VW Emissions Control & Environmental Compliance 2 — Questions and Answers
Question 1: Which VW-specific scan tool parameter indicates the NOx storage catalyst has reached its sulfur poisoning threshold?
- Adaptation value exceeding 100% (Correct answer)
- NOx sensor output above 200 ppm at idle
- Lambda deviation greater than 0.05
- EGR valve duty cycle below 20%
Correct answer: Adaptation value exceeding 100%
When the NSC adaptation value exceeds 100%, the ECM flags sulfur saturation and initiates a desulfation request.
Question 2: On a VW TDI with AdBlue, what is the consequence of allowing the DEF tank to run completely empty?
- Engine power is reduced by 25% and a no-restart interlock activates after the next key-off (Correct answer)
- The DPF regeneration cycle is disabled permanently
- Fuel injectors are protected by shutting down cylinders 3 and 4
- The turbocharger wastegate defaults to open, limiting boost
Correct answer: Engine power is reduced by 25% and a no-restart interlock activates after the next key-off
EPA regulations require a no-restart interlock when SCR DEF fluid is absent, and VW implements a torque reduction followed by a start-inhibit after refill confirmation.
Question 3: A technician reads an active fault P2002 (DPF efficiency below threshold) on a VW Golf TDI. Which measurement most directly confirms DPF restriction?
- Differential pressure across the DPF exceeding specification at a given exhaust mass flow (Correct answer)
- Exhaust back-pressure at the turbocharger outlet above 0.5 bar
- Fuel consumption increase greater than 10% in closed-loop operation
- Lambda sensor reading lean beyond 1.05 during active regeneration
Correct answer: Differential pressure across the DPF exceeding specification at a given exhaust mass flow
The ECM calculates DPF efficiency using the differential pressure sensor readings relative to exhaust mass flow; high differential pressure at normal flow confirms restriction.
Question 4: What is the primary purpose of the lean NOx trap (LNT) on VW GDI engines during highway driving?
- To store NOx during lean operation and reduce it during brief rich purge cycles (Correct answer)
- To oxidize HC and CO continuously using a platinum catalyst wash coat
- To cool exhaust gases before they enter the main three-way catalyst
- To recirculate exhaust gas back to the intake under steady-state cruise conditions
Correct answer: To store NOx during lean operation and reduce it during brief rich purge cycles
The LNT stores NOx as nitrates during lean operation and releases and reduces them during timed rich regeneration events.
Question 5: A VW 2.0 TSI engine fails a state I/M test for high HC at idle. The ECM shows no DTCs. Which component failure is MOST likely?
- Crankcase ventilation valve stuck open causing oil vapor bypass (Correct answer)
- MAF sensor reading 15% low causing mild enrichment
- Downstream oxygen sensor slow response
- EVAP purge valve stuck closed preventing fuel vapor combustion
Correct answer: Crankcase ventilation valve stuck open causing oil vapor bypass
A stuck-open PCV valve allows unmetered oil vapor into the intake, raising HC emissions without necessarily setting a DTC.
Question 6: Under EPA OBD-II rules, within how many driving cycles must a VW ECM illuminate the MIL for a confirmed emissions-relevant fault?
- Two consecutive failing trips (Correct answer)
- Three failing trips within one warm-up cycle
- One failing trip if the fault is safety-related
- Five trips if the monitor runs less than once per trip
Correct answer: Two consecutive failing trips
EPA regulations require the MIL to illuminate by the second consecutive failing trip for most monitored components.
Question 7: On a VW with an active carbon canister, what sensor input does the ECM use to detect an EVAP system gross leak during a natural vacuum leak detection test?
- Fuel tank pressure sensor signal after sealing the system at cold soak (Correct answer)
- Differential pressure between intake manifold and fuel tank
- MAF sensor reading during a forced idle purge event
- Downstream HO2S voltage shift when the purge valve opens
Correct answer: Fuel tank pressure sensor signal after sealing the system at cold soak
During natural vacuum EVAP testing, the ECM seals the system and monitors the fuel tank pressure sensor for a vacuum build, confirming system integrity.
VW Emissions Control & Environmental Compliance 2
Which VW-specific scan tool parameter indicates the NOx storage catalyst has reached its sulfur poisoning threshold?