# What kind of oxygen sensor does my car need? (Need help fast!) **URL:** \ **Category:** Factual Questions\ **Created:** [August 3, 2011, 8:43am UTC](https://boards.straightdope.com/t/what-kind-of-oxygen-sensor-does-my-car-need-need-help-fast/591301 "2011-08-03T08:43:29Z")\ **Posts on this page:** 9\ **Page:** 1
**Author:** ![Reply](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/reply/32/15952_2.png) [@Reply](https://boards.straightdope.com/u/Reply)\ **Post date:** [August 3, 2011, 8:43am UTC](https://boards.straightdope.com/t/what-kind-of-oxygen-sensor-does-my-car-need-need-help-fast/591301/1 "2011-08-03T08:43:29Z")
I have a 2001 Mazda Protege 2.0 LX that’s about to go on a long road trip this week, and I want to fix the check engine light before then. I got the code run and it’s a heated oxygen sensor issue, but it seems like my car has two of them and the online OEM replacement sensors are either “pre-catalytic” or “post-catalytic”. I have no idea what that means, but I did manage to dig up a service manual for the 02 protege (which should be similar enough). I found a seemingly relevant page that shows the two oxygen sensors as part of the exhaust system: [http://www.asdfv.com/ho2s.pdf](http://www.asdfv.com/ho2s.pdf) The part I need to replace is #5, the heated o2 sensor (rear). Is it possible to tell from that schematic whether it’s post or pre-cat? Thank you in advance! ---
**Author:** ![jz78817](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/jz78817/32/1028_2.png) [@jz78817](https://boards.straightdope.com/u/jz78817)\ **Post date:** [August 3, 2011, 9:32am UTC](https://boards.straightdope.com/t/what-kind-of-oxygen-sensor-does-my-car-need-need-help-fast/591301/2 "2011-08-03T09:32:16Z")
“rear” is likely to be the downstream sensor, or post-cat. All cars have two (or four, for vee engines) now; the pre-cat one is used by the engine management and the post-cat one is to make sure the catalytic converter is working properly. **ETA:** looked at the PDF you linked. Yes, part #5 is the downstream (post cat) sensor. #11 is the pre-cat sensor which screws into the exhaust manifold. ---
**Author:** ![Reply](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/reply/32/15952_2.png) [@Reply](https://boards.straightdope.com/u/Reply)\ **Post date:** [August 3, 2011, 9:51am UTC](https://boards.straightdope.com/t/what-kind-of-oxygen-sensor-does-my-car-need-need-help-fast/591301/3 "2011-08-03T09:51:46Z")
Perfect, thanks so much! Is that “WU-TWC” the cat, then? ---
**Author:** ![Reply](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/reply/32/15952_2.png) [@Reply](https://boards.straightdope.com/u/Reply)\ **Post date:** [August 3, 2011, 9:59am UTC](https://boards.straightdope.com/t/what-kind-of-oxygen-sensor-does-my-car-need-need-help-fast/591301/4 "2011-08-03T09:59:11Z")
I ordered the part and I’m going to try to replace it myself. I have zero car repair experience, but my friend said it should be a simple job with a wrench. I hope I don’t blow anything up, heh… ---
**Author:** ![jz78817](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/jz78817/32/1028_2.png) [@jz78817](https://boards.straightdope.com/u/jz78817)\ **Post date:** [August 3, 2011, 10:04am UTC](https://boards.straightdope.com/t/what-kind-of-oxygen-sensor-does-my-car-need-need-help-fast/591301/5 "2011-08-03T10:04:20Z")
warm-up three-way cat. It’s the one closest to the engine so it gets up to operating temp quickly (catalysts don’t work unless hot.) Three-way cat means it cleans up HC, CO and NOx. your car actually has two catalysts, part #3 in your diagram is the second one. **ETA:** > [@](#): > > I have zero car repair experience, but my friend said it should be a simple job with a wrench. I hope I don’t blow anything up, heh… it’s pretty simple. unplug, unscrew, screw the new one in, plug it in. it gets complicated if the old one is seized in the pipe, but for an '02 I’d think it should come out. ---
**Author:** ![MikeG](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/mikeg/32/4329_2.png) [@MikeG](https://boards.straightdope.com/u/MikeG)\ **Post date:** [August 3, 2011, 12:30pm UTC](https://boards.straightdope.com/t/what-kind-of-oxygen-sensor-does-my-car-need-need-help-fast/591301/6 "2011-08-03T12:30:38Z")
You will need a special o2 socket with a slot cut in it to remove and install. They are widely available. ---
**Author:** ![Hairy\_Bob](https://avatars.discourse-cdn.com/v4/letter/h/ecc23a/32.png) [@Hairy\_Bob](https://boards.straightdope.com/u/Hairy_Bob)\ **Post date:** [August 3, 2011, 11:52pm UTC](https://boards.straightdope.com/t/what-kind-of-oxygen-sensor-does-my-car-need-need-help-fast/591301/7 "2011-08-03T23:52:51Z")
> [@MikeG](#): > > You will need a special o2 socket with a slot cut in it to remove and install. They are widely available. Or a good set of vice grips.😉 ---
**Author:** ![jz78817](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/jz78817/32/1028_2.png) [@jz78817](https://boards.straightdope.com/u/jz78817)\ **Post date:** [August 4, 2011, 12:03am UTC](https://boards.straightdope.com/t/what-kind-of-oxygen-sensor-does-my-car-need-need-help-fast/591301/8 "2011-08-04T00:03:53Z")
> [@MikeG](#): > > You will need a special o2 socket with a slot cut in it to remove and install. They are widely available. eh, a box-end wrench (or flare-nut wrench) will usually work just as well. no need to buy special tools. ---
**Author:** ![Reply](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/reply/32/15952_2.png) [@Reply](https://boards.straightdope.com/u/Reply)\ **Post date:** [August 4, 2011, 1:24am UTC](https://boards.straightdope.com/t/what-kind-of-oxygen-sensor-does-my-car-need-need-help-fast/591301/9 "2011-08-04T01:24:51Z")
At work I have a wide assortment of wrenches, pliers, grips, and implements of pain, so I’ll try those first. Should I disconnect the battery?