How to Fix a Check Engine Light Before a Smog Check
Test-only guidance: pull codes, get repairs at a licensed shop, complete readiness monitors, then test.
Why a Lit MIL Can Trip You Up
Your check engine light (MIL) is a major warning during a California Smog Check. While the state doesn’t print “MIL = automatic fail,” an illuminated MIL with stored trouble codes usually prevents a pass.
Important: if your OBD readiness monitors aren’t complete, our testing equipment will return a “Not Ready” result (not a fail). You’ll need to complete monitors first, then retest to get an official pass/fail.
Step 1 — Pull the Codes (We Share, You or Your Mechanic Can Too)
Find out why the light is on before anything else.
Code Read: We can scan and share your diagnostic trouble codes (DTCs) with you. Your mechanic will also be able to pull the same codes.
Test-Only Policy: By state law we do not diagnose or repair. We provide testing and code information so you can take it to a licensed repair facility.
Don’t just clear codes: clearing resets OBD readiness and leads to a “Not Ready” test result instead of a pass.
Step 2 — Get Repairs at a Licensed Shop
Once you know the fault, have a trusted mechanic repair it.
Use OEM or CARB-approved parts: Non-approved emissions parts can prevent a pass.
Ask for a pre-check: have the shop verify the MIL is off and there are no current or pending DTCs.
Step 3 — Complete a Drive Cycle (Set Readiness)
After repairs or a battery disconnect, your vehicle must re-run its self-tests (“readiness monitors”).
Follow the manufacturer’s drive cycle if available (mix of cold start, city, and highway).
General rule: several trips totaling ~150–200 miles helps many vehicles set all monitors. Diesels or certain systems may take longer.