Check engine codes and your Nevada smog check

A check engine code is a short label, a letter and four digits, that your car's computer stores when one of its self-checks finds a problem. The codes follow the SAE J2012 standard, so a P0420 means the same thing on every make. For the smog check, the code tells you why the lamp is on; the lamp is what decides the test.

Why the lamp decides the test

The Nevada DMV's OBD test reads the car's computer through the port under the dash. If the check engine light is on, the DMV says the vehicle fails. A flashing light is more serious: the DMV calls it a severe malfunction that can damage the catalytic converter, so slow down and get service.

Clearing the code with a scan tool right before the test does not help. The computer's self-checks reset too, and the DMV rejects the car as not ready until they finish.

How the code pages are grouped

Each code has its own page, and the pages sit in seven groups by system: the catalytic converter, the evaporative emission (EVAP) system, fuel mixture, misfire and knock, oxygen sensors, air flow and idle, and a last group for temperature, cam timing and the transmission.

Before you come in with a code

Have a repair shop read the code and find the cause. Federal law covers some expensive emission components up to 8 years or 80,000 miles, so check with the dealer about coverage or a recall first. If the light came on suddenly, the DMV says a loose gas cap is a common cause.

Testing with the light on still has a use: the failed report is the legal document that starts the repair, retest and waiver path, and the DMV does not count repair money spent before that first failed test toward a waiver.