The rear O2 sensor literally serves no other purpose that to tell if your CAT is functioning properly so I'd try and find the cheapest one possible. DENSO is an OEM and IIRC makes the sensor for Mitsu as well. You could even get a universal and splice it in if youre trying to go the uber cheap route...

 

My car was tuned in 09(have Dino sheets,but no info on what tune was done, GST tuned, but they no don't tune evos since 6 years ago, so I have no way of finding out) , if the rear o2 sensor was disabled would the o2 sensor still read on my scanner? Or would it just show 0 volts and what not? I have 2 foulers, so I figured it could just have the mechanical fix but not the ecu disabled o2.... Would I need to re-tune it?

 

Im confused...what are you trying to accomplish? Do you need to pass inspection? Id imagine that if you have the spacer on the rear 02 then you dont have it deleted in the ECU. Do you have a cat on your car or is it a test pipe?

 

My bad for confusing you lol. Yes I need to pass inspection. That's what I assume because I had 2 spacers on. But my monitor hasn't set after 100 miles, with the drive cycles. Currently I put the cat back on, and removed the spacers.

 

Ahh OK now im picking up what youre putting down haha!

So for starters if you have the cat back on then forget about the spacers, as you have done. Generally it takes about 50 miles for the system to go through its checks and verify readiness however realistically it can be more or less as it need to go through different conditions drive cycles various RPM's loads etc. So it can take a little while to reset.

However, if it's still not flipping it's one of 3 things. 1 it was deleted when you had your car tuned so you wouldnt get a CEL. In this case it wont go to ready status. 2 your rear o2 is bad. 3 your cat is bad. If I had to guess I'd put my money on #1.

If thats the case you need to find someone local with a laptop a tactrix cable ecuflash and a little bit of knowledge to help you out. I'd offer but you're 3000 miles away so not exactly feasible.

 

You need to see if Bryan can help you out. He is still on the forums but he no longer tunes. Maybe he can look at it for you.

 

I'm also leaning to #1 lol and thank you for your willingness to help! I came to a local tuner, mostly does subarus. But also tunes nissan's and evo's, says he is going to check to see if the map/rom(whatever it's called) is locked. So if it's not he said no charge for me yay! But if it is than a new tune would be option 2. Till Monday I'll know

Do you happen to know his contact info? Username, or email?

 

Sn is razorlab. Goodluck.

 

For what it's worth, I swapped my O2 sensor today and it's still reading 0v. And my tuner confirmed its not shut off in the ROM. Super annoying.

 

I'm a smog tech and mastertech. The rear o2 should show a slow and steady voltage it should not mimic the front o2. If the computer sees the rear o2 switching like the front then that usually means the cat is deteriorating and the cat monitor test will fail and make the cel come on. It usually takes several drive cycles and some miles of driving at a steady speed. One drive cycle is starting the car it warms up you drive it and shut it off. The computer has to see the coolant temp drop below about 165 degrees F. I've had to drive some cars 2 or 3 cycles and 100 miles or more.

 

I find that the rear o2 goes to ready status very quickly. However its EGR and EVAP that tends to take some time. I tried to get my sticker yesterday but both EVAP and EGR wouldnt flip. I let the car sit overnight and now I got my EGR to flip but I also got a misfire code so after clearing that im back to both EVAP and EGR needing to go back to ready status.

There is a defined drive cycle process in the manual however its next to impossible to meet those conditions on the road; you'd need to leverage a dyno. I wish there was a way via Evoscan or some other tool to force the tests to run. I don't mean force them to pass but force them to go through their system of checks.

 

When the rear 02 switches it's not fast like the upstream one. The readiness monitor was incomplete before trying with a working cat on. Now it still hasn't set. I've put about 120 miles with cat, but will continue driving and trying the drive cycles.
I took my car to a local tuner shop. They said my tune didn't disable the rear 02 so it could be the 02,cat,ecu problem. Most testing on Friday will keep this update for others.

 

ive also seen when a monitor takes too long to pass and when it finally does. the cel comes on. sometimes ive driven the car for up to about 200miles.

 

To get the Evap to set, cold morning turn on ac and rear defrost and idle, that's how mine set quickly. But my 02 monitor still won't set 🙄 Still trying to avoid replacing the sensor just a bit longer because 400$ ugh.Cat on, 02 stock position in cat, all other monitors set and tuner didn't disable rear 02 has me lost lol


Anyone around North Bay Cali who can flip ready through evoscan or tephra???

 

To pass the smog check you do not need evap to set. You need everything else to set. I think you guys got some issues in your tune. I remember when i first got a tune my o2 would not set as well. I called and got the same answers as you guys. Need to play with the force pass patch. Its on this forum but its confusing and requires alot of trial and error to get it to set right. Yes it sucks but it is possible to accomplish. Goodluck guys and if you need help you can message me and i can see what i can do to help out. Im no expert, just an evo owner passing on what i learned about our cars.