Published on 2026-09-14
P0420, P0171, P0300, P0455, P0401… the OBD2 codes most often pulled on U.S. vehicles, with likely causes and priority checks.
In the United States, emissions testing makes certain codes especially costly: one active DTC can fail a smog check outright. Here are the Check Engine codes most frequently read on U.S. vehicles, including large SUVs and pickups.
P0420 / P0430 – Catalyst efficiency below threshold (Bank 1 / Bank 2). Number one in the U.S., and a direct emissions-test failure. Check O2 sensors and upstream exhaust leaks first.P0171 / P0174 – System too lean. Intake vacuum leak, dirty MAF, failing PCV valve.P0300 / P0301–P0308 – Misfires. Very common on high-mileage V6 and V8 engines: plugs, coils, valve cover gaskets flooding the plug wells.P0455 / P0442 – Large or small EVAP leak. Often the gas cap, otherwise the purge valve — the number-one part on many GM models.P0401 – EGR insufficient flow. Carbon-clogged EGR passages.P0128 – Thermostat stuck open.P0141 – Downstream O2 sensor heater circuit.
The ChatMecano OBD2 connector reads these in seconds, including permanent codes that stay in memory after a clear — exactly the ones that fail an inspection even with the light off. The AI assistant then explains likely cause, severity and the order of checks. See AI Check Engine OBD2 analysis.
Tip: never clear codes right before an inspection. OBD readiness monitors must reset through a drive cycle, and a "not ready" vehicle is rejected just like one with the light on.
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