GM P0633 - Mobile Tech Story

Robert Mobile Technician Delaware Posted   Latest  
Case Study
2007 Chevrolet Silverado 2500 HD Classic LS 6.6L (D LBZ) 4-spd (4L80-E)
P0633 — Immobilizer Key Not Programmed - ECM/PCM
Crank / No Start

As a mobile diagnostic / module programming tech I tend to get some jobs from time to time that have me second-guessing myself. I'm posting this as a tech tip because I don't want any one of you to fall into this trap. 

So I got a call from a shop to program a new powertrain control module on an 07 Silverado 2500 6.6 Duramax. This is a job that I have done many times in the past and is a pretty simple one. 

  • Load the software after correcting the VIN
  • Perform a vehicle theft deterrent relearn

Now if the vehicle fails to start and has a theft light on, the VTD relearn is just a hand shake between the security module and the new PCM. It doesn't require all of the keys to the vehicle, etc, etc. 

So I get it all done and the vehicle cranks but won't start. The security light comes on as it should for bulb check then goes out. I now realize that they must have misdiagnosed it so I scanned the vehicle again to see if I can give them a quick tip to get them pointed in the right direction. I only had a "P0633 - Immobilizer Key not Programmed ".

I have seen this before on these C/K Chassis after a new PCM is installed. The fault will be there as a current fault and cannot be cleared. It only goes to a history code that can be cleared with a scan tool after a successful engine start. GM document PIT4660B explains this in great detail. 

To paraphrase from that document: "You may experience a vehicle that has a P0633 DTC set in the ECM. Also, the Tech 2 will not be able to clear the DTC. DTC P0633 will set as an indicator that the ECM is in learn mode and not as a fault indicator. Any malfunction that causes a no crank condition will prevent DTC P0633 from clearing. DTC P0633 will clear upon a successful engine crank and start."

So at this point I go to pickup my check and ask the service advisor why the PCM was replaced. He said it was towed in for a no start and had generator L terminal faults. I told him that he still has a no start and needs to diagnose farther. The tech working on the vehicle had already went home for the day so I could not explain to him why there was a security fault in the PCM. I explained it to the service advisor as I did not want the tech to think that the P0633 fault was related to his no start. 

So fast forward about 15 hours later I get a phone call from the tech telling me: "his scan told him that the truck is not starting because I didn't program the keys". I referred him to the PI Document from GM and how the light will go out once it starts. He said ok and hung up. 

In the interest of saving time I'll skip the details but I got 4 more calls from them that day basically telling me the same thing. "You need to come back and program this thing correctly" was basically what every convo was about. 

I try to be very polite in these situations, however, sometimes it's frustrating when you try to address the situation as professional as possible but they still can't understand that they misdiagnosed it. Honestly, they had me starting to second guess myself so I called on two of my good tech friends: John Rogers and Michael Burmester. Both agreed that I was correct and that the code would clear itself or go into history once it's started. 

So I called them back and told them that I would not be back unless it was to diagnose the vehicle and that they would be paying my diagnostic fee. As I was talking to them I had an idea. I told them to hit it with a little shot of ether so that it would start and possibly see if the code went away. 

They tried it and said that the code is still there. So, 0700 the next day I show up, hook up the Tech 2 and find a P0633 fault as passed on this ignition cycle. I cleared the fault, (which could not be cleared before the engine was ran on ether) cycled the ignition, cranked it, rechecked and there's no presence of DTC P0633. The tech did not tried to clear the code after starting it. He was too concerned with the code and how all of the data in his tool said that the keys aren't programmed. 

So I stood back and asked the tech to recheck it for codes. He found no DTC's present yet he still had a crank, no-start condition. I didn't tell him yet that I already noticed the fuel rail pressure live value pid was sitting at only 124psi while cranking with the desired > 5,000psi. 

I pulled a wiring diagram and started my diagnostics by verifying the supply pump was working. Right away I found power going to the pump but no continuity to ground on terminal 87 which feeds b+ to the pump. I went right to the pump and found an open in the windings of the pump motor. 

A new lift pump fixed this one but this job taught me a lot. See I never really paid much attention to exactly how or when the P0633 code went away. It's usually not a big deal because the vehicle starts when we are done programming. That is if they diagnosed it correctly. 

Have a great weekend gentlemen, and always remember to Dream big, work hard, stay focused, and surround yourself with good people. 

Maurice Business Development Manager

Thanks Robert it just show you the ignorance that some auto techs have, they all want to blame the last person who worked on the car and are not willing to admit that they were wrong.

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Dean Mobile Technician
New York

So true, sad but true.

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Terry Diagnostician

Great write up Robert! I too have been where you were at! we do diagnostics and reprogramming for some local shops in the Pensacol, Fl area and have seen the misdiagnose a vehicle as a PCM issue, and either tow the vehicle to our shop or I go to their shop and program the PCM, only to realize it didn’t fix the no start condition! They automatically assume its “your incorrect programming

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Eric Mobile Technician


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Marlin Technician

Great info. Thank you. Just to clarify, though. Why does this vehicle have an electric lift pump? They do not come from the factory that way, so it must be a special up-fitter add-on. Your post could send people looking for a lift pump that doesn't exist.

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Robert Mobile Technician

Thank you for pointing that out. I suppose it could have been an add on. I didn't stick around for the repair. I guess my post was more focusing on the fact that you shouldn't try to diagnose a p0633 code if the pcm is new and the vehicle will not start.

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Martin Instructor
British Columbia

Hi Robert. I understand the P0633 rationale, but other than Express/Savana models and newer L5P models, Duramax 6.6L pickups never used a factory installed lift pump for fuel supply from the main fuel tank. Is it perhaps a truck with dual fuel tanks and the rear tank was not pumping fuel to the front tank, resulting in a loss of fuel available from the front tank? That is the only fuel pump on

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David Technician

Thanks for sharing that Rob, sometimes it's easy to get tunnel vision, I am very guilty of that.

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Florin Technician

Great detailed information that can be very helpful not necessary with this particular code but generally with the principle of diagnosing or performing a procedure based on another technician's diagnostic. We have been thru similar situations doing diagnostics for other shops before and it is not funny when you try to explain yourself to a tech who is sure he is right. Thank you for the posting.

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