Signs you need check engine light diagnostics
- Check engine light is on steady
- Check engine light is flashing
- Engine runs rough or shakes at idle
- Loss of power or hesitation when you accelerate
- Worse fuel economy than usual
- Smell of fuel or rotten eggs from the exhaust
- Vehicle failed an emissions test
- Other warning lights came on with it
- Engine stalls or idles unevenly
If the check engine light is flashing, the engine is misfiring and can overheat the catalytic converter, so avoid driving and have it looked at where it sits. Pull over right away if the light comes on with overheating, low oil pressure, smoke or a strong fuel smell.
Common causes
Loose or faulty gas capA cap that does not seal can set an evaporative emissions code. It is one of the simplest causes, but it still needs to be confirmed.
Ignition misfireWorn spark plugs, a failing ignition coil or a bad plug wire can cause a misfire. A flashing light often means an active misfire that can damage the catalytic converter.
Oxygen sensor faultOxygen sensors report how rich or lean the engine is running. A slow or failed sensor can affect fuel economy and emissions.
Mass airflow sensor problemA dirty or failing airflow sensor gives the computer bad information about incoming air, which can cause hesitation and rough running.
Vacuum or intake leakA cracked hose or leaking gasket lets in unmetered air and can set lean codes. Leaks are often found with a smoke test.
Catalytic converter efficiencyA converter code can mean the converter is failing, but it can also be caused by a sensor or an engine problem upstream, so testing matters.
EGR or emissions valve faultStuck or clogged exhaust gas recirculation and purge valves are common sources of codes on many vehicles.
Wiring or connector damageCorroded, chafed or rodent-chewed wiring can make a good sensor look bad. Checking the circuit prevents replacing parts that are not broken.
What gets done where it's parked, and what needs a shop
Usually done on site
- Scanning stored, pending and permanent trouble codes
- Reviewing freeze frame and live sensor data
- Visual inspection of hoses, wiring and connectors
- Smoke testing for vacuum and evaporative leaks
- Testing ignition coils, spark plugs and fuel trim
- Replacing many sensors, coils, plugs and valves
- Clearing codes after repair and confirming the fix
- Checking for open recalls or service bulletins related to the code
May need a shop or lift
- Exhaust or catalytic converter work that needs the car raised on a lift
- Internal engine faults such as low compression that need teardown
- Programming that requires dealer-only software or a subscription the mechanic does not have
- Faults that only appear on a long road test or under heavy load
Every mechanic's setup is different. The one who accepts your request will tell you if your job needs a shop.
Before the mechanic arrives
- Note when the light came on and what the vehicle was doing at the time
- Write down any changes in how it drives, sounds or smells
- Park on a flat, open spot with room to open the hood
- Have your VIN, year, make, model and engine size ready
- Keep any recent repair receipts or previous scan results handy
- Do not clear the codes yourself before the mechanic arrives
Photos of warning lights, leaks or damage help the mechanic bring the right parts. You can add up to five with your request.
Questions to ask when you get a quote
- Is the diagnostic fee separate from the repair, or applied to it?
- What did the codes and live data show, and what did you test to confirm the cause?
- Is this code a root cause or a symptom of something else?
- Is it safe to keep driving until the repair is done?
- Can the repair be done here, or will it need a shop?
- What parts are you quoting, and are they new, remanufactured or used?
Trackara never handles payment for the work. You agree the price with the mechanic before they start, and you pay them directly. How to read a repair quote.
Vehicles this applies to
Check engine light diagnostics: questions and answers
If the light is steady and the vehicle drives normally, it is usually safe to drive for a short time while you arrange a diagnosis. If the light is flashing, or it comes with overheating, low oil pressure or a loss of power, stop driving and have it checked where it is parked.
No. A code tells you which system reported a problem, not always which part failed. A proper diagnosis uses the code as a starting point, then looks at live data and tests the circuit or component before recommending a repair.
Many can. Mechanics list the vehicle types they work on, and some carry the heavy duty or diesel scan tools those engines need. When you send a request, include the engine type so the right mechanic sees it.
Clearing a code only turns the light off until the computer sees the fault again. It also erases data the mechanic could use. Leave the codes stored so they can be read and tested.
Send a free repair request with your vehicle details, what you have noticed and a photo of the dash if you can. A local mobile mechanic who takes the job will quote before starting, and you pay the mechanic directly.