Check Engine Light On Before Your Smog Check? Read This

Your registration renewal says a smog check is due, and the check engine light just came on. That timing is no coincidence for a lot of drivers, and it's a problem, because in California an illuminated check engine light is an automatic smog failure. Here's what you need to know.

Why the light means a fail

The check engine light, officially the malfunction indicator lamp, comes on when the engine computer detects a problem that can affect emissions. The smog check reads that status directly. If the light is commanded on, the car fails, even if it seems to run perfectly.

The "not ready" trap

The computer also runs self-tests called readiness monitors on systems like the catalytic converter, oxygen sensors and evaporative system. If the battery was recently disconnected or replaced, or someone cleared the codes, those monitors reset. Until they complete, the car can be rejected. That's why clearing the code right before the test doesn't work. The car needs to be driven normally, usually over several days and a mix of conditions, before it's ready.

Common causes of a check engine light

• Loose or bad gas cap: triggers an evaporative leak code.
• Disconnected or cracked vacuum hose: sometimes a one-minute fix. One customer's Nissan Rogue had a hose that simply wasn't connected.
• Oxygen sensors: slow or failed sensors set fuel trim and sensor codes.
• Misfires: worn plugs or failing coils. See engine tune-up.
• Catalyst efficiency: P0420 or P0430, often with an underlying cause.
• EGR and EVAP faults: clogged passages, stuck valves, leaking hoses.
• Wiring and sensor circuits: including rodent damage. See electrical repair.

Don't trust the parts-store printout blindly

A free code read is useful information, but it's a symptom, not a diagnosis. One of our customers came in with a printout that said his oxygen sensor was bad. Arthur looked at the paperwork, said that might not be the problem, and found a disconnected hose under the hood. A real diagnosis looks at live data and tests parts before replacing them.

A smart timeline

• As soon as the light comes on, or as soon as you get your renewal notice, have the car diagnosed.
• Repair the confirmed cause.
• Drive normally for several days so the readiness monitors complete.
• Then take the smog test.

Waiting until the week your registration expires leaves no time for the monitors, so start early.

Flashing light? Act sooner

A flashing check engine light usually means a severe misfire. Unburned fuel can overheat and destroy the catalytic converter quickly, turning a modest repair into a much bigger exhaust repair. Drive gently and get it checked soon.

We'll help you pass

Walnut Smog & Repair diagnoses check engine lights, repairs the cause, and performs the smog check at our STAR Certified station in Pasadena. Call (626) 449-6252 or visit 1323 E Walnut St #2.

Book a visit at Walnut Smog