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Discussion Starter · #1 ·
Hey everyone...

I've had this A3 since 101K miles. Has dieselgate warranty until 151K or 2025. Currently has 141K miles. It's with Audi now for the second time DPF Warning light that wont turn off. I'm able to do a forced DPF Regen... the car will do auto regens, but the DPF warning light stays on.

I don't think its the DPF. I think what's causing the DPF warning like to stay on is this fault code that I found:

006744 - NOx Catalyst
P1A58 - 000 - Regeneration Required - Intermittent

This is the second time this exact thing has happened. The first time was about 2 years ago. Same thing. Took it to Audi under warranty. Their solution was "drove it 90 miles... etc etc" which sounded like they did "a force NOx regen while driving." So this time, I did the same thing... I learned how to do that on the VCDS... drove it for 100 miles, but light stayed on.

Did more research online and came across a VW tsb related to this fault code. The tsb also said to do a "force NOx regen while driving", but before that it said to check the temp on MVB 46/2... while driving for 6 miles on the freeway... and if the temp drops below 87C, then replace the thermostat before attempting a force regen. So get this: It takes me about 10 miles to get to the freeway... so the car is plenty warmed up by the time I get there. But it still took 4 miles on the freeway before it finally got to 87C... and then as soon as I got off back to side streets, it went back to hovering around 85 and 86. I don't know how the thermostat impeded NOx regen, but apparently its a factor based on the tsb.

So I take it into Audi, assuming they'll follow the same tsb and find the same issue - its the thermostat, and I was afraid they would say, "that's not covered by the emmissions warranty." But to my surprise, they said, "we will rerplace the NOx converter at no charge - under warranty."

All great right?

But I am concerned my thermostat is failing. Here's why: In all my other cars... when the car gets up to operating temp (ie the coolant gauge is on center between C and H), IT STAYS THERE. No matter if I'm going 80mph, or standing still at the stop light. Not the A3. The needle will drop below center whenever the car is at a stop... and even when driving steady... it's just shy of center. And MVB 46/2 reflects this. It'll drop as low as 84C on a really cold day if I'm just at a stoplight. I haven't tested this in summer though and see how it behaves since I'm just now discovering all this. But regardless of the season... I'm thinking if the thermostat is working right... temp should be pretty steady.

But at the same time, I figured if the thermostat is going bad... I would think Audi would have identified it (if they follow the same VW tsb about this fault code).

It's great I will have a new NOx converter... and I'm sure the code will go away as soon as they replace it... but if somehow a bad thermostat negatively effect NOx converter ... I could be "ruining" this new one pre-maturely.

So my real question is... do you think my thermostat is failing? Anyone got any experience with failing thermostat on their TDI?

Thanks in advance.
 

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When was your coolant changed tested? You’re over 100k miles/ 5 years.
A thermostat is a mechanical device that will open and close throughout the engine operation to TRY and maintain the same temperature. Remember this is a mechanical device and your gauge is electrical and more accurate. Common signs of failure is the thermostat stays closed and the engine over heats. The thermostat stays open and you have no heat in the car (winter), or never get to operating temperature. Your thermostat is working because the temperature is changing. Your coolant may not have the capacity to absorb engine heat and dissipate that through the radiator (heater core too).
As far as the TSB goes, if the engine operating temperature is not hot enough, the regen cycle wouldn’t be effective or even incomplete.
Regen with a stuck open thermostat is like cooking eggs on a warm pan instead of burning them off the pan. What the regen is supposed to do to soot in the DPF, burn it off.
 

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Discussion Starter · #4 ·
Update: After calling around different dealerships, Audi confirms that a bad thermostat would not be covered by dieselgate even if it is affecting aspect of the emissions system. After the replaced the NOx converter, I told Audi to also replace the thermostat ($950).
 

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Am I understanding this correctly? In that you had this work done at a dealer for $950 and the issue is resolved?
 

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Discussion Starter · #6 · (Edited)
Yes and no.

The Audi diagnosed my P1A58 code and determined the NOx converter and replaced it. That made P1A58 code/DPF warning disappear... but thats no surprise since they would have had to reset the ECU to tell it of NOx replacement. [I havent confirmed this, but I assume its similar to a DPF replacement... theres an adaptation in VCDS to inform the ECU of a new DPF replacement, which also resets the wanting light - this I have confirmed].

