What should be checked first if a vehicle fails an emissions test?

When a vehicle fails an emissions test, the first thing that should be checked is the Malfunction Indicator Lamp (M.I.L.) status. The M.I.L., often referred to as the "check engine light," is designed to alert the driver to any issues that could affect vehicle performance or emissions.

If the M.I.L. is illuminated, it indicates that the vehicle’s onboard diagnostic system has detected a problem that can lead to increased emissions. The first step in diagnosing why the emissions test failed is to retrieve diagnostic trouble codes (DTCs) from the vehicle's computer. These codes provide valuable information about specific issues within the engine or emissions control system that need to be addressed.

By starting with the M.I.L. status, technicians can identify issues directly related to emissions failures, ensuring that repairs target the root cause of the problem rather than addressing unrelated components that might not be impacting emissions.