Ignition or misfire fault

• Spark plugs worn past their gap spec, which is normal at 60,000 to 100,000 miles
• An ignition coil breaking down under heat and load, often intermittent before it is constant
• A fuel injector clogged or leaking, which shows up as a rich or lean trim on that bank
• Carbon buildup on direct-injection intake valves restricting airflow into one cylinder
• Mechanical loss — a burnt exhaust valve or a failed head gasket — which compression testing finds

• Note whether the light is steady or flashing; flashing means raw fuel is reaching the catalytic converter
• Note when it happens: cold start only, under acceleration, or at idle
• Look for a service record — if plugs have never been replaced past 80,000 miles, start there
• Smell the exhaust for a strong fuel odor, which points at an injector or a very rich mix

• Misfire counts per cylinder read live on a scan tool, not just the stored code
• Freeze-frame data showing engine load, rpm and coolant temperature when the fault set
• A coil or injector swap test, moving the suspect part to another cylinder to see if the misfire follows
• Fuel trim values on both banks, which separate an ignition fault from a fuel or air problem
• A compression or leak-down test on the affected cylinder before any expensive repair

Repair costs vary by vehicle, location, labor rate, and parts. Treat every range as a starting point for a conversation, not a quote.

Estimate your repair cost

• All coils and all plugs replaced when the data named one cylinder
• A catalytic converter quoted before the misfire causing it is fixed
• A fuel-system cleaning sold as a misfire repair
• Any repair over $400 without a compression number on a cylinder that keeps misfiring

• What did you find, and what test confirmed it?
• Is this a safety item, or can it wait?
• What happens if I wait a month?
• Which part actually needs replacing, and why that one?
• Can you show me the failed component or the measurement?
• Was anything else damaged by this?
• What is the diagnostic fee, and does it come off the repair?
• Is any of this quote optional or recommended rather than required?

Can I keep driving with a misfire?

Short distances with a steady light, yes. With a flashing light, no — you are actively destroying the catalytic converter, which is the single most expensive emissions part on the car.

Will a misfire fail a California smog check?

Yes. An illuminated check engine light is an automatic failure at a California smog station regardless of what the tailpipe reads, and a stored misfire code will not clear itself just from driving.

Do I need premium plugs?

Use what the manufacturer specifies — iridium where iridium was fitted, correct heat range, correct gap. There is no performance gain from an upgraded plug in a stock engine, and the wrong heat range causes the exact problem you are trying to fix.

• Vacuum or intake leakAir sneaks into the engine after the sensor that measures it, so the computer's fuel math is wrong — worst at idle.
• Heat-sensitive sensor failureA crankshaft or camshaft position sensor that works cold and drops out once under-hood temperature climbs.
• Fuel delivery and fuel pump faultsA weak pump, clogged filter or failing injector that cannot keep up under load — and often leaks vapor as it ages.
• Rich running fuel mixtureThe engine burns more fuel than it can use — black smoke, poor economy, and a catalytic converter on borrowed time.