the cylinder 8 misfire counter should remain at or near zero during idle, acceleration, cruise, and hot restart.
Typical Verification Tests
- Inspect the cylinder 8 spark plug
- Perform a spark-output test
- Perform a controlled ignition-coil swap when appropriate
- Verify coil power, ground, and control signal
- Verify injector power and PCM command
- Measure injector resistance and compare with other cylinders
- Perform an injector balance or contribution test
- Inspect the injector and coil connectors for spread terminals or corrosion
- Perform compression and leakdown testing
- Check for a local vacuum or intake leak
- Inspect for oil or coolant contamination
Before You Condemn
- Do not condemn these components until testing confirms the failure:
- Ignition coil
- Fuel injector
- PCM
- Cylinder head
- Catalytic converter
- Verify spark quality, coil command, injector command, injector flow, wiring integrity, compression, leakdown, and mechanical timing first.
Before Replacing Parts
- Save freeze-frame data before clearing codes
- Review cylinder 8 misfire counts and Mode $06 data
- Inspect the spark plug and plug well
- Verify coil output or perform a controlled swap test
- Check coil and injector connectors for corrosion and terminal tension
- Verify injector power and PCM command
- Compare injector resistance and balance with other cylinders
- Perform compression or leakdown testing if ignition and fuel tests pass
Diagnostic Workflow
- Confirm P0308; save current, pending, history, freeze-frame, readiness, misfire/fuel-trim data where applicable, and the complete powertrain scan before clearing codes.
- Verify the exact definition — Cylinder 8 Misfire Detected — and review VIN-specific system design, enabling criteria, fail-safe strategy, wiring, component locations, bulletins, and test procedures.
- Prioritize companion DTCs and basic prerequisites: battery/charging voltage, module powers/grounds, engine mechanical condition, coolant temperature plausibility, and any shared reference or network faults.
- Recreate the freeze-frame condition safely and graph cylinder 8 misfire counters, ignition energy, injector command/delivery, compression/leakage, relative compression, fuel trim, crankshaft speed variation, and operating-condition dependence with RPM, load, temperature, vehicle speed, and system voltage.
- Inspect the relevant harnesses, connectors, hoses, pipes, grounds, vacuum lines, fuel/exhaust components, and recent repair areas before disconnecting or replacing anything.
- Compare commanded and actual data. Decide whether the failure is electrical, hydraulic/pneumatic, mechanical, combustion-related, or a sensor-plausibility problem before choosing the next test.
- Perform loaded electrical tests on affected powers, grounds, reference, signal, solenoid, injector, relay, or actuator circuits; static continuity alone is not sufficient for current-carrying circuits.
- Use bidirectional controls where supported to command the affected actuator or system and compare command with current/voltage feedback, sensor response, and actual physical operation.
- Independently verify the physical condition with the appropriate test: calibrated fuel-pressure equipment, smoke/EVAP test, compression/leak-down, ignition waveform, injector balance, airflow/pressure response, or another OEM-approved method.
- Separate bank-specific or cylinder-specific faults from shared-system faults by comparing the opposite bank, neighboring cylinders, redundant sensors, or common feeds under the same operating condition.
- Check for heat-, vibration-, and load-dependent failures by repeating the test as the system warms and by carefully manipulating suspect harness sections while monitoring data.
- Do not condemn a sensor because its reading is abnormal until an independent measurement establishes whether the sensor is wrong or accurately reporting a real physical fault.
- Do not condemn an actuator because it fails to respond until power, ground, control authority, mechanical freedom, and the load side of the circuit are proven.
- Before condemning the PCM, prove stable module powers/grounds, terminal tension, external loads, circuit capability, prerequisite inputs, network integrity, and current calibration/software.
- Repair only the fault supported by test evidence, then perform any required relearn, adaptation, fuel-trim reset, or monitor-specific service procedure.
- Repeat the original enable condition and verify P0308 remains absent current and pending, the monitored data is plausible, driveability is normal, and readiness progresses.
