Last year when we brought our 2010 Q7 TDI in to the dealer they found codes both for a glow plug and a nox sensor (this was a courtesy scan they did while working on our malfunctioning headlamp). We didn't have the money for that repair at the time so we brought it back more recently. When we brought it in a couple weeks ago we had them do just the glow plug and its associated cable (they once described it as a boot).

It's been a couple weeks, and we haven't seen the check engine light come back on for the nox sensor. I'm not complaining, as it was over $600 for the glow plug and its cable, but I'm wondering if the nox sensor issue was related to the cylinder whose glow plug wasn't working?

I'll admit I am not a Diesel mechanic. My understanding of a glow plug is on par with what it posed in the entry on the same on wikipedia. It would seem that once a cylinder is heated up by combustion - and the glow plug is off - that combustion should be as close to optimal as it can get, and hence the nox sensor shouldn't be triggered. Or could it be that the nox sensor is only read when the car first starts, and then the error remains regardless of how emissions look later in the drive?

 

A NOx sensor (there are two) is really just a constant feedback loop provider...it just reads how much of the 'bad stuff' is in the exhaust stream at NOx sensor 1 (upstream of the emissions gear in exhaust), then again at NOx sensor 2 (downstream of the emissions gear in exhaust system), and that drives how frequently and for what duration the urea solution gets injected into the exhaust, amongst other things.

Same basic idea as with the O2 sensors, except these are focused on highly specific emissions molecule detoxing...so impacts EGR duty cycle and the SCR reductant system (Adblue) urea injection frequency/duration, etc. Both NOx and O2 sensors can be cleaned and using a defined process, but Audi isn't going to tell you that. Those NOx and the O2 sensors, and even the little EGT (temp) sensors for this car's exhaust are pretty pricey too.

What a bad glow plug does is screw up the NOx emissions levels produced from cold-start and beyond due to the one cylinder not being 'managed' in synch with the rest. These glow plugs don't just pre-heat the cylinder to ready it for supporting spontaneous combustion when you turn the key on; they will actually stay 'lit' at a reduced heat output level after the cold start for up to a couple minutes to stabilize the combustion process while the engine is cold. Of course the glow plugs function is depending heavily on ambient temps and other variables/measurements the ECU has to work with including barometric pressure, outside temp, fuel temp, intake air temp, etc. etc. As you might imagine, in the Winter, the glow plugs get a lot more action than during the Summer months. Your fault/error code for the NOx sensor was directly related to the emissions increase/fallout from the broken glow plug.

TDIs are fun! Spontaneous combustion happens...every time I drive my TDI. Bad luck on paying $600 for a single glow plug swap though. Might want to find you a good indie mechanic to help you out with basic parts-swap maintenance stuff like this.