VW Advanced Diagnostics & Troubleshooting 3 — Questions and Answers
Question 1: A VW Touareg with air suspension shows fault P0627 – Compressor relay circuit open. Relay tests good. What is the next diagnostic step?
- Test voltage at the compressor motor feed wire under load (Correct answer)
- Replace the air suspension control module
- Bleed and re-initialize the ride-height sensors
- Check for a blown fuse in the engine compartment fuse panel
Correct answer: Test voltage at the compressor motor feed wire under load
After confirming the relay is functional, voltage drop testing under load at the compressor motor connector identifies wiring resistance or ground faults that appear only under current draw.
Question 2: During an EVAP system leak test on a VW using the onboard leak detection pump (LDP), the system fails to hold pressure. Smoke testing shows no external leak. What is the most likely cause?
- Carbon canister purge valve stuck open
- Fuel cap gasket deteriorated
- Leak detection pump diaphragm cracked
- Vent solenoid not closing during the test (Correct answer)
Correct answer: Vent solenoid not closing during the test
If the vent solenoid does not close when commanded, the EVAP system remains vented to atmosphere, causing the LDP to fail to build pressure regardless of canister or cap integrity.
Question 3: A technician scopes the crankshaft position sensor signal on a 1.8 TSI and notices one tooth gap appears wider than the others. What does this indicate?
- The 60-2 reluctor wheel has a missing reference tooth — this is normal (Correct answer)
- A cracked reluctor tooth is causing signal distortion
- The sensor air gap is too large at that position
- EMI interference from the ignition system is distorting the signal
Correct answer: The 60-2 reluctor wheel has a missing reference tooth — this is normal
The 60-2 trigger wheel intentionally has two consecutive missing teeth as a reference gap to establish TDC, and this wider gap is normal on all VW 60-2 engines.
Question 4: On a VW with electromechanical power steering (EPS), the steering angle sensor requires recalibration after an alignment. Which procedure is correct?
- Drive straight ahead at 40 mph for 30 seconds to auto-calibrate
- Use VCDS Basic Settings to perform steering angle sensor zero-point calibration with wheels straight (Correct answer)
- Center the steering wheel and cycle ignition three times
- Calibration is not required; the ESC module recalibrates automatically
Correct answer: Use VCDS Basic Settings to perform steering angle sensor zero-point calibration with wheels straight
The steering angle sensor zero-point must be set via VCDS Basic Settings with the front wheels in the straight-ahead position to give the ESC and EPS modules an accurate reference.
Question 5: A VW Passat B8 presents with fault P0299 – Turbocharger underboost. Boost pressure measured at 1.4 bar, spec is 2.0 bar. N75 valve is functioning. What should be checked next?
- Intercooler for internal coolant contamination reducing airflow
- Charge air pipe for cracks or disconnected couplings under boost (Correct answer)
- Oxygen sensor for a lean shift causing ECM to limit boost
- Oil separator (PCV) for excessive pressure in the crankcase
Correct answer: Charge air pipe for cracks or disconnected couplings under boost
After confirming the wastegate solenoid is working, boost leaks from cracked charge air pipes or loose couplings are the most common cause of underboost on the MQB/B8 TSI platform.
Question 6: A scan of a VW shows fault B10D5-82 in the comfort module. The '82' sub-type indicates which type of fault?
- Signal too high / above range
- Component faulty – signal implausible (Correct answer)
- No communication / bus off
- Short circuit to ground
Correct answer: Component faulty – signal implausible
In VW/Audi fault code nomenclature, the sub-type '82' denotes an implausible signal, meaning the module receives a signal but its value is outside expected parameters relative to other sensor data.
Question 7: Which oscilloscope pattern correctly represents a healthy VW piezoelectric fuel injector during its command pulse?
- A square wave with a sharp rising edge followed by a sustained flat dwell
- A rapid voltage spike to ~150V for charging, followed by a discharge spike and ring-down (Correct answer)
- A PWM signal varying between 0-12V with increasing duty cycle
- A sine wave with amplitude proportional to injection duration
Correct answer: A rapid voltage spike to ~150V for charging, followed by a discharge spike and ring-down
Piezoelectric injectors require a high-voltage charging pulse (~150V) to expand the crystal stack and open the nozzle, followed by a discharge event when closing, producing a distinctive spike-and-ring pattern.
VW Advanced Diagnostics & Troubleshooting 3
A VW Touareg with air suspension shows fault P0627 – Compressor relay circuit open.
Relay tests good.
What is the next diagnostic step?