Code 56 - GM Forum - Buick, Cadillac, Chev, Olds, GMC & Pontiac chat


1992-1999 Series I L27 (1992-1994 SE,SLE, SSE) & Series II L36 (1995-1999 SE, SSE, SLE) and common problems for the Series I and II L67 (all supercharged models 92-99) Including Olds 88's, Olds LSS's and Buick Lesabres Please use General Chat for non-mechanical issues, and Performance and Brainstorming for improvements.

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Old 08-15-2005, 03:51 PM   #1
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Default Code 56

History code 56
Quad driver Module
(QDM) B Fault

Other than the engine light comming on when I let off the gas (doen't matter weather it'* in town or on the highway) it runs fine. It comes on ~ 4.5 seconds after I let my foot off of the accelerator. But once I accelerate again it turns back off, it is very repeatable. What does this mean, how can I fix it. Thanks for any help.
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Old 08-15-2005, 06:32 PM   #2
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thats kinda ironic i have the same problem and the same quad driver problem not sure what it is yet still investigating it but i will keep u updated
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Old 08-15-2005, 11:16 PM   #3
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You guys need to have someone with an FSM tell you what is controlled by that QDM and systematically determine if each item is working properly.

To give you an idea. I had a QDM code on my 95. Soleniods controlled are boost, evap purge and TCC.

Not wanting to pull the tranny I changed the charcoal canister...nope. Then I read the book and figured out how to test it. Worked fine. Boost was working good..so it must be good right? Must be the TCC... NOPE it was the boost soleniod. While working or seeming to work properly, it was feeding bad stuff back to the pcm and causing the code.
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Old 08-15-2005, 11:30 PM   #4
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Won't an EGR cause a similar symptom?
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Old 08-16-2005, 08:59 AM   #5
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Last night when I was comming home from work I noticed I was only getting about 3 pounds of boost under full throtle, but this morning comming to work it seemed to be fine, and the ses light only came on a couple time. Of course the wife says I don't need more boost than that, whatever
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Old 08-16-2005, 09:07 AM   #6
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We need a 92-93 manual to get which soleniods are controlled. Sounds like one might be boost and it might be messing up.
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Old 08-16-2005, 10:23 AM   #7
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i'll be back on tonight with the 93 manuals. I'll see what I can come up with for you.
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Old 08-16-2005, 10:41 AM   #8
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code 56 is the boost control module code.

Personally I'd hit up a junkyard and grab the another boost soleniod. I just fixed my light with one. It clearly sounds like either your having ECM problems or the BCS. The BCS is probably a cheaper first try. Plus you mentioned it doesn't seem to be functioning properly.
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Old 08-16-2005, 10:42 AM   #9
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Code 56 is NOT the QDM fault. It'* a Boost Control soloinod fault. Check your vacuum lines. Techinfo has a listing of the faults.

For QDM faults: http://www.bonnevilleclub.com/forum/...ht=quad+driver You'll notice that 26 is the QDM fault.

BCS'* don't typically fail. It'* more likely a vacuum problem to it. It'* the little box mounted to the front of the boost control actuator on the front of the SC.
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Old 08-16-2005, 03:42 PM   #10
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The little printout @ the parts store from the scan tool says exactly what I put in my first post, Code 56 Quad Driver Module, that'* the only reason I put that in my post, sooooo, is the parts store wrong or what??? I have no idea, maybe you guys can hash it out.

Thanks a bunch
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