Warning lights
Flashing check engine light: stop as soon as it is safe
A check engine light that blinks is the most urgent message the emission system can send. It means the engine is misfiring badly enough, right now, to damage the catalytic converter.
Can I keep driving with this light on?
Check engine light: flashing
Severe: stop when safe
Stop as soon as it is safe: a misfire is damaging the catalytic converter.
- Ease off the throttle and avoid towing or climbing.
- Pull over and switch off when it is safe.
- Have the car towed or driven only a short distance at light load.
40 CFR 86.1806-05(c): must show "Check Engine", "Service Engine Soon" or an engine symbol; must blink during catalyst-damaging misfire. Your owner's manual prevails for your car.
A flashing check engine light has one meaning, set by federal rule rather than by each maker. Under 40 CFR 86.1806-05, the malfunction indicator must blink at least once per second whenever misfire reaches a rate that the manufacturer has determined will damage the catalytic converter, and it may return to steady when the misfire falls below that rate. In other words, cylinders are not firing, unburned fuel is reaching the converter and burning there, and its temperature is rising far above normal. Driving on can melt the converter's internal structure within minutes to hours, turning a repair of an ignition coil or plug into one that includes a converter, and in extreme cases overheating parts under the car. Slow down, avoid hard acceleration, and pull over when it is safe. If the light keeps flashing at idle, switch off and arrange a tow. The usual causes are an ignition coil, worn spark plugs, a fuel injector, or a vacuum leak.
Coil swap test: does the misfire follow the coil?
Result
Coil
| What it means: The misfire moved with the coil: replace that ignition coil. |
Swap only with the engine off and the coil connectors clipped back. Read the per-cylinder misfire counters, or wait for the new P030x code, before concluding.
Codes, rules and labor data checked by Radif Partners · Editorial policy · Method and sources
What happens inside the converter
A catalytic converter normally runs hot, well above the boiling point of water, as it cleans the exhaust. When a cylinder misfires, its unburned fuel and air pass into the exhaust and burn in the converter. With one cylinder in four or six misfiring repeatedly, the converter can far exceed its design temperature. The ceramic honeycomb inside can crack or melt, which restricts the exhaust and later sets P0420.
The usual causes
- An ignition coil failing, often on one cylinder (P0301 to P0304).
- Worn spark plugs, which misfire under load first.
- A fuel injector stuck or clogged.
- A large vacuum leak or very lean mixture, which causes random misfire (P0300).
- Less often, low compression from a valve or head gasket.
Once the car is somewhere safe, the coil swap test on this page shows whether a coil is the cause on a single-cylinder code.
How a misfire is detected
The engine computer measures tiny changes in crankshaft speed as each cylinder fires. A cylinder that does not fire gives no push, and the crankshaft slows for a moment. The rules that apply from model year 2017, which incorporate California's requirements (40 CFR 86.1806-17), keep this logic and the duty to warn the driver at once when the misfire risks damaging the converter.
What it costs
Typical fixes are small: an ignition coil at a fraction of an hour of labor, a set of plugs at half an hour to three and a half hours depending on the engine. The estimates on the misfire code pages use the starting rate for each state. A converter damaged by driving on is a much larger bill, and a new converter on an engine that still misfires will be damaged again.