# Dreaded DEF Countdown



## revjpeterson (Oct 2, 2013)

When the DEF warning came on, did any new codes come up along with the P21AE? The DPMS wouldn't be related to the DEF warning, so if there aren't codes indicating a problem with the DEF system, my first stop would be the DEF injector (since it doesn't set codes of its own when it fails/clogs, just results in increased NOX2 readings), then next stop would be to see if NOX2 is misbehaving, but not completely failed to set a code.


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## MP81 (Jul 20, 2015)

If there's a DPF sensor code, that's potetially the issue...and is also free under special coverage warranty.

The NOx2, though, if reading high enough, will set the countdown, as we experienced earlier this year.


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## SethMarshallAllen (Oct 16, 2014)

So, @revjpeterson no other codes. The DEF light came on about 48 hours after I first saw the P21AE. No codes aside from that one are present or pending. I don't (unfortunately) have a clue what the correct NOX sensor readings are so even if I could see it there'd be no point in me trying without knowing the variable for which I am looking. My dealer and I have a good relationship and they service three CTDs so I trust their word. Fortunately all these parts are relatively inexpensive to repair/replace and are (mostly) readily available. 

As an aside, my MPG has been steadily decreasing lately. I used to see about 41mpg on average and in the last few months it's only been about 36mpg. It also does spend a decent amount of time idling with no load on the engine. I'd say about 1 hour per day is idle time for this car.


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## SethMarshallAllen (Oct 16, 2014)

Update: car is sitting at the dealer now; has not been moved since I dropped it off there yesterday morning. I am driving the family “backup car” now, and will have a loaner on Tuesday first thing. Gave them my new Bilstien B6 front struts (finally off backorder) to install and they’ll be doing the timing belt and tensioner as well. I am myself fairly confident that it is the rear Nox Sensor or the DEF injector, but we’ll see.


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## sailurman (Sep 29, 2016)

Have you already done your water pump? If Timing Belt and tensioner are coming off, the water pump should be right there with them.


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## SethMarshallAllen (Oct 16, 2014)

Water pump was replaced under warranty last year. They didn’t reccomend the belt because the car had 85k miles and I wanted to wait.


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## SethMarshallAllen (Oct 16, 2014)

Well guys. Ready for this? It was the differential pressure sensor. There was a P2453 code I wasn’t expecting. But, hey. It’s fixed now.


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## boraz (Aug 29, 2013)

hour a day of idling?

get used to goin to the dealer


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## SethMarshallAllen (Oct 16, 2014)

boraz said:


> hour a day of idling?
> 
> get used to goin to the dealer


Perhaps that wasn't quite right. Surely it's about 25-30 minutes a day. Maybe not even every single day.


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## MP81 (Jul 20, 2015)

All in a row, or just in total for the day?


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## SethMarshallAllen (Oct 16, 2014)

That’s my total for the day.


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## Rivergoer (Mar 30, 2017)

Idling and traffic to the CTD is like the Grinch to Christmas...get ‘er out on the highway and she’ll be a happier camper.


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## MP81 (Jul 20, 2015)

SethMarshallAllen said:


> That’s my total for the day.


Oh, that's not that bad, if it's for the whole day. Ours is probably similar, maybe a touch less, depending on traffic.


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