About 2 months ago I replaced my factory Def cap with an aftermarket cap, no "tighten cap" warning, filled the Def tank at the same time.

Yesterday I got a P203B00 code on my 2015 tdi for the level sensor. I find it very coincidental that it stopped using def after replacing the cap.

Two questions. Would the aftermarket cap cause this issue, second if the cap isn't the issue would the level sensor cause the system to not use the def despite not getting the low def warning on the dash.

Level sensor is getting voltage

 

This car will throw out a very specific error code if the Adblue/reductant filler cap is involved, as you likely had that one showing when you replaced the OE cap with an aftermarket one. Otherwise, why would you replace that overpriced plastic cap, even with an aftermarket one?

Here's the thing: If your TDI didn't get any of its' SCR system replaced under 'dieselgate' warranty coverage, then you should expect the entire SCR system will fail piece-by-piece, bit by bit, at some future point, or perhaps starting now.

On our '15 TDI, nothing on the SCR system got replaced except for one of the NOx sensors during the settlement warranty, so we knew that the SCR system on it was gonna become an issue since it had 120k miles on car at that point. Hopefully you understand all this.

On my '10 TDI, it began having issues with SCR after 108k miles, and piece by piece it got replaced via the TDI settlement warranty; literally the entire system. Depending where you live and what, if anything in the SCR has already been replaced, you may want to consider all your options for repairing vs tuning the car.

Here's an example of the details provided via VCDS (grabbed from Ross-Tech site), and below I've linked an overview 'naked' pic of the SCR system and how it is distributed through the car. The working tank is right there by the cap you replaced, just under the wheel liner, and it has a pressure pump and sensors package internally, as well as a level sensor, which may or not be individually replaceable. If you could post up what's already been replaced on the SCR system and your car's year, mileage, that'd be a great help towards getting advice about it.

 

Here's a video detailing some of most likely causes for the P203B fault code. Some of them may surprise you, like using sub-par DEF to refill your tanks, etc.

Enjoy!

 

2015
210,000 miles

The entire scr system had been replaced, as far as I know there have been no repairs to the scr since it was done.

 

Now we know it's a 210k miles car, all is becoming much clearer.

The SCR exhaust treatment parts/system is much less reliable than the rest of the car, as it is utilizing a highly caustic fluid (DEF) to chemically react with NOx particles and render them non-harmful. In a nutshell, it's a ticking time-bomb for when it will fail, as it is full of a highly corrosive fluid that if it leaks out or oxidizes inside the system it forms large crystals, which can physically block movement, clog up the DEF injector, and/or corrode/damage wiring connections, etc., etc.

You're not really going to know the extent of it until you remove the RR wheel liner to be able to see/access the active/working tank and determine if there is leak or crystallization issue going on. Just based on the car's mileage, it could definitely be that time again, but definitely consider what DEF you added to the car most recently, and how soon the symptoms began afterwards, as a lot of owners have reported having SCR system problems after filling up with DEF from a questionable source, or just old DEF. DEF has a rather short shelf-life, so even name-brand stuff goes bad if it sits on a shelf long enough or gets handling damaged and leaks, etc.

I'm TDI tuned via Malone/Tunezilla, so simply changing my tune parameters to address this problem rather quickly/bluntly costs me nothing additional. I already have that variant stored, as I'm sure it'll come in handy at some future point. I'm also TX registered, where they just stopped doing any and all annual emissions control/smog inspections checks on passenger vehicles, not that they could do a full-time AWD TDI smog check prior to that decision anyhow, but I digress. Neither of our TDIs ever got smog-checked based on where lived and registered. Every state is different, so know what the rules are where you reside/register your vehicle.

Simply based on your TDI's mileage (210k miles), my thinking is that your SCR system is likely due for some problems as those components operate in a highly caustic environment, and any leakage whatsoever will result in wiring damage/corrosion/ faults and/or hardware damage and possibly even a physical blockage due to oxidation/ crystallization action of the reductant fluid once exposed to air, etc. That may even be what's actually creating the issue for your tank level sensor, but you won't really know until you pull back the wheel well liner to have a gander at what is going on in there, IMHO. Is it fixable? Sure, it's just parts after all, but those are specialized emissions parts, and so more expensive items in general.

There are quite a number of archival threads you can locate using that fault code, so pretty easy to research what others have gone through, and remember this same Gen 2 TDI engine in your Q7 was also used in VW T-Reg, Audi A6, Q5, etc., etc. and so the same exact stuff happens to those cars with their SCR systems...same engine/same NOx management system (SCR) from the VW AG parts bin.

Hope this helps you out. I've already future proofed my TDI knowing it'll happen sooner or later, so maybe that could work for your situation too.

Here's my loyal customer code for 10% off at Malone/Tunezilla should any of that appeal to you (good through 12/13/2025), just in case they don't have any other discount offers running currently. Around major holidays they usually do offer up to 15% off, and you would need the flashing tool to DIY this. As a bonus, for your '15 TDI, with tune purchase you receive the free option to roll your 'dieselgate' TCU tune on the 8spd tranny back to the original factory TCU tuning, which gets rid of the 'laggy' feel at take-off in D-mode on transmission, as well as the propensity for the immediate upshifts. That really adds a ton of value, IMO, and it'll show you that option the first time you connect the Flashzilla Pro flash tuner to your car, etc.

Here's that code for 10%:

FD40211-3713ZQ

Good luck!!!