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Why is my check engine light on?
Read it. Fix it.
Fits North American (US and Canada) vehicles with OBD-II. Not VIN-matched.
OBD-II guide for US and Canadian cars: read the code, prove the cause before you buy, fix gas caps, MAF, vacuum leaks, O2 sensors, misfires.
Free on this page. Match the row, then stop. Illustrated steps, torque and the job card stay in the PDF.
What you see Light FLASHING, engine shaking
Points to Active misfire
What you see Steady light after filling up
Points to Gas cap loose, worn or missing (EVAP leak)
What you see P0171 / P0174, rough idle, hesitation
Points to Running lean: vacuum leak or dirty MAF
What you see P0101–P0103, hesitation, poor mileage
Points to MAF sensor dirty or failing
What you see P0130–P0167 (sensor or heater circuit, e.g. P0135, P0141)
Points to Oxygen (O2) sensor or its heater circuit
What you see P0420 / P0430
Points to Catalyst efficiency below threshold
What you see P0128, weak heater, temperature gauge reads low
Points to Thermostat stuck open
What you see EGR, transmission, U (network) or other shop codes
Points to Systems that need maker data or tools
What you see Light on but no code, or monitors 'not ready'
Points to Fault in a module a basic reader can't see, or monitors reset by a code clear or battery disconnect
What you see Light won't stay off
Points to The fault is still there, or a new one
What you see Light came on, then went off
Points to Intermittent fault
What you see ABS or traction light, not the engine light
Points to Different system (a common cause is a wheel speed sensor)
What you see Red oil or temperature warning
Points to Low oil pressure or overheating
General information for the vehicles this guide covers. You do the work at your own risk; follow the vehicle maker's service information and the emissions and roadworthiness rules where your vehicle is registered.
A 30–60 second clip from the Repair Guider channel. The illustrated steps stay in the PDF.
Four previews. Torque specs and the printable job card stay in the PDF.
Read the code.
Job card
Is this your problem? This guide is for you if you're asking:
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A steady check engine light means the on-board diagnostics stored a fault, typically emissions-related; a flashing light means an active misfire that can damage the catalytic converter, so ease off and pull over safely. Read every code and the freeze frame with an OBD-II scanner, confirm the cause with a test, fix it, then clear the code and drive until the readiness monitors complete.
Tools have exact part numbers; wear parts are matched by VIN or the old part. Borrow what you can.
General information for the vehicles this guide covers. You do the work at your own risk; follow the vehicle maker's service information and the emissions and roadworthiness rules where your vehicle is registered.
Chapters that say when to stop:
Illustrated steps, torque specs and the one-page job card are in the download.
I treat a steady light as get it checked soon. A flashing light means a misfire that can damage the catalytic converter: ease off, pull over safely and get it checked promptly.
Yes. A loose, damaged or missing cap can cause an EVAP leak code. Tighten it until it clicks; the light can take several days of driving to go out.
No. O2 sensors, exhaust leaks and misfires can also cause it. Rule those out and have the converter tested before buying one.
No. It can clear codes and reset readiness monitors but fixes nothing, and it can erase learned settings. Fix the cause, then clear with a scanner.
General information for the vehicles this guide covers. You do the work at your own risk; follow the vehicle maker's service information and the emissions and roadworthiness rules where your vehicle is registered. Car guide terms · Edition 5.0 · 2026-10-07
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