Scotty I have a 2016 Toyota Avalon Touring with about 89,000 miles.

This past summer I noticed occasional hard shifting at low speeds. Very low speeds at around 5MPH. I would notice it only in dead-stop freeway traffic and in parking lots. Otherwise everything seemed fine. No lights were present on the dash.

However one day the Check Engine Light and the VST indicator came on. The car seemed fine and I was almost home so I finished the half-mile drive home. The next day the Check Engine Light and the VST indicators were still on. I disconnected the battery for a couple of minutes and reconnected, and the lights went away. The car drove perfectly around town.

Until more recently, the only consistent behavior was after 45-60 minutes on the highway the check engine and VST indicators on the dash would come on again. Once again I successfully turned them off by disconnecting the battery for a 2 minutes spell.

It seemed I was noticing a pattern of short trip city traffic not triggering the lights, while 45-60 minute trips on the highway would trigger the lights. But I am not certain about that anymore.

When the lights once again came on I took the car to an independent Toyota specialist and left it for a few days.

The mencanic replaced the transmission fluid, noting its appearance may have been on the dark side, but not bad or really unexpected for the mileage.

I took the car and drove it on the highway, and again after about 45-60 minutes on the interstate the check engine and VSC lights came on. I took the car back to the Toyota mechanic.

His scope returned two possible causes:
1. The Transmission Temperature Sensor, requiring replacement of the entire Valve Body Assembly - $1,674.48
2. The transmission Control Module - $1,164.82

The mechanic and I aren not sure which to try first. I don't know if the computer is drunk, or if the transmission temperature sensor is misbehaving. I mostly lean toward the transmission temperature sensor, but a few days before the lights started coming on I did drive through a torrential rain storm for nearly 4 hours. However, considering the period of hard low speed shifts before the lights ever came on I still tend toward the transmission temperature sensor.

I have since picked up the car and have been driving it, so far entirely with short-trip city driving consisting of many stops. After 2-3 days the lights have come on, but they also go off on their own. This is the mechanic's experience when he drove the car too.

I do notice now that when the lights are on I experience hard shifting at all normal city speeds (>50 MPH). But when the lights are not on everything seems normal.

By the way, I think this mechanic drove the car home or on errands for a couple of days. This I think is great and is going above and beyond. But I think he topped off the gas tank and reset my trip A on the odomoter without telling me or charging me for the gas.

Do you have any insight on which could be the cause? Both are quite expensive repairs.