P0740 code for 2000 Grand Voyager SE

I have a 2000 Grand Voyager SE with 3.3L engine and 4 speed automatic and 150,000 miles. I have a code P0740 indicating a problem with torque converter lockup. I don't have any problems with the car except for the check engine light. I can drive the car to about 35mph and feel it shift & rpm decrease for all of the gears. I can then test it by manually downshift to 3rd and see the rpm increase. I can shift back to D and see the rpm decrease. So I know I have gone through all of the gears. I can slowly accelerate to about 45 mph and the rpm decreases again. I suspect that is when the torque converter locks as it should.

So why do I have the check engine light?