There is no OBD connection on a 1990 Nissan as that is an OBD1 vehicle not OBD2. However there is a way to read the codes. https://codes.rennacs.com/Petrol-Engine/Japanese-Korean/Nissan-Engines.php
Check your engine. Check your fluid levels and add if needed.
your oxygensensor is bad ,its screwed in to your exhaust manifold, about 30.00
engine size would help. check with your local auto parts shop.
check out ebay.
I don't own a 1993 Nissan Pickup Truck, stupid! check your thermostat to see if it opening
haynes repair book at auto parts store or get a Nissan truck service manual,check eBay .the dealer wants 75.00.
check eBay or Nissan dealer ,they are pricey at Nissan ,about 75.00 bucks.
first check your bulbs to see if they are loose or you might have some rust and its not getting agood connection,and check your fusible link,its tied into your positive battery cable.
Depends on which year/model. Check http://www.edmunds.com/used/nissan/index.html for specifics.
check both of your pulleys,they have marks on them.
call auto parts and check part numbers on both years to see if they match.
check engine