2020 GMC Sierra 2500HD Code question

where would look for the wire that sets off P20B9 which tsb 21-na-006

21-NA-006 1…2 (nhtsa.gov)

your vehicle should still be under warranty so just visit the dealer.


You have a warranty so use it and that way you will not void the free repair by messing with it yourself.


Good advice above, best to just take it to a dealership. Otherwise you are risking a GMC claim of failing to observe your vehicle warranty terms.

If you want to do something yourself anyway, I presume your GMC is a diesel, right? Don’t you have to put something in a tank in the engine compartment from time to time, a substance used to reduce diesel emissions? If you know where that tank is located, the connector in question is likely in that same area.

Okay so the problem I have is that in the last 32 months as of right now I’ve only been able to drive it 23 of them

GM has no parts and no eta, no one rents a truck that is set-up for a goodseneck, so family trips are void as in all are other plans, I bought this truck to use not sit in the driveway,

I did locate connector and am trying to bypass a function that I don’t need going into summer, finding wire schematics has been a challenge so far

Thanks for advise

It’s not drivable with code?

I expect the truck might be put in reduced power mode w/that code.

Not sure what you mean. What is a goodseneck? Some sort of towing gadget?

This is a common covid-era complaint we get here these days, not just for GM but all vehicles. Only path forward is to investigate if there’s a way to work around it. I expect that’s exactly what you a doing with your postings here. Be cautious however to avoid voiding truck’s warranty. Might want to keep the dealership shop apprised in writing.

Here’s a few ideas:

  1. Try Googling GMC 2500 Diesel engine wiring schematics, you may get lucky and find exactly what you are looking for. Be sure to click the “image” tab too.

  2. The GMC dealership may be able to provide you a page or two of schematics of a particular section of the circuity, presumably the section for the reductant tank. Stop by when they aren’t too busy and ask. Worse they can say is “no”.

  3. Ask your local public library what resources they might have. If they have All Data, that database will have the truck’s wiring diagrams.

  4. Ask at your auto parts store if there’s a Chilton’s or Haynes repair manual for this vehicle out yet.

  5. Inquire at All Data if they offer a short term diy’er subscription just for your truck. They used to do this, not sure if they still do.

  6. GMC almost certainly offers a subscription service that provides the service info, including wiring diagrams. Ask if there’s a diy’er version, some manufacturers offer 2 day access to their service-data website for a relatively small fee.

Suggest to explain in more detail what exactly you are trying to bypass.

Did you disconnect it from the tank connector & inspect the associated wiring harness carefully on each end? Any burn marks etc?

Suggest to also study how that functions works. Make yourself an expert. Presumably a computer sends a signal to the tank to inject some reductant into the exhaust pipe, either all the time or periodically. How does that injector work? Is it just a simple solenoid that allows the reductant to flow under gravity? Or is the reductant pressurized with a pump of some kind in order that it come out as a spray? If it is just a solenoid, look for signs of burn marks near the solenoid coil. If there’s a pump involved, look for a burned pin at the electrical connector to the pump…

As mentioned already, anything you do could void your truck’s warranty.

I haven’t been able to drive my Corolla for the past 28 months for a similar covid-related problem. I understand your frustration.

Ask your local auto recycler if they have parts for this vehicle from wrecked trucks w/the same engine. They may have the exact wiring harness you require.

What has GM said? Unfortunately, you do need this in the summer, as the truck tests to make sure the different heater circuits are functioning- even if it isn’t cold out. Bypassing this system will be very difficult, and quite illegal, as it is programmed into the computer and required by the EPA.

I feel your pain, as many International trucks are having similar issues- and I have a fleet of these things that could conk out on me at anytime, and parts are no where to be found.

Have you escalated this issue and lack of repair parts to higher up the GM chain? Some newer vehicles have been granted a “pass” of sorts and a computer reprogram that will bypass some of the systems since parts are in such short supply. When parts are available, repairs will be made. I have no idea if this relates to your truck or not, but it doesn’t hurt to ask.

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