Published on 2026-09-14
P0420, P0300, P0455, P0401 — these are the codes U.S. techs pull constantly, and the ones most likely to fail a California CARB, Texas or New York emissions inspection.
Get a car through a California CARB smog check, a New York emissions test, or a Texas safety inspection with even one active DTC, and it's often an automatic fail. That reality shapes which codes matter most in American shops. Below are the ones techs pull constantly, from compact sedans like the Toyota Camry to full-size trucks like the Ford F-150 and Chevy Silverado.
P0420 / P0430 – Catalyst efficiency below threshold (Bank 1 / Bank 2). The single most common failure reason at inspection stations nationwide, and the most expensive: a CARB-legal converter for a California-registered vehicle can run $300-$500 more than a standard part. Rule out a weak O2 sensor or an upstream exhaust leak before condemning the converter itself.P0300-P0308 – Misfire detected. Extremely common on high-mileage Silverado and F-150 V8s that spend most of their life on the interstate, particularly when valve cover gaskets start leaking oil into the spark plug wells.P0455 / P0442 – EVAP leak, large or small. Start with the gas cap; if that's fine, the purge valve is usually next.P0401 – Insufficient EGR flow. Carbon buildup clogging the EGR passages after years of highway mileage.P0135 / P0141 – O2 sensor heater circuits. Common on vehicles past 150,000 miles.
The ChatMecano OBD2 connector reads all of these in seconds, including the permanent codes that stay stored even after a code clear — the exact codes that cause a re-fail at the test station even with the light off. The AI assistant explains the likely cause, how urgent it is, and the order to check things for that specific year, make and model. See how AI-powered Check Engine analysis works.
One tip from ASE-certified techs that saves customers money: never clear codes right before a CARB smog check or a New York emissions test. The readiness monitors need a complete drive cycle to reset, and a vehicle flagged "not ready" fails just as fast as one with the light on.
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