A/t oil temp light come on 1999 isuzu npr diesel

Check your trans fluid and inspect for leaks asap. If level is good and leaks are none to minimal, I would suggest a new trans cooler. Your current one could be clogged.

If I'm not mistaken, the A/T cooler on some of these trucks has a separate electric fan. If that is disconnected or not working, you might see high trans fluid temp even with good circulation.

Adding to the previous comment: Check the condition of the trans oil when you do the level/leak test. If the oil is discolored or has a burned or fishy odor, it's time to do a proper flush and service (note, NOT a 'reverse flush' - check proper procedures...) Drop the pan, change filters, clean sludge, etc. (That would be a good time to clean the transmission oil cooler too)