Q: Q: limp mode

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hello . i have a 2008 GMC 2500 HD . it goes into limp mode off and on . 1st time it did i was on the highway and it down shifted into 2nd gear .locked up the rear end and revved up so i put it in neutral . the door locks went crazy going up and down .. lost speedo , lost trans temp gauge . lost tow haul , the brake pedal makes a clicking sound when you depress it . i have a friend with OBD2 and it said lost communication with TCM . i then cleaned All fuses , fuse box , and connectors , added a ground wire to engine . cleaned all ground wires i could find . replaced the alternator .. i am lost now what to do because it still goes in and out of limp mode ?

My car has 50000 miles.
My car has an automatic transmission.

This problem may be a wire connection or TCM or PCM issue. The first thing that has to be done is to get communication with the TCM. The different computers communicate all to the engine computer and then to the data link. If one or more modules do not communicate then the engine goes into limp mode. I can only recommend having a mechanic like one from YourMechanic to go through complete diagnosis for the no communication error you have to isolate the cause of the problem on this one since the wiring is a major factor on these vehicles along with TCM.

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