My 2012 Chevy Sonic transmission is slipping after the car warms up when it's cold you can started get into it and drive a couple of blocks but once it warms up it slips and will eventually not will fail

that seems like the filter in the tranny pan is packed up in gunk.
as in HOT ATF flows better past junk there.
cold not.

Your tyranny is either low on fluid or getting ready to kick the bucket... one of the signs you watch for is the car Jerks hard when changing gears while cold but seems to be OK when warmed up... this can go on for months... one thing to try asap.. get the transmission flushed... and don't be a lead foot... if she is slipping it's not going to help if you give it more gas... but a lot of the time a good flush will help ( filter gets plugged and the transmission fluid isn't doing its job

Sounds like a servo seal (o-ring) in the trans itself. If the seal leaks, it will not allow the proper pressure to build in the circuit that engages bands and clutches. I see a trans rebuild in your future.

is it slipping,?
if the rpms rise and car dont speed up , in direct proportions that is slip.
is it?
if the engine has full power in 1st, the the transmission is bad.

not sure what shift manually means,
Drive to low? low to high. what? be exact in statements/
better stated what it does good and bad, and when. all modes.
are you saying the trans shirts perfectly in AUTOMATIC when cold./?

check level transmission oil

Its cold start issues:----
Go through the list of help links mentioned below:----

Click the link :----

Cold start problem and vehicle shudder around 2500 to 3500 rpms

http://repairhelpcenter.blogspot.com/2012/10/cold-start-problem-and-vehicle-shudder.html

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Low idle when cold out side?

http://repairhelpcenter.blogspot.com/2011/12/low-idle-when-cold-out-side.html

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Ford explorer 4.0 will not idle when cold?

http://technoanswers.blogspot.in/2012/02/ford-explorer-40-will-not-idle-when.html

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Automatic transmission shift lever hard to move on cold mornings?

http://technoanswers.blogspot.com/2012/01/automatic-transmission-shift-lever-hard.html

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Car won't start when cold?

http://schematicsdiagram.blogspot.com/2011/12/car-wont-start-when-cold.html

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These will help.
Thanks.

Sounds like you have definitely an internal problem in the transmission if its slipping ...

It could have an electrical issue as well like a faulty PCM .

How could the mechanic say its fine if it slips.....
I would start by going to a different shop/mechanic and have them scan the data to see whats going on electrically..but i would think at this mileage and with it slipping you're probably going to be looking at an overhaul/replacement transmission.

For Automatic transmission problem,Check the list of help links below.click the link below:----Automatic transmission troubleshooting:----

http://howtobyme.blogspot.com/2011/11/automatic-transmission-will-not-go-into.html

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Automatic transmission shift lever issues?-------

http://technoanswers.blogspot.com/2012/01/automatic-transmission-shift-lever-hard.html

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AUTOMATIC TRANSMISSION FLUID LEVEL? --

http://schematicsdiagram.blogspot.com/2011/07/how-to-check-automatic-transmission.html

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AUTOMATIC TRANSMISSION FLUID AND FILTER?

http://schematicsdiagram.blogspot.com/2011/07/how-to-change-automatic-transmission.html

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Transmission fault light on car dash?

http://schematicsdiagram.blogspot.com/2011/12/transmission-fault-light-on-car-dash.html

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Transmission shifting problem?

http://schematicsdiagram.blogspot.com/2012/01/transmission-shifting-problem-on-toyota.html

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Transmission Slips out of 2nd gear?

http://repairhelpcenter.blogspot.in/2012/02/transmission-slips-out-of-2nd-gear.html

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OD light flashing and transmission fluid leaking?

http://repairhelpcenter.blogspot.in/2012/02/od-light-flashing-and-transmission.html

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Why Transmission fluid leaks near radiator?

http://repairhelpcenter.blogspot.in/2012/02/transmission-fluid-leaks-near-radiator.html

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transmission fluid leaking?

http://technoanswers.blogspot.in/2012/02/ford-windstar-transmission-fluid.html

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****** wont go in reverse?

http://technoanswers.blogspot.in/2012/02/dodge-neon-wont-go-in-reverse.html

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Automatic transmission will not go into reverse?

http://technoanswers.blogspot.in/2012/02/dodge-automatic-transmission-will-not.html

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Transmission is making buzzing noise?

http://technoanswers.blogspot.in/2012/02/transmission-is-making-buzzing-noise.html

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This will help.

Thanks.

Silly question: Have you checked the transmission fluid level, and is it to manufacturer's spec?

put in a bottle of lucas transmission conditioner will fix it