How does a fault code arise in the Emissions and Exhaust group?
The emissions group is the reason OBD-II exists: the system was designed to detect faults that could push a vehicle’s exhaust emissions above a defined threshold. Lambda (O2) sensors, catalytic converter efficiency, evaporative emission control (EVAP), exhaust gas recirculation (EGR) and the secondary air system all sit under this heading.
Most of these codes are comparisons, not direct measurements. A catalyst efficiency code, for example, does not measure the catalyst; it compares the signals of the upstream and downstream lambda sensors. If the downstream signal starts to look like the upstream one, the module concludes that the catalyst is no longer storing oxygen. So the real cause of a catalyst code is sometimes the sensor itself, and sometimes a small exhaust leak ahead of it.
Emissions codes are directly tied to roadworthiness testing, and clearing codes is especially costly here: clearing also resets the readiness monitors, and the vehicle cannot pass a test until those monitors have completed again.
Fault codes in this group
The database holds 54 codes in this group. Each code has its own page with the meaning, possible causes and diagnostic steps:
Read fault codes on your own vehicle
With an ELM327-based BLE OBD2 adapter, CarCoding reads confirmed, pending and permanent fault codes separately, pulls the freeze frame and explains every code in plain language. Clearing codes is done from the app as well. The app is free to download.
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Data source: the CarCoding fault code database. Code definitions follow the SAE J2012 standard; manufacturers may use different wording for their own codes. Everything on this page is generated from the code-specific database records.