Chevrolet Repair Questions? Ask a Mechanic for Answers ASAP
I am sorry your having this problem
did all those codes set out of the blue one day ?
has anything been done to correct this issue ?
was anything done to where now you have this issue ?
are you planning on fixing it yourself or going to shop and just want an idea whats wrong ?
how does the transmission slip and when and what speed ?
5.3 or 6.0 engine ?
the 336 code is usually a bad crank sensor and or the connection or wiring at the crank sensor
the 121 and the 2138 are both electronic throttle codes which can be bad gas pedal assemblies or throttle bodys or the connections at the throttle body
as for the transmission slipping those codes can alter the engine power enough to cause it to feel like its slipping but would hate to see you spend the money on fixing those things and have a bad transmission