Labor & Inspection Checklist
- Record freeze-frame and Mode $06 data
- Inspect cylinder 8 spark plug and plug well
- Test or swap the ignition coil
- Verify coil power, ground, and control
- Verify injector power and command
- Check injector resistance or balance
- Inspect connectors and harness routing
- Check for a local intake leak
- Perform compression or leakdown testing if needed
- Road-test and confirm zero repeat misfire counts
Common Repairs
- Replace a failed cylinder 8 spark plug or ignition coil only after testing proves the fault
- Repair cylinder 8 injector wiring/connector or replace a proven faulty injector
- Repair an intake/vacuum leak affecting cylinder 8
- Correct compression, valve, valvetrain, head-gasket, or other mechanical fault proven by testing
- Repair fuel-delivery or shared electrical faults when data shows a system-wide cause
- Repair PCM driver/input faults only after ignition, fuel, wiring, and mechanical causes are excluded
Shop Notes
- A cylinder-specific code identifies where the PCM detected the misfire, not which part failed.
- Oil or coolant in the spark-plug well can damage the coil boot and cause repeat failures.
- A coil swap is only useful when the part and connector design allow a safe, controlled comparison.
- If the misfire does not move with the coil, stop replacing ignition parts and continue testing.
- A leaking injector can cause a cold-start or hot-soak misfire before setting other fuel-trim codes.
- Low compression can mimic both ignition and injector faults.
- A flashing MIL means the misfire is severe enough to threaten the catalytic converter.
Final QA diagnostic boundary for P0308: Cylinder 8 Misfire Detected. The technician should center the diagnosis on cylinder 8 misfire counters, ignition energy, injector command/delivery, compression/leakage, relative compression, fuel trim, crankshaft speed variation, and operating-condition dependence rather than treating the DTC as a replacement instruction.
Evidence hierarchy: preserve freeze-frame first; confirm prerequisites second; compare command/input with loaded circuit behavior third; verify the physical system independently fourth. This order prevents a biased PID or unloaded continuity test from driving the repair.
Failure-pattern note: separate faults that follow engine load, heat, vibration, bank, cylinder, key cycle, purge/EGR command, or pressure demand. A repeatable pattern is often more diagnostic than a single static reading.
Before-parts rule: sensors require plausibility proof, actuators require power/ground/control plus mechanical-response proof, and modules require external circuit/load proof. Do not use a code description as proof of component failure.
Safety/driveability note: Do not continue normal driving with an active severe misfire or flashing MIL. Unburned fuel can rapidly overheat the catalytic converter, and a mechanical misfire can worsen engine damage.
Verification standard: reproduce the original enable condition after repair, not merely a warm idle in the bay. Confirm the monitor's related PIDs remain plausible and that no pending code returns.
P0308 QA case-pattern 1: Reproduce the original operating condition and compare cylinder 8 misfire counters, ignition energy, injector command/delivery, compression/leakage, relative compression, fuel trim, crankshaft speed variation, and operating-condition dependence with an independent physical or electrical measurement. If the scan value changes but the physical system does not, investigate sensor interpretation, control authority, or a mechanical restriction. If the physical system changes but scan feedback does not, investigate the sensing/reference path. If neither changes, prove power, ground, command, and prerequisite conditions before escalating to the controller. Document the before-and-after data so the repair is verified by evidence rather than by the temporary absence of a code.
P0308 QA case-pattern 2: Reproduce the original operating condition and compare cylinder 8 misfire counters, ignition energy, injector command/delivery, compression/leakage, relative compression, fuel trim, crankshaft speed variation, and operating-condition dependence with an independent physical or electrical measurement. If the scan value changes but the physical system does not, investigate sensor interpretation, control authority, or a mechanical restriction. If the physical system changes but scan feedback does not, investigate the sensing/reference path. If neither changes, prove power, ground, command, and prerequisite conditions before escalating to the controller. Document the before-and-after data so the repair is verified by evidence rather than by the temporary absence of a code.
P0308 QA case-pattern 3: Reproduce the original operating condition and compare cylinder 8 misfire counters, ignition energy, injector command/delivery, compression/leakage, relative compression, fuel trim, crankshaft speed variation, and operating-condition dependence with an independent physical or electrical measurement. If the scan value changes but the physical system does not, investigate sensor interpretation, control authority, or a mechanical restriction. If the physical system changes but scan feedback does not, investigate the sensing/reference path. If neither changes, prove power, ground, command, and prerequisite conditions before escalating to the controller. Document the before-and-after data so the repair is verified by evidence rather than by the temporary absence of a code.
