Why does my 2000 frontier shift OUT of overdrive when it accelerates past 40 mph

If it (doesn't) slip & works just fine

I would change the fluid & be sure to use
the correct fluid

I would add one 10 oz bottle of RED LUBGARD
to it,nothing else,no other brand or additive

May just be the cold temp shrinks the valve body,
until warmed up

40 mph to OD, light on the gas, is correct,more gas later
into OD

An expert at a shop ought to look at this---could be a shift module or electronic or mechanical problem.

It might also be a sign something bad is about to happen.

Electronic or mechanical issue?

Roger, that doesn't even make a whole lot of sense to me. If the overdrive is causing the transmission to slip, wouldn't it also cause the fuel mileage to go down? Are you sure the transmission is slipping? Or is the transmission simply shifting in and out of overdrive? If it is, is it shifting at the proper speeds? Driving at around 40 - 50 Mph will sometimes make the transmission go in and out of overdrive repeatedly. This can be perfectly normal.

Have you checked your transmission fluid level?

Is your overdrive off light flashing? If the transmission is actually slipping the OD OFF light should be blinking at you to let you know there is a transmission problem. Either that, or the Check Engine light will be coming on. In either of these cases, you need to scan your onboard computer network for possible fault codes.

A common problem with most overdrive units.Overdrive should only be used above 40 mph. With the mileage odds are that you need to have transmission looked at professionally.It maybe time to consider rebuilding it.I do not see a trans service of being a benefit.

Well if you would elaborate some more on what kind of car you have,...that would be helpful as my ESP is very low today....

simple tune up could do it, or a bad MAF or MAP sensor could give similar problems

The ETC switch is a transmission control device. When it is not active, the transmissions shifts normally. When the ETC is engaged, it alters the shift points.
Example: the 1-2 upshift occurs normally between 18-20 mph. Now, say you are towing something, or are carrying a heavy load...engage the ETC, and the trans computer will "hold" the 1-2 upshift until 22-24 mph. It is just for that, prolonging the shift points, due to load.

The switch on the side of the shifter is for the Overdrive. Engaged (the OD light is off) the transmission shifts into overdrive. This saves you gas. When the button is dis-engaged (the OD OFF light is on) , the transmission does NOT shift into overdrive. It is best to leave the overdrive ON, and the ECT OFF to obtain maximum fuel mileage. I hope this has answered your question. Please feel free to comment back if you have any other questions. Thanks for chosing FixYa for assistance.

You my friend have a problem. I suggest a trusted transmission shop or a salvbage yard replacement.

you may have damaged the overdrive gear, a local transmission shop would be your best bet. for good troubleshoting and estimate.