I have a 2004 Jeep Grand Cherokee. It has over 100,000 miles on it. When I drive down the street or the highway, the over drive off light and the trans temp light come on. It then goes off within a second or two. The longer the trip, the greater the chance it will happen again. Recently, it happens more often. In fact, yesterday in a 5 mile trip it happend 4 times. When it first happend about 7 months ago, I took it to my mechanic who could not duplicate it (wasn’t happening that often). A month or two later I took it back to the mechanic. He suggested a valve (made me think of something like a thermostat) could be stuck. They replaced that. I didn’t even get home and the two lights came on. By the way, the two lights always come on together at the same time. A few months after that it started happening more often. One day it happened when I just happened to be within 5 minutes from the mechanic shop. I took it to them and they put a laser temp gun on it and said the trans fluid temp was normal. They now think it is a sensor, that will cost $350 to replace. Anyone agree with the diagnosis and does the cost sound reasonable.
I am not going to be the guy who can give you intelligent advice about the repair.
However, I will tell you that it would be in your best interest to take it to a reputable, local transmission shop (not a big chain) and have them do a full diagnostic on the condition of the transmission. (I am only assuming that the mechanic you mention isn’t a transmission specialist). If they do that, make note of any malfunction codes that the transmission spits out & you can post those. I probably can’t sort any of it out but there are others here who probably can.
The idea I have in mind is to avoid continuing to put band-aids on by guesses rather than getting a good thorough diagnosis.
And just out of curiosity, how have you been about having this transmission serviced? This just sounds a lot like difficulty induced by too little maintenance.
It does sound like a sensor to me, but I’m no expert. The reason the two lights come on simultaneously is likely that the transmission computer turns off the overdrive to help the transmission cool down, thinking it’s overheating.