What type of failure can trigger a retest requirement?

A retest requirement is triggered by any failure related to emissions systems. This is because the core purpose of a smog check is to ensure that a vehicle complies with California's stringent emissions standards. If any component of the emissions control systems, such as the catalytic converter, oxygen sensors, or exhaust gas recirculation system, fails during the inspection, it indicates that the vehicle may release harmful pollutants beyond allowable limits.

When an emissions system fails, it often necessitates repairs before the vehicle can be retested to confirm compliance. Thus, it directly affects the vehicle's eligibility to pass the smog check, prompting the need for a retest once appropriate corrective actions are taken.

Other issues like a light bulb failure or using low-quality fuel, while they might affect vehicle performance or driving safety, do not directly relate to the vehicle's emissions systems and, therefore, would not trigger a retest requirement. Identifying a problem without performing the necessary tests also does not meet the criteria for retesting, as it lacks an actual failure confirmation related to emissions systems.