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I have a Caterpillar GC40k forklift that the engine won't rev up past 1300 RPM. It drives slow and all hydraulic functions are also slow. I already swapped out the ECM from another forklift but did not fix the problem. Anything else it could be?

Hello and welcome to fixaforklift

I have had this issue before but my situtation was pretty obvious as to what the issue was.
Inside of the muffler there is a honey comb/catalytic converter. If that gets damaged or dented it would prevent full throttle revolutions.
Another drive speared the muffler and dented it I tried to drive it to a safe location to work on it but it rev'ed up very slow and obvious that it was not all the way to the max.
When I removed and replaced it the truck ran like it was previously.
When I hooked up diagnozer before all the readings looked normal and confused me as to why.
after I removed and replaced the muffler the readings remainded the same but the engine rev'ed as it was.

So check your muffler for any dents, or even worst case scenario clogged muffler.
A sure way to find out is to remove the 02 sensor and that should relief some of the tension inside the muffler causing it not to rev up.