I need some assistance. I have a 93 del sol s with a d15b7 motor. I purchased an upstream o2 sensor to replace but I can’t figure out where it installs. I’m wondering if a previous owner swapped out the header with a different one at some point. I was under the impression that this motor being obd1 only has one sensor. But if you look st the pic below it looks like another sensor bung on the header near where it mates with the catalytic converter. Do the stock headers for these motors have another bung on them that doesn’t get used? And the upstream sensor should install straight into the header and plug in to the connector near the distributor correct?

 

In theory the primary (upstream) sensor can be installed anywhere before the cat. Old Toyota trucks had them under the floor.

The stock setup is a cast iron manifold (not header) with the sensor screwed into it. The manifold then attaches to a front pipe which turns under the engine and goes to the cat. An aftermarket header replaces both of those parts with one piece.

You will need a wiring extension to mount the sensor in your header setup. Other than that it will work.