Where is the stage 1 O2 Sensor on a '98 Pontiac Montana?

Answer 1

the stage 1 sensor, actually called the pre-converter sensor, is located in the exhaust system before the catalytic converter. the post-converter is after the catalytic convertor.

AnswerI have a 2003 Montana but I believe it's the same. Official service manuals describe a procedure whereby you disconnect and rotate the entire engine forward, then remove various bits from the top of the engine in order to reach the stage 1 sensor from above. I gave up on that and was able to do it by jacking up the front of the van (please use wheel blocks and appropriate jackstands!), and reaching up from underneath the van along the exhaust pipe. The stage 1 sensor is screwed into the top side of the exhaust pipe, facing slightly rearward, about 3/4 of the way up. Go get a real O2 sensor socket (has a slot on the side for the wire). I guarantee that sensor will be seized up VERY tight, so use the largest socket wrench you can fit up there to get enough leverage. It was a tight fit, and I had to do most of it by feel, but it IS possible. I'm a big guy, 6 foot and I don't have skinny arms, and I was still able to reach up there. Definately use some anti-seize on the new sensor threads!

Make sure that you know which O2 sensor your van needs: the California emissions sensor or the Federal emissions sensor. If you get the wrong one, you won't be able to plug it in (beware of plugging the wrong kind in backwards and thinking that it's done correctly!). Remove the old sensor first and take it with you to the parts store, that way you'll be able to directly match the old plug with the new plug. You can distinguish plugs by looking first at the color of them (Gray was California emissions for me), and then **most importantly** by looking inside the plug itself. There are two differently sized plastic pieces in the plug in the California sensor, and only one in the Federal sensor.

Also, you'll need to buy an O2 sensor socket, and there were two at the auto parts store I went to. Only the short one worked for me. The tall one wouldn't let me get a socket wrench on it without hitting the firewall.

Good luck!!