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P0700 and P0872 Fault Codes on 2004 GMC Duramax with Allison Transmission - Diagnosis and Solutions

Model: 04 GMC Duramax Fault Code: P0700 Posted: 2014-06-16 13:49

I'm experiencing trouble codes P0700 and P08702 on my 2004 GMC Duramax diesel truck equipped with an Allison transmission. The issue started recently, and I'm looking for guidance on where to begin troubleshooting the problem. P0700 is a generic OBD-II code indicating a general transmission malfunction. P0872 is a specific transmission code related to shift solenoid 'E' — it indicates that the solenoid is stuck in an active position when commanded to deactivate. The transmission fails to enter hydraulic default mode, the MIL (Malfunction Indicator Lamp) illuminates, and the TCM (Transmission Control Module) freezes shift adaptations. Additionally, torque converter clutch (TCC) engagement is inhibited when this code is present. I've already had the transmission fluid, filters, and pan serviced about four months ago at a GMC dealer — no prior issues up to 127,000 miles. After taking it to a transmission shop for a C-switch replacement (which cost $300 including installation), the problem persists. The truck only engages into drive or reverse when warmed up, and even then, shifting is rough or delayed. It sometimes enters limp mode — getting stuck in one gear — especially under load or during normal driving conditions. However, it performs well when not triggering the fault codes (e.g., smooth shifts in tow haul or regular modes). The shop suggested possible stuck valves inside the transmission. I'm now unsure what to do next. Since this is a complex system involving both electronic and mechanical components, I'd appreciate any advice on further diagnostics — especially whether a bidirectional scanner is required for accurate reading of Allison transmission data. Any insights or recommended steps would be greatly appreciated.

Related fault codes
P0700P0872
Comments (4)
Anonymous 2014-06-16 14:07

Where is the Transmission Control Module (TCM) located in a 2004 GMC Duramax with Allison transmission? How can I safely test it using diagnostic tools or a scan tool? Any guidance on basic troubleshooting steps would be helpful.

Anonymous 2014-06-16 14:23

P0700 is a generic code indicating a broader transmission issue. P0872 specifically points to the shift solenoid 'E' being stuck in an active state when it should deactivate. When the TCM detects P0872, it performs several actions: • Illuminates the Malfunction Indicator Lamp (MIL) • Prevents hydraulic default mode activation • Stores the code in TCM memory history • Freezes shift adaptation settings • Inhibits torque converter clutch (TCC) engagement A bidirectional diagnostic scanner is required for accurate reading and control of Allison transmission parameters. This is essential to verify solenoid commands, check valve positions, and assess real-time transmission behavior. For reliable diagnosis, consider checking the TCM wiring harness, ground connections, and potential internal solenoid or valve issues — especially if the truck shifts poorly when cold or enters limp mode unexpectedly.

Anonymous 2014-06-16 15:53

Dan, no need to create multiple threads. I've consolidated all related discussions into this single thread for better organization and easier navigation. Thanks for your input!

Anonymous 2018-10-04 15:07

I have the same P0700 and P0872 codes appearing suddenly. After a C-switch replacement at a transmission shop, the issue didn't resolve — the truck still fails to engage into drive or reverse until warmed up, and shifting is rough or slips when engaged. The transmission only operates properly after warming up or with engine revving in gear. It occasionally enters limp mode, getting stuck in one gear. The shop said they checked fluid levels and oil condition — it looked good, especially since I had a full service at the GMC dealer months ago. I paid $300 for the C-switch replacement and installation. Despite this, symptoms persist. The transmission performs well when not triggering codes (smooth shifts in tow haul or normal driving), but fails during other conditions. The shop suspects stuck valves inside the transmission. I'm now considering further diagnostics — such as a full TCM scan with bidirectional communication or a valve body inspection. Any recommendations on next steps would be appreciated, especially for DIYers working on this system.