2000 Nissan Maxima won't shift into overdrive
 

I have a 2000 Nissan Maxima with an automatic transmission. When turning the car on, the OD light blinks 16 times. When driving, you can be up to about 65 and at 3500 rpms. The overdrive button is on but the transmission doesn't shift into overdrive. What might be the problem?

The flashing OD light indicates that there is a problem with one or more sensors providing input to the TCM; it could be the TPS, VSS/AT, ATF Temperature Sensor, and several other components.

Have you checked your AT fluid level? At any rate, follow the diagnostic procedure starting on page AT-49 of your FSM. You can download it here: http://boredmder.com/FSMs/Nissan/Maxima/ .