Diagnostic code p21dd

Car54 · May 7, 2019

My check engine light came on so I ran code check which showed DEF reluctant heater failure. I got a letter sometime back from GM stating that this issue was now covered under an extended warranty. My problem is my closest dealership 62 miles, told me they are booked until May 24. I saw on other forums that the truck may go into "limp mode" if I drive it? Has anyone dealt with this issue out there, I have other dealerships 250 miles away just don't want to pay a tow bill trying to get there

Hillwood · May 8, 2019

If you haven't seen the message on the DIC yet, it might not go into limp mode.. The DIC will state the message when it does.

You might want to post this on the HD section. This is the LD 1500 section.

EJCarter · May 9, 2019

I had the same P21DD code come up about three months ago. My dealer ran the check and the DEF heater / pump assembly would need to be replaced...but it was on a nationwide back order. They cleared the code and let me know they would call when the part came in (they had 10 on order as a couple other trucks had the same code and needed the same part, plus they wanted some in stock). Approx a month later, they received a few heater / pump assemblies and replaced mine.

During that month wait, my check engine light did come back on a few times. I asked the dealer about the truck going into limp mode. They said there is a chance, but it should be a rare occasion from this issue. I wasn't too confident in their answer. They also stated, like Hillwood stated above, it will come up on the DIC and give you a number of miles before going limp...I think they said like a 50 mile "warning."

The dealer also knows I have a CTS2 monitor that will run and clear codes. They said to clear it if it returns. This will keep from driving it too long with an active code (CEL), thus reducing the chance of it going into limp mode. I never had any limp issues, limp warnings, etc...and my CEL was off when I took it in for replacement. Since it's been replaced, no further CELs have come up...

I would confirm your closest dealer has the part in stock. If so, see if they can put one aside for you so you don't waste a trip. If they don't have any in stock, see if they can order some now...

It should only take about an hour or two to replace if they have a good diesel mechanic.

I wouldn't worry too much about the limp mode. It sounds like you were able to read your code ? If that's the only code you are seeing, I'd clear it. If it comes up again, confirm it's the same P21DD and clear it again...

And yes, this part is covered under warranty. GM is having so many problems with our DEF / emission system, they extended the warranty on it. I believe I heard it was 10 years /125K miles...but double check that...

markwadeewen · May 9, 2019

Mine gave me the same code yesterday. I took it into the dealer today & they put it on there scanner to confirm code. Now I am getting a message saying that I have 160 km till the 105km/hr max speed. Took it back to the dealer & was told that they cannot clear the code cause the truck needs to go into regen mode to clear & they cannot do a force regen because they need tank heater for it to happen. So now I got 160 km till Speed is reduced & they figured another 200 km at reduced speed then truck will only idle. No def tanks around so it will be over 3 weeks before they get one.
Anyone else ever have this happen? Seems weird to me that GM never thought this through & had a fix in this kind of situation.

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Car54 · May 10, 2019

Eric thanxs for the info. I will definitely call the dealership and see if they will order the heater now.

Car54 · May 17, 2019

Just some quick follow up on the P21 DD code. Dealership took care of it under warranty and they had the heater in stock! Drove my truck approx. 100 miles prior to the repair and I never got a DIC message relating to limp mode.

Erodz · April 21, 2021

I have a 2014 2500hd, I had the codes P21DD and P20B9 with a “service exhaust fluid system” warning. I replaced the heater pump my self and cleared the codes with my scanner. Ran the truck for a little while, codes haven’t come back, but the “service fluid system” light won’t go away. Any thoughts?