Engine Misfire: Single-Cylinder vs Random — Evidence-Driven Framework

Symptom-first framework

1. Clarify complaint — under load? at idle? cold? flashing MIL?
2. Confirm operating condition — load, RPM, temperature
3. Retrieve DTCs — P0300 (random) vs P0301–P0312 (cylinder-specific)
4. Inspect freeze-frame / live data — counters, trims, load
5. Identify affected system — ignition, fuel, air, mechanical, sensor
6. Form differential
7. Select decisive test
8. Update hypothesis
9. Confirm cause
10. Repair
11. Verify — under the load that misfired

Single-cylinder vs random

The first split:

• P0301–P0312 / one cylinder counter high → cylinder-specific: that cylinder's coil, plug, injector, compression, valve
• P0300 / counters spread → random/whole-engine: air leak, fuel pressure/volume, MAF, fuel quality, timing, crank-signal integrity

This single distinction reshapes the hypothesis list.

Ignition

• Coil / plug / lead — the leading single-cylinder cause
• Swap test — move the coil/plug to another cylinder; does the misfire follow?
• Coil trigger — confirm the ECM commands the coil (scope/test light)
• Under load — coil breakdown under load is a classic

Fuel

• Fuel pressure / volume — low under load → random misfire
• Injector — restricted (lean on one cylinder) or leaking (rich)
• Fuel quality — water, old fuel, wrong octane → random misfire
• Fuel trims — lean trims + misfire → air/fuel, not ignition

Compression

• Compression / leak-down — low on one cylinder → misfire on that cylinder; on several → timing or wear
• Relative compression — cranking-current test, fast screen

Air leak

• Vacuum leak → random misfire, lean trims at idle
• Smoke/propane test
• See P0171.

Timing

• VVT / cam phase mistimed → random misfire, correlation codes
• Timing chain stretch → misfire + rattle
• See the Crank/Cam Correlation guide.

Mechanical

• Burnt valve — low compression on one cylinder, leak-down confirms
• Valve seat / carbon (DI intake ports) — random or cylinder-specific
• Head gasket between cylinders — two adjacent cylinders low

Swap tests

The swap test isolates a part:

• Coil swap — misfire follows the coil → coil; stays → injector/compression/wiring
• Plug swap — same logic
• Injector swap (where accessible) — misfire follows the injector → injector

Swap tests are faster and more decisive than replacing parts on a guess.

Relative compression

A relative-compression test (scan-tool cranking-current test) compares cylinders' cranking current — a low-current cylinder has low compression. It is a fast screen before a full compression test.

Scope

• Crank sensor — a noisy crank signal corrupts misfire detection; scope confirms signal integrity before trusting misfire data. See the Crank Sensor Waveform guide.
• Ignition — coil primary/secondary waveform (where accessible) shows coil breakdown
• Injector — injector current/voltage waveform confirms command and driver

Relationship with P0300

P0300 is the random/multiple-cylinder misfire code. This guide is the symptom-first framework behind it. The code reports the symptom; the counters, trims, and tests find the cause.

Links

• P0300 — random misfire DTC
• P0171 — lean (a common misfire co-cause)
• Fuel Trims — mixture evidence
• Rough Idle — the idle-quality framework
• Crank Sensor Waveform — misfire-detection integrity

Common mistakes

• Replacing all coils/plugs without reading per-cylinder counters
• Not distinguishing single-cylinder from random
• Ignoring fuel trims (lean misfire is often air/fuel, not ignition)
• Not swap-testing before replacing
• Trusting misfire data when the crank signal is questionable
• Overlooking fuel quality