[ChevyTech]

It should be a one wire sensor on your 1990 truck and the wire should be Purple.

On that truck the O2 sensor should have a single wire to a connector the in several inches from the actual sensor.

I can’t remember which years have the sensor in which positions. It may be that the exhaust system on your truck is from something else. Look at the driver side exhaust manifold and make sure there is not oxygen sensor there also, close to the outlet.

The single wire O2 sensors have to be very close to the engine to stay warm enough to function well.