But intel on the web suggests that a failing thermostat and the engine not maintaining normal operational temps can hurt the NOx (and possibly the DPF as well) because engine temp is a factor in successful regens. And the VW TSB supports this. So knowing this... and having seen for myself and how my coolant temps fluctuates... I wanted to replace the thermostat... for good measures in case the intel is right in that... a bad thermostat not only keeps regen from being successful, but it can prematurely cause NOx (and possibly DPF) to prematurely fail (ie get clogged).

I'm really surprised that Audi didn't catch the thermostat was an issue. If they had followed the same TSB, it would have clearly tested the thermostat first. It's possible that the NOx was beyond saving even if they replaced the thermostat only. And my fear was... if the NOx was the only thing that got replaced... I'd be back in the situation in soon again... but by then, my dieselgate warranty would have expired. so I decided to pay for the new thermostat now. The labor charges was $524; parts/coolant was $320, rest was taxes. I was ok with paying the $524 after reading of what a pain this repair can be. Audi was going to charge me an extra $180 to confirm my suspicions about the thermostat... I opted not to. But told the master tech all the symptoms I saw... and he agreed that the thermostat is going bad.

I'll be getting my car back this afternoon.

I'll try to update this after so many thousands of miles on the car again.

Looking back... it makes sense why this happened the first. Over the 40K miles Ive had this car... I always thought the "long time it takes to warm up" must be a diesel thing. Its very likely that when I bought this car (at 100K), the thermostat was already bad. And that's why this is the second time I've gotten the P1A58 code... and this time... it couldn't be cleared even by Audi... and likely did push the NOx over the edge. After reading other intel... others have seen the same temp behaviors as I have... and as soon as they replaced the thermostat... the behavior was gone - ie, doesn't take long to warm up; doesn't drop in temp at a stop, etc etc.
 

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Discussion Starter · #8 ·
Wow... that article looks very interesting. Ive been wanted to really understand all this DPF and NOx business every since. Can't wait to sit down and read that carefull. Thanks for the info. I'll chime back after I get to read.
 

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I have the exact same car which I got at about 42K miles. Same issue, I have taken it in about 4-5 times in the past two years for the PA158 error code. On the second visit they installed a new catalytic converter, but the DPF lights keeps on coming back on. It's frustrating because the car is in really good shape. Even though it's still under the extended emissions warranty I finally got the VCDS interface. So last time the light come on I just did a forced nox regen myself and after a 50 minute drive the light turned off. It seems like there are so many cars with that same issue, they should hand out a free diagnostic tools with them 😆. My thermostat seems to be ok, so hopefully I can get by with doing the forced regen once in a while.
 

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Discussion Starter · #10 ·
Did you check the thermostat per the TSB? Ie... "... take a reading of MVB 46/2 while driving 6 miles on the freeway and see if the temp holds at 87C."

I havent had time to re-check mine after getting the thermostat replaced, but now the temp needle on the dashboard stays dead center once the car reaches operating temp... regardless if I'm at a stop. And it's winter now here in CO, so it's very cold right now too. However... I do find it still takes a long time to "warm up" (ie get the needle to move) compared to my other, gas vehicles. While I had the 2021 A4 as a loaner... I also noticed the same thing on that car. It may just be an Audi/German thing.

I would just make sure your thermostat is still within spec. My hunch is (based on what I've read out there) repeated conditions that cause P1A58 is bad for the NOX over time. If you have a lot of mileage left on your dieselgate warranty... it may need replacement before it's up... in which case it's would work out fantastic for you.

OR... like what I initially feared... when it does fail... the dealership may find that it's the thermostat that caused it and MAY NOT cover the NOX at all! Again... not sure why Audi did not detect the thermostat on mine, but instead replaced NOX under warranty. And I have only 10K miles on my warranty.

At any rate, I would just make sure to check the thermostat per the TSB ("...drive it on the freeway for 6 miles while reading MVB 46/2") to be sure.

If it really isn't your thermostat... then I would be curious why you keep getting PA158. It's quite possible that it's simply because of your driving habits ... ie very frequient, short commutes, which is known to cause regen issues for these TDIs (in case you didn't know).

Keep us posted.
 
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