The NEW Aftermarket Magazine Troubleshooter quiz – October 2026
23 September 2026 
Think you know your diagnostics? Put your fault-finding skills to the test with the Aftermarket Magazine Troubleshooter quiz. Each issue, in partnership with DiagnoseDan TSB (DDTSB), we’ll give you a real-world diagnostic challenge. Study the evidence, work out what’s really going on and tell us what you think is causing the fault.
Get the Aftermarket Troubleshooter challenge right and you could earn something money can’t buy: an Aftermarket Troubleshooter mug.
Yes, a mug. But not just any mug.
These are won, not bought and are only available to winners of the Aftermarket Troubleshooter quiz. So when it appears on your toolbox, desk or tea round, everyone in the workshop will know you’ve earned it.
And there’s more than bragging rights at stake. Each issue’s winner will also receive a six-month subscription to the DDTSB technical bulletin database, worth €230 + VAT.
This month’s challenge
A 2019 Volkswagen Touran 2.0 TDI enters the workshop with the engine management lamp illuminated and an AdBlue system warning message displayed on the instrument cluster. The customer reports that the vehicle drives normally, with no loss of power.
The technician finds:
- Fault code: P20EE00 – SCR NOx Catalytic Efficiency Below Threshold (Bank 1), stored in the ECM.
- Visual inspection shows no physical exhaust damage, external AdBlue leaks or crystallisation around the dosing injector or SCR catalyst converter.
- An AdBlue quality check confirms the correct concentration of 32.5% urea.
During an initial road test at a steady 80 km/h, live data shows:
NOx Sensor 1 (upstream): 45 ppm
NOx Sensor 2 (downstream): 72 ppm
Under full acceleration:
NOx Sensor 1: 854 ppm
NOx Sensor 2: 317 ppm
Back in the workshop, the technician wants to eliminate a leaking or malfunctioning AdBlue injector as the cause of misleading conversion data.
The AdBlue supply line to the dosing valve is temporarily clamped, and an extended overrun (engine braking) road test is carried out.
The readings are:
NOx Sensor 1 (upstream): 0 ppm
NOx Sensor 2 (downstream): fluctuates between 32 and 45 ppm
What is the most likely cause?
A) Mechanically degraded or poisoned SCR catalyst
B) Faulty downstream NOx Sensor 2 over-reading actual ppm
C) Faulty upstream NOx Sensor 1 under-reporting actual ppm
D) Crystallised or restricted AdBlue dosing injector nozzle
Think you’ve cracked it?
Submit your answer and details in the form below and if you are correct, you’ll be entered into the draw .
Entries for this issue close at 11.59pm on 31 October 2026.
All correct entries will go into the draw, and one winner will receive: The exclusive Aftermarket Troubleshooter mug – you can’t buy it; you have to earn it. A six-month subscription to the DDTSB technical bulletin database, worth €230 + VAT.
We’ll reveal the correct answer on this page shortly after entries close, together with an explanation of the diagnosis. The answer will also appear in the next issue of Aftermarket, and the winner will be contacted by email.
Competition terms
One entry per person. The competition closes at 11.59pm on 31 October 2026. One winner will be selected at random from all correct entries received before the closing date and contacted using the email address supplied with their entry. No cash alternative is available and prizes are non-transferable. The editor’s decision is final. By entering, entrants agree to their name and/or business name being published in Aftermarket and on the Aftermarket website if selected as the winner. Full competition terms and conditions are available on the Aftermarket website.