5r55w No forward at engine hot

[axallcom]

I have problem with my 5r55w no forward gear if the engine is go to hot but the shift gear is normaly at cool engine what the problem about this case. but is have reverse gear with hash shift kit

 

[974X4BLACKSPORT]

First thing to know when diagnosing transmission problems is what the hydraulic pressure is when running.
There is a hyd. test port on the driver side, just above the oil pan rail behind the "PNP" switch. A 0-300psi gauge
would need to connected to this port, and the hydraulic pressure measured as the engine is running, and during
all the selections of the gear shift lever.
Things to check:
1) proper fluid level
2) fluid not burned
3) filter not clogged
4) when was the trans. last serviced