P0308 QA case-pattern 4: Reproduce the original operating condition and compare cylinder 8 misfire counters, ignition energy, injector command/delivery, compression/leakage, relative compression, fuel trim, crankshaft speed variation, and operating-condition dependence with an independent physical or electrical measurement. If the scan value changes but the physical system does not, investigate sensor interpretation, control authority, or a mechanical restriction. If the physical system changes but scan feedback does not, investigate the sensing/reference path. If neither changes, prove power, ground, command, and prerequisite conditions before escalating to the controller. Document the before-and-after data so the repair is verified by evidence rather than by the temporary absence of a code.
Mechanic's Tip
Use the misfire's behavior to guide the next test. A fault that appears only under load favors ignition breakdown; a fault strongest at idle may favor vacuum leakage, injector imbalance, or compression loss.
Common Mistakes
- Replacing the ignition coil without testing or a controlled swap
- Replacing the injector without checking command, resistance, and balance
- Ignoring the spark plug condition or plug-well contamination
- Skipping compression and leakdown testing
- Clearing codes before recording freeze-frame data
- Assuming a new part is automatically good
- Continuing to drive with a flashing Check Engine Light
Tools Used During Diagnosis
- Professional scan tool
- Spark tester
- Digital multimeter
- Noid light or oscilloscope
- Compression tester
- Leakdown tester
Customer Explanation
Cylinder 8 is not producing power consistently. The cause may be the spark plug, ignition coil, fuel injector, wiring, an air leak, or an internal engine problem. The diagnostic process tests each system so the actual fault is repaired instead of replacing parts based only on the code.
Frequently Asked Questions
What does P0308 mean?
P0308 is Cylinder 8 Misfire Detected. The monitor is focused on cylinder 8 misfire counters, ignition energy, injector command/delivery, compression/leakage, relative compression, fuel trim, crankshaft speed variation, and operating-condition dependence.
Does the code prove a part has failed?
No. It identifies a failed monitored relationship. Electrical integrity, physical system behavior, and sensor plausibility must be separated before parts are replaced.
Is it safe to drive with P0308?
Do not continue normal driving with an active severe misfire or flashing MIL. Unburned fuel can rapidly overheat the catalytic converter, and a mechanical misfire can worsen engine damage.
What should be checked first?
Save freeze-frame and the complete scan, verify the exact system design by VIN, then reproduce the failure while comparing commanded data with actual electrical and physical response.
What freeze-frame clues matter most?
RPM, load, vehicle speed, coolant temperature, system voltage, fuel trims, pressure/airflow data, command state, and companion DTCs show whether the failure occurs at idle, startup, cruise, acceleration, deceleration, or high load.
What live data is most useful?
Graph cylinder 8 misfire counters, ignition energy, injector command/delivery, compression/leakage, relative compression, fuel trim, crankshaft speed variation, and operating-condition dependence plus system voltage, RPM/load, temperature, and relevant redundant or opposite-bank/cylinder data. Look for the first relationship that breaks.
Can another system create this code?
Yes. Shared voltage, airflow, fuel delivery, exhaust, vacuum, mechanical, or combustion faults can make the named monitor fail even when the named component is functional.
Should I replace the most common part first?
No. Common-failure statistics are useful for prioritizing inspection, not for skipping tests. Expensive or invasive parts should have at least two consistent forms of evidence.
When is professional equipment justified?
Enhanced graphing data and bidirectional controls are strongly recommended. Depending on the code, a pressure transducer/gauge, scope, current clamp, smoke machine, injector balance equipment, or compression/leak-down tools may be justified.
How do I verify the repair?
Repeat the original operating condition, confirm electrical and physical results agree, check current and pending codes, and complete enough of the drive cycle to show stable data and readiness progress.
Related Atlas Resources
Related Atlas Resources
Understanding Fuel Delivery Systems
Related Reference Chapters
Diagnostic Confidence
High - cylinder-specific misfires can usually be isolated with scan data, controlled ignition tests, injector testing, wiring inspection, and compression or leakdown testing.
Related Codes
Project Atlas Feedback
Was this article helpful?
Needs Improvement
Working on this repair?
Create a professional estimate in minutes.
Why Trust This Guide?
Built by mechanics. Improved by mechanics.
This guide is maintained for working mechanics, mobile technicians, fleet maintenance professionals, and independent repair shops.