Check this first
Notice whether the revs flare on the motorway – that's the lock-up clutch slipping.
What usually fixes it
In the order a good shop checks them. Typical total: $236–$3,100 at $130 an hour.
Transmission fluid and filter
$276–$644Old or low fluid stops the clutch applying.
Lock-up solenoid
$392–$888A stuck solenoid never applies the clutch.
Torque converter
$1,232–$3,100Worn out; the transmission has to come out.
Code titles are the generic SAE definitions. Prices are typical US parts and labor for a common car or light truck – a guide for judging a quote, not a quote. Always confirm a repair against the vehicle in front of you.