Published on 2026-09-14
P0420, P0300, P0455… in California, Texas or New York, just one of these active codes can fail an emissions inspection outright. Here's how ASE-certified techs diagnose them on Silverados, F-150s and Camrys.
In the U.S., geography decides the rulebook: California enforces strict CARB standards, New York and a dozen other states require an annual emissions test, while most Texas counties only require a safety inspection. The same code can mean a retest in California and go unnoticed elsewhere. These are the ones ASE-certified technicians see most, on high-highway-mileage Chevrolet Silverados, Ford F-150s and Toyota Camrys.
P0420 / P0430 – Catalyst efficiency below threshold. The number one failure reason in California and New York. A CARB-compliant replacement converter for California often costs $300-$500 USD more than a standard part — rule out the O2 sensor and exhaust leaks first.P0300-P0308 – Misfire detected. Very common on Silverado and F-150 V8s past 100,000 highway miles: worn plugs, tired coils, valve cover gaskets leaking oil into the spark plug wells.P0455 / P0442 – EVAP leak. Gas cap or purge valve, roughly $50-$400 USD depending on the model.P0401 – Insufficient EGR flow. Carbon buildup from the long highway commutes typical of American fleets.P0135 / P0141 – O2 sensor heater circuits. Common past 150,000 miles.
The ChatMecano OBD2 connector reads these in seconds, including permanent codes that stay in memory after a clear — exactly the ones that fail a CARB or New York inspection even with the light off. The AI assistant then explains likely cause, severity and the order of checks for your exact Silverado, F-150 or Camry. See AI Check Engine OBD2 analysis.
ASE tech tip: never clear codes right before an emissions test. OBD readiness monitors must reset through a full drive cycle, and a "not ready" vehicle is rejected in Texas just as fast as in California.
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