Diagnostic Overview
Meaning: Fault code P0308 indicates that your vehicle's Engine Control Module (ECM/PCM) has registered a threshold discrepancy for cylinder 8 misfire detected.
Severity Level: High
Drivability Guidance: Driving should be minimized. Continued driving under load can cause internal engine wear or catalyst overheating.
What does code P0308 mean?
Diagnostic Trouble Code (DTC) P0308 is defined under SAE J2012 / ISO 15031-6 standards as Cylinder 8 Misfire Detected. The onboard Engine Control Module (ECM/PCM) continuously evaluates sensor signals and circuit electrical integrity during vehicle operation.
ECU Detection Mechanism
The Engine Control Module (ECM/PCM) measures microsecond-level rotational acceleration of the crankshaft reluctor wheel following each cylinder firing stroke using the crankshaft position sensor. When a cylinder fails to accelerate the flywheel according to calibrated torque-gain tables, the internal misfire counter increments. If the misfire frequency exceeds emissions failure thresholds (typically 1.5% to 3.0% of total firings) or catalytic damage thresholds, this code is set.
When the fault criteria are met across consecutive drive evaluation cycles, the system illuminates the Check Engine Light (MIL) and freezes sensor telemetry into non-volatile memory.
Common Symptoms of Code P0308
Depending on driving conditions and vehicle engine architecture, symptoms may include:
- V8 engine roughness
- Misfire code
Diagnostic Evidence & Cause Verification
Rather than guessing component failures, examine live sensor PIDs and electrical values to confirm the root cause before replacing parts:
| Likely cause | Confirming evidence | Test / PID to read | Expected value if this is the cause |
|---|---|---|---|
| Ignition coil failure on Cylinder 8 | Misfire follows coil when swapped to Cylinder 7 | Misfire Cylinder 8 Count |
> 30 counts per 1000 revs |
| Worn spark plug or low fuel injector flow on Cylinder 8 | Plug electrode gap worn; lean condition on rear cylinder bank | Cylinder 8 Power Contribution |
RPM drop deficit > 15% |
Recommended Diagnostic Procedure
Follow this systematic mechanical checklist to isolate the fault:
- Step 1: Swap coil #8
- Step 2: Check plug #8
Related Technical Diagnostic Guides
Deepen your understanding of live sensor diagnostics and ECU communication protocols: