This tool provides general guidance — not professional mechanical advice. Always have a qualified mechanic inspect your vehicle before making any repair decisions.
#1 Most Likely
Check engine light on (solid)
Typical Cost
$0 – $1,500
Cost depends entirely on the fault code. Loose gas cap: free fix. Oxygen sensor replacement typically costs $434–$537 per industry repair data (2024). Catalytic converter replacement typically costs $900–$2,500 per industry repair data (2024). A mechanic code scan is the necessary first step to determine the cause. Prices vary by vehicle make and region.
Wide range because the exact repair depends on what's causing the symptom.
- Loose or missing gas cap (EVAP system leak)
- Oxygen sensor reading out of range
- Catalytic converter below efficiency threshold
- Mass airflow sensor or throttle position sensor fault
- Misfiring cylinder from a worn spark plug or ignition coil
A solid (non-flashing) check engine light indicates the engine control module has detected a fault and stored a fault code. The cause can range from a loose gas cap to an oxygen sensor, catalytic converter, or emissions system failure. Because the light is solid rather than flashing, immediate engine shutdown is not typically required — but the vehicle commonly warrants inspection soon to identify and address the underlying fault.
What a mechanic will check
A mechanic will likely connect a scan tool to read stored fault codes, inspect the relevant system flagged by the code (oxygen sensor, evap system, ignition system, etc.), and may perform component-specific tests based on the code. They will also check for any secondary symptoms that narrow the likely cause.
What to say to your mechanic
“My check engine light has been on solid for a few days. Can you read the fault codes and tell me what system is flagged and what the most likely causes may be? I want to understand the issue before we talk about repairs.”
Script based on industry repair procedures
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