Emissions & EVAP Components
Emissions control is a reliable chunk of the ASE A8 test, and it's where 'replace the part the code names' goes wrong most often. EVAP, EGR, PCV, secondary air, and the catalytic converter each have a job, a failure mode, and a signature code. This reference explains what each system does, the trap codes (a P0420 isn't a fuel problem, a P0442 is usually a cap), and how to confirm the cause before condemning a converter.
What the emissions systems do
These systems clean up what combustion produces and capture what evaporates. EVAP traps tank vapor and burns it; EGR lowers NOx by cooling combustion; PCV recycles crankcase gases; secondary air helps the catalyst warm up; and the catalytic converter does the final cleanup, with the computer grading it via the oxygen sensors. A8 tests whether you can tie a code or symptom to the right system instead of guessing.
Component reference
| System | Job | Common failure | Signature code / symptom |
|---|---|---|---|
| EVAP | Capture + burn tank vapor | Loose cap, leak, bad purge/vent valve | P0440-series; P0442 small leak |
| EGR | Recirculate exhaust, cut NOx | Clogged passages, stuck valve | P0401 (low flow) / P0402 (high flow), pinging or rough idle |
| PCV | Vent crankcase blow-by | Stuck open/closed valve | Rough idle, oil leaks, skewed trim |
| Catalytic converter | Final exhaust cleanup | Aged/contaminated substrate | P0420 (efficiency below threshold) |
| Secondary air | Cold-start catalyst light-off | Pump or valve failure | P0410-series, cold-start emissions |
EVAP small leak vs. EGR mix-up
Two pairs techs confuse on A8 emissions:
EVAP small-leak code (P0442)
- Monitor result: system was pressurized and found a leak smaller than 0.040"
- Most common cause: loose, worn, or wrong gas cap
- Confirm with a smoke test before replacing the canister, purge valve, or vent valve
- No driveability symptom — the monitor failed, not the engine
EGR low-flow (P0401) vs. high-flow (P0402)
- P0401 = insufficient EGR flow → usually clogged passages
- P0402 = excessive EGR flow → usually stuck-open valve causing rough idle
- The repairs are opposite — read the specific code before acting
- PCV faults (stuck valve) can also cause rough idle + oil weeping + skewed trim — not an EGR code
The call: Match the code to the system and direction of the fault before ordering parts. A P0442 is not a simple gas-cap swap until you've confirmed it; a P0401 and P0402 call for opposite repairs.
The parts techs mix up
- P0420 is not a fuel code. It’s a catalyst-efficiency verdict made by comparing the upstream and downstream oxygen sensors. Confirm fuel trims are clean and the downstream sensor is good before condemning the converter — a fuel problem can mimic or cause it.
- EGR low-flow vs. high-flow. P0401 (insufficient flow) usually means clogged passages; P0402 (excessive flow) points to a stuck-open valve causing a rough idle. The repair is opposite, so read the code.
- EVAP small leak. A P0442 is most often a loose, worn, or wrong gas cap — check and smoke-test before replacing the canister or valves.
Failure patterns worth memorizing
| Symptom | Real cause | Exam trap / correct call |
|---|---|---|
| P0442 small EVAP leak, no driveability complaint | Loose, worn, or wrong gas cap (most common) | Smoke-test to confirm before replacing the canister or purge/vent valves |
| P0420, fuel trims clean | Catalyst efficiency below threshold — downstream O2 confirms | Not a fuel-control code; verify the downstream O2 before condemning the converter |
| Pinging, rough idle, P0401 | Clogged EGR passages restricting flow | P0401 = insufficient flow → clean passages, not necessarily replace the valve |
| Rough idle, oil weeping, skewed fuel trim | Stuck PCV valve | PCV fault mimics EGR; check the valve before pulling the EGR |
| P0402, rough idle at all speeds | EGR valve stuck open (excessive flow) | P0402 is opposite of P0401 — stuck open, not clogged |
ASE traps
- A P0420 is a catalyst/monitor story, judged by the downstream O2 — not a fuel-control code (see engine sensors & diagnostics).
- EGR low-flow and high-flow codes call for opposite repairs — don’t assume “replace the EGR valve.”
- A small EVAP leak is most often the gas cap; confirm with a smoke test before parts.
Work the full test through the A8 study guide and pair this with engine sensors & diagnostics for the control side.