Emissions Control Systems Diagnosis and Repair
The control devices
Emission systems control HC, CO, and NOx. The PCV system returns crankcase vapors; a stuck-open PCV leans the mixture, a plugged one pressurizes the crankcase. The EVAP system stores and purges fuel vapor - test with a smoke machine and check the purge and vent valves, canister, and fuel cap. EGR lowers combustion temperature and NOx; insufficient flow sets P0401 and causes ping or high NOx. The AIR system adds air to the exhaust during warm-up to speed converter light-off. The catalytic converter oxidizes HC and CO and reduces NOx; the downstream O2 sensor monitors its efficiency.
Reading the evidence
Use a five-gas analyzer: high HC at idle suggests misfire or rich mixture; high CO indicates rich; high NOx points to high combustion temperature, often from a lean mixture, excessive advance, or failed EGR. O2 sensor waveforms - upstream should switch rapidly, downstream should be relatively steady. A downstream sensor that mirrors the upstream signals a failed converter.
Traps
P0420 is a converter efficiency code, but an exhaust leak or aging upstream O2 sensor can set it. Do not replace the converter without checking fuel trim and misfire history. P0455 is a large EVAP leak, P0442 a small one - check the fuel cap and filler neck first.
Rule of thumb
High NOx with normal HC and CO points to EGR or a lean condition; high HC and CO together point to a rich mixture or misfire.