Car stalled when thrown in reverse

[Dr_No]

I added 350ml of brand x transmission treatment.
I threw the car in reverse and the whole car stalled, right there. Started it up and it took off easy..
What happened? what could have caused this?

Help please, I am very worried.

Just for reference, car is 1991 pontiac grand prix. 3.1l version
128k

 

[Quest]

Assuming that your engine is running fine, check for modulator valve issues on your A/T trannie.

Q.

 

[Chris142]

Those have problems with the lockup torque converter not unlocking. Very common.

 

[Dr_No]

Is it a costly repair? or should I just ignore it?

 

[Chris142]

Quote:

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Is it a costly repair? or should I just ignore it?

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Trans has to come out.

 

[Tom300M]

"Thrown in reverse" as in going from drive still rolling forward a little and putting it in reverse? If that is what you mean, that could be the reason.