A8 emissions control, system by system

Incorrect: A dirty air filter restricts airflow but adds no oil or fuel residue to the valves.

Correct: Crankcase oil vapor reaches the intake valves through the PCV system; a faulty valve or oil separator lets in excess oil that bakes into carbon.

Incorrect: On port injection the fuel spray washes over the intake valves, so the injectors are not where valve carbon starts.

Incorrect: Spark plug condition shows how combustion is going, but plugs do not put deposits on the intake valves.

Incorrect: A faint hiss of air through the valve at idle is normal flow.

Incorrect: A clean, dry valve is a good sign, not a reason to replace it.

Correct: A healthy PCV valve has a plunger that moves freely and rattles when shaken; no rattle means it is stuck with sludge and should be replaced.

Incorrect: A loose grommet should be fixed to stop a vacuum leak, but it says nothing about whether the valve works.

Incorrect: A faulty position sensor could misreport, but the idle-only misfires on several cylinders show exhaust gas really is flowing.

Correct: The valve reads 15% open with 0% commanded, and exhaust gas at idle dilutes the mixture enough to misfire several cylinders, only at idle where EGR is never wanted.

Incorrect: A valve stuck closed gives the clean idle you want; its symptoms appear under load as knock or high NOx.

Incorrect: A clogged EGR passage blocks flow, causing pinging or high NOx under load, not idle misfires.

Incorrect: An intake manifold vacuum leak would explain the trim and rough idle, but not the EGR position reading 15% with the valve commanded closed.

Correct: The valve opens on command and holds vacuum, yet nothing changes; a clogged EGR passage is keeping exhaust gas from reaching the intake.

Incorrect: The valve has just shown that it holds vacuum and moves, so the valve itself is working.

Incorrect: The diaphragm held vacuum and the valve opened, so the vacuum line is doing its job.

Incorrect: A position sensor only reports valve movement; with the valve opening, gas would still flow if the passages were clear.

Correct: Technician A is right: too little EGR flow lets combustion run hot and form NOx. Technician B is wrong, because a clogged PCV valve affects crankcase pressure and oil vapor, not combustion temperature.

Incorrect: Technician B's clogged PCV valve leads to oil leaks and sludge, not high NOx, and this choice drops Technician A's valid EGR point.

Incorrect: Both cannot be right: only the EGR fault raises NOx, while the clogged PCV valve affects crankcase ventilation.

Incorrect: Choosing neither would need Technician A to be wrong, yet an EGR valve that fails to flow is a leading cause of high NOx.

Incorrect: A downstream sensor shorted to battery voltage would sit high and flat, not switch between 150 and 800 mV.

Correct: A healthy catalyst keeps the rear sensor fairly steady; here it swings almost as often as the front one, 7 against 8 cross-counts, so the converter no longer stores oxygen.

Incorrect: The upstream sensor makes eight full swings in 10 seconds, so it is responding normally.

Incorrect: An exhaust leak ahead of the converter would skew readings lean; it would not make the rear sensor copy the front sensor's switching.

Incorrect: An engine running continuously rich would hold both sensors high, but both are switching across the full range.

Correct: 250 misfires on cylinder 3 send raw fuel into the converter, where it burns and overheats the substrate; this misfire-induced catalyst damage is why the rear sensor now mirrors the front.

Incorrect: Calling it normal aging ignores 250 misfires on cylinder 3; a new converter fitted without fixing them would fail the same way.

Incorrect: A biased downstream sensor would read off-center, not switch in step with the upstream sensor.

Incorrect: An exhaust leak would show as lean readings, and the freeze frame already points to a heavy cylinder 3 misfire.

Incorrect: Nothing in the data supports soot from contaminated fuel; the misfire count is the documented cause.

Correct: Secondary air feeds fresh air into the exhaust right after a cold start, burning off HC and CO and bringing the catalyst up to temperature faster.

Incorrect: Air is pumped into the exhaust, not the cylinders, so it adds no engine power.

Incorrect: The added oxygen burns with the hot exhaust and heats the catalytic converter; cooling it is not the goal.

Incorrect: Running the air pump costs a little energy and does nothing for fuel economy.

Incorrect: A pressure gauge can confirm the system loses pressure, but it gives no location.

Incorrect: A visual inspection finds obvious damage, but small EVAP leaks stay invisible without smoke.

Incorrect: The scan tool reads the code and can seal the system for a test, but it cannot show where the leak is.

Correct: A smoke machine fills the sealed EVAP system with visible vapor that escapes at the leak, showing exactly where it is.

Incorrect: Technician A's stuck-closed purge valve is a cause, but so is Technician B's clogged canister, so this choice is incomplete.

Incorrect: Technician B's clogged canister restricts purge flow, but Technician A's stuck-closed purge valve does too, so this choice is incomplete.

Correct: Both technicians are right: P0441 means the PCM saw the wrong purge flow, and a purge valve that never opens or a canister too clogged to pass vapor each block it.

Incorrect: Choosing neither ignores that each technician names a way to block purge flow, which is what P0441 detects.

Incorrect: Technician A's restricted fuel filter affects fuel delivery to the engine, not the sealed vapor system the EVAP monitor tests.

Correct: Technician B is right: a disconnected canister vent hose leaves a large opening, so the system cannot hold its test vacuum. Technician A is wrong, because a restricted fuel filter sits on the liquid fuel side, outside the vapor system.

Incorrect: Both cannot be right: only the vent hose opens the vapor system, while a fuel filter is outside it.

Incorrect: Choosing neither ignores Technician B's disconnected vent hose, a classic cause of a large leak code.

Incorrect: Steady cruise is one of the conditions many EVAP tests look for, so it supports the monitor running rather than stopping it.

Incorrect: Normal battery voltage is inside every monitor's enable range.

Incorrect: A tight, seated fuel cap is what the test needs; it does not block the monitor.

Correct: EVAP leak tests run only inside the manufacturer's fuel level window; a nearly full tank leaves too little vapor space for a valid test, so the monitor waits.