Misfire Monitor

Continuous

Typically sets ready within 2-5 minutes of normal driving after engine start.

What This Monitor Checks

The Misfire Monitor continuously tracks crankshaft rotational velocity via the crankshaft position sensor (CKP) to detect combustion irregularities in each cylinder. It identifies both Type A misfires (catalyst-damaging severity) and Type B misfires (emissions threshold exceedances) by measuring minute variations in crankshaft acceleration between firing events. The PCM correlates misfire counts against engine speed and load to determine if the misfire rate exceeds acceptable limits.

Why It Matters for Emissions

Misfires allow unburned fuel and air to pass through the catalytic converter, dramatically increasing hydrocarbon (HC) emissions and potentially overheating and destroying the catalyst substrate. Severe misfires can raise tailpipe HC emissions by 10-100x and cause irreversible catalyst damage within minutes.

Drive Cycle Steps

Start the engine and allow it to reach normal operating temperature (coolant temp above 160°F / 71°C).

Drive at varying speeds between 25-65 mph for at least 5 minutes, including moderate acceleration and steady cruise periods.

The monitor runs continuously during all engine operating conditions — no special drive pattern is required.

Maintain smooth, consistent driving; avoid lugging the engine at very low RPM under heavy load.

Prerequisites

Engine coolant temperature must reach normal operating range (above 160°F / 71°C).
No active crankshaft position sensor (CKP) or camshaft position sensor (CMP) DTCs present.
Battery voltage must be above 11.0 volts to ensure accurate sensor readings.

Common Failure Reasons

Worn or fouled spark plugs causing inconsistent combustion events.
Failing ignition coils or cracked spark plug boots creating intermittent spark loss.
Vacuum leaks causing lean misfires, especially at idle and light load.
Low fuel pressure from a weak fuel pump or clogged fuel filter starving individual cylinders.
Worn valve seats or burned valves causing low compression in one or more cylinders.

Pro Tips

If the misfire monitor won't set ready, use Mode $06 data to check misfire counters per cylinder — this pinpoints which cylinder is borderline even before a DTC sets.
Rough road surfaces can cause false misfire detection due to crankshaft speed variations from drivetrain vibration; some vehicles have a rough road detection strategy that suspends misfire monitoring.
On coil-on-plug systems, swap the suspect coil to a different cylinder — if the misfire follows the coil, you've confirmed the failure without buying parts.
Type A misfires (flashing MIL) require immediate attention as they indicate catalyst-damaging severity levels.

Frequently Asked Questions

What does the Misfire Monitor check?

How do I get the Misfire Monitor to set ready?

Follow the drive cycle: Start the engine and allow it to reach normal operating temperature (coolant temp above 160°F / 71°C). Drive at varying speeds between 25-65 mph for at least 5 minutes, including moderate acceleration and steady cruise periods. The monitor runs continuously during all engine operating conditions — no special drive pattern is required. Maintain smooth, consistent driving; avoid lugging the engine at very low RPM under heavy load. Estimated completion: Typically sets ready within 2-5 minutes of normal driving after engine start.

Why does the Misfire Monitor keep failing?

Common failure reasons include: Worn or fouled spark plugs causing inconsistent combustion events.; Failing ignition coils or cracked spark plug boots creating intermittent spark loss.; Vacuum leaks causing lean misfires, especially at idle and light load..

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Quick Info

Type

Continuous

Completion

Typically sets ready within 2-5 minutes of normal driving after engine start.

Resets on Clear

Yes