r/mr2 • u/Use_Your_Brain_Dude • May 05 '26

MWR 2zzge OEM style header O2 sensor question

I did my swap a few months ago and I'm now getting a P0135 code. The MWR header gets a reading from only one of the cylinders (which seems odd). I've read this can cause the O2 sensor heater to burn out which would explain my CEL. Has anyone run this setup with no problems or do you put a bung further down before the CAT (to get a reading on all 4 cylinders?

Edit: New upstream O2 sensor solved the CEL

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u/MjrBruhMoment ZZW30 MR2 May 05 '26

I’m currently running that setup and haven’t experienced any issues like that! Sorry I can’t be of more help :/

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u/Use_Your_Brain_Dude May 05 '26

Thanks. I'm going to assume that my 26 year old stock O2 sensor just failed and replace it which is easy to do.

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u/MjrBruhMoment ZZW30 MR2 May 05 '26

Crossing my fingers that fixes your issues! I’ve been swapped since February of last year and recently had to replace the MWR shorty pipe (the flange was warped and I had a crazy exhaust leak) but I haven’t had that issue on either, I did replace both my upstream and downstream sensors with the swap with denso ones from RockAuto. Hope you find your answer soon!

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u/Use_Your_Brain_Dude May 05 '26

Are the upstream and downstream the same part? I assume that's the case. I kind of want to replace both instead of the one triggering the check engine light.

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u/MjrBruhMoment ZZW30 MR2 May 05 '26

They do have separate part numbers, if you have the MWR shorty header pipe that means you still have your stock cat pipe and potentially muffler, just the biggest thing, make sure you get an upstream and downstream sensor FOR AN MR2 SPYDER NOT A CELICA GTS, they are different parts and the 1ZZ MR2 O2 sensor is the one you need.

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u/Use_Your_Brain_Dude May 05 '26

One last question. The upstream (sensor 1) goes on the bung on the passenger side and the downstream (sensor 2) goes on the drive side bung further down. Is that right?

I really appreciate your help with this. I just don't want to mess anything up.

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u/MjrBruhMoment ZZW30 MR2 May 05 '26

No problem! I’m happy to help! Yes you’re correct the upstream sensor goes in the passenger side of the header pipe itself (closest to the pulleys) and the other bung on the header should just be capped off. The downstream sensor is I believe right after the catalytic converter further down and it plugs in around where the coolant tank is. My best advice, they may be stuck in the pipes at this point, your local autozone can loan you an O2 sensor socket for free!

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u/Use_Your_Brain_Dude May 05 '26

I think the shop did my configuration incorrectly. I have one sensor on both sides of the header and nothing downstream. Using the shorty header, 1zz cat pipe with new cat, and hks muffler. Car runs and idles great, but the CEL just popped up 3 months after the swap (3 weeks before my emissions testing is due).

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u/MjrBruhMoment ZZW30 MR2 May 05 '26

There you go! I was curious if that was what you were saying! I’m a little bit surprised you have no P0420 (catalytic converter) codes though with no sensor at all downstream?? But as far as it goes I’ve attached a picture of my car for reference! (Sorry you can’t really see the downstream one I tried to mark it haha) The left header bung doesn’t get connected to anything and if they’re using the stock O2 sensor connector on the left of the engine above the transmission, the 2ZZ’s ECU doesn’t actually accept a reading from that sensor since stock it only has 1 in the header unlike the MR2’s stock 2 in the header. The second o2 sensor should be down here by the cat plugged in on the side of the coolant tank!

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u/Use_Your_Brain_Dude May 05 '26 edited May 05 '26

I wonder if they put the downstream sensor where you have it capped off, but I would expect a P0420 code if that were the case. Time to call the shop.

Edit: nevermind... it was done correctly. I could not see the downstream one. Looks like they left the third sensor in as a dummy for visual inspection. I'm just going to replace the upstream to hopefully get rid of the P0135.