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Ask GM Tech (Cam) Your Own Question
GM Tech (Cam)
GM Tech (Cam), GMC Mechanic
Category: GM
Satisfied Customers: 29515
Experience:  GM Grand Master Technician 2007. 14 years experience.
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I have 2006 gmc2500hd 4wd pu 110,000 miles code p894 check

Customer Question

i have 2006 gmc2500hd 4wd pu 110,000 miles code p894 check engine light. oil lvl good pink dont smell bad overy last 2 weeks twice felt a klunk while driving.
Submitted: 1 year ago.
Category: GM
Expert:  GM Tech (Cam) replied 1 year ago.

Hi there, I am Cam. Along with this clunk, is the check engine light on?

Customer: replied 1 year ago.
check engine light on, also been have codes po171 andpo174 that comes and goes
Expert:  GM Tech (Cam) replied 1 year ago.

Ok, those will cause shifting issues. The fuel system on yours does not have an external filter. If the integral filter is plugged it reduces the actual power the engine can put out, you want to go faster, it does not respond so what is your natural reaction? Push the fuel. Since the shifting is controlled buy a combination of 3 things, engine speed, vehicle speed and throttle position. Normally you push the throttle down all the way you want more speed so it down shifts and goes to high RPMs. IF there is not enough fuel to fully make the power at that RPM, the engine will simply run at that RPM but make little power and when you do not feel it moving or accelerating, it gives the feeling of slipping. So a plugged filter can do this, or be the cause of the larger issue. A plugged filter will eventually damage the fuel pump due to the strain of trying to push the fuel past the restriction and that in turn makes the pump weak. So to accurately measure this we need to see what the fuel pressure is doing when it is trying to go faster. So when driving. IF that is fine then the next option is the exhaust. If the filter was plugged it can cause a lean condition, which is no power and that can damage the catalytic converter which will cause the engine to plug up in a way, meaning it cannot push the exhaust out and that will also cause a lack of power. The test for that is a back pressure test done by taking the front oxygen sensor out and either driving it or testing back pressure. Anything more than 1 psi at all RPMS is too much.

Customer: replied 1 year ago.
have had o171 and o174 come and go, they both come and go tegether, had some1 say mostlikly a sensor since both come on at same time, pickup has good power and pickup from stop to top speed nad not dead spots and very good aceleration. just this week felt heard like a kluck or mometary slip. 2 times this week. checked and now have transmission slipping alarm. what are the chances bands needs adjusting or bad transmission., will follow up still with other 2 alarm testing.
Expert:  GM Tech (Cam) replied 1 year ago.

The bands are not adjustable so it's not that. It might be the torque converter clutch. If you take the lower cooler line off at the transmission, is there debris in there? These are know for that clutch coming apart and the clunk/shifting would be the result.