96 chevy blazer 3rd gear and overdrive slipping

Great question.
PRNDL (is the cable good both ends) shifer>?
R good
L good?
Drive , selected and 1st gear car will not move ?
or 1st gear car moves good but will not up-shift.
1,2,3,4 gear. 1st slips or works??????????????
name 4T60 (transverse 4speed box) GM./
DIY?
check fuse first
scan the PCM (the PCM has TCM) the TCM tranny brain and if TCM is not happy it tells you why
so all carsChev 1992+ must be scanned, first.
the 4t60E box is electronic shifted. scan it.
what is miles on car.? 100k,300Kwhat?

causes are vast, (ever see inside any A/T box)
the next test in the book is pump pressure test
PRNDL

1 Ck A/T fluid level & quality
2 Scan/ck for code(s)
3 With scan tool connected,road test vehicle/Ck parameters....
....time for a pro !!

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I have a 92 Crown Vic first and second works fine reverse works fine when it shifts in the third gear it goes to slipping and overdraft can you help me fix this problem thank you

rebuild or put a used trans in it they have hd zpack made for 3and 4 gear

I say band adjustment in transmission, have done by shop. might be just a vacuum hose or modulator, but I don't think so

Most likely the second gear section of the transmission has gone bad. There are 3 or 4 different gear sections or assemblies in an automatic transmission, when they go bad, they go bad.

Normally while driving in overdrive (4th gear) you will feel it start slipping and finally slide into third gear. Then 3rd gear slips out into second, and you will never see 3rd or 4th gear again. I'm guessing this was the problem? If so its time for an overhaul

no you have a failed transmission on your hands, it is slipping.

Yes, it could very well be a bad transmission.

Check both O2 sensor(s) and catalytic coverter.. P0420 means catalyst below threshold. Also check the sensor at the transmission and solenoid.