What do I do if I used the wrong transmission fluid?

My friends mom bought fluid for a Honda, but I drive a Hyundai. The gears are shifting hard and the check engine light came on and it's because of the transmission oil temperature sensor. But how do I fix that besides the sensor? I've probably driven no more than 30 miles on the wrong fluid. Is there a way that can be fixed? Will I be able to drain it and replace the filter and use the right fluid? Because I have a feeling that that's why it's shifting hard. Although at first it was because I was low on fluid. But I don't know what to do. I don't want my car to have permanent damage to it.

You should call a repair shop they should give you free advice over the phone

First of all, make sure you really do need a different type of fluid. There's "dex/merc", and there's "type f". Both cars might need the same type. If the wrong kind of fluid is in there, you can just undo the drain plug, let it all come out, and add the right kind. I don't see any need to change the filter. Hopefully that will clear up the problem you are having, because a replaced or rebuilt transmission is $$$$$$$$$$$$.

Mo

Do a drain and fill service 3 times using the correct Hyundai fluid. Cycle the gears 3 times before each change.

Hi so gears shifting hard can have several causes lubricant is not one of the reasons. Wear however is in the gearbox.
so it needs to go to a honda dealer.

I agree with Erik. First check the specifications for the fluid required, you may find that the "Hyundai" fluid meets the required specs no matter how it is badged.

If it is wrong simply drain it and refill with fluid to the correct specs. Make sure you get the level right.

Hyundai takes an SP III rated ATF. Castrol makes an import vehicle ATF that is SP III rated.
Why is a friends Mother buying you ATF?
Didn't you read the info on the bottle before pouring it in?
If it turns out that the Honda ATF is not SP III compatible you'll need to do a drain and fill two or three times with fresh SP III ATF every change. The ATF filter is not serviceable.
The clutch discs in the automatic tranny absorb the ATF so you won't get immediate results with better less harsh shifting.

Your Hyundai has a 100,000 mile drivetrain warranty, which includes the transmission, so apparently it's past that point, and it was leaking fluid and shifting hard before you added any fluid, right?
Bad news. While the trans may continue to operate for awhile, because of the low fluid and overheating, it is SEVERELY damaged and will need to be rebuilt soon.
The Honda fluid did not cause this damage whatsoever.

You want to blame (change) the sensor?

If the ATF is the wrong type, you need to remove it! Unfortunately, draining the transmission fluid from the pan only gets out SOME of the fluid.

You would have to drain it several times, to thin out the bad fluid.

Some transmissions don't have a drain plug in the pan. You would need to remove the pan to drain it the first time. Then install a drain plug in a good spot. Then you won't have to take the pan off again. You'll need a new pan gasket.

I'd run the car in the driveway with both front tires safely off the ground and put it in drive for a few minutes and let the car shift gears a few times and keep the speedometer at maybe 20 mph. Stop the tires and transmission from turning with the brakes before you put the transmission in park. You'll need 2 jackstands placed properly. If you're not familiar with repairing cars, have a mechanic do this.

Drain the fluid and refill with new. Repeat the process.

Repeat it a third time.

There's more wrong than the transmission just leaking, since it was low on fluid, but check with the Hyundai service dept to see if Honda transmission fluid can make it act up. If they say yes, have the transmission flushed and refilled with the correct fluid, but also fix the leaks. It should never be low on fluid from normal operation.