darbs242

iTrader: (5)

So for the first time ever with headers in 4 years I failed inspection this year. For some reason my o2 heater and o2 sensor monitor would not set. We scanned the car which had no active codes and a few inactive for voltage. We cleared the systems codes and I had a 10 day pass from NYS to get the monitors to set. So I put about 1000 miles on the car across that 10 days with dozen drive cycles. The monitors still did not set and the car had not popped a check engine light. My friend used a scan tool and did a test on the o2 sensors when the computer richness and leans the car out to watch how the o2 sensors react and came to the conclusion that the sensors were not reacting correctly to the computers changes and were on their way out. I replaced both front o2 and in one drive cycle they monitors have set and I'm back in business. He said it is common on toyota/scion/Lexus vehicles. They have extremely sensitive o2 sensors but the cel range is pretty far from the actual failure range. He said they were working but not very well.

MileHIFcar

If I understand correctly when you put on aftermarket headers (whether they are sikky or ppe or novel etc..) do you loose 2 of the O2 sensors with the headers install like loosing the 2 main cats?? I guess what I'm trying to say is you can put on a secondary aftermarket set of high flow cats with header and exhaust install, but does that mean you have 4 O2 sensors just like the stock 4 cats? Or does the car only have 2 O2 sensors and where are they located?

Sorry for all the questions

darbs242

iTrader: (5)

You don't loose them, there are still 4 o2 sensors. But the rear two won't actually be working because there's a simulator telling the ecu that the rears are reading a good cat which has actually been deleted.

MileHIFcar

^^Ok, makes sense then the front two are still reading the untreated exhaust stream coming off of the aftermarket headers correct? So how is the engine making adjustments for A/F ratios if the fronts are going to be reading differently since the primary cats have been removed? I guess I'm still trying to understand how the ECU makes adjustments on the fly if the simulator is feeding a static reading? Maybe I'm off on this so enlighten me on where I'm confused.

Thx

darbs242

iTrader: (5)

Fronts read and tell car to make adjustments. Rears tell the car the cats are working. If cats have been removed you need a sim to lie to the computer telling it you have cats. This keep the cel off. Pretty standard stuff if you deleting cats on any car.

MileHIFcar

^^So if the primary cats are gone with the installation of aftermarket headers, then the front O2 sensors can still make adjustments to the A/F ratio if need be and wouldn't be affected because the exhaust stream is not flowing through primary cats correct?

jat0223

iTrader: (1)

The primary sensors come before any cats. Removing any cats doesn't affect the primary a/f sensors.

MileHIFcar

^^Ahh ok that makes sense that they are located before the primaries.

Thx

Joe Z

iTrader: (10)

The sensors attached to the OEM Manifolds / Headers are the A/F sensors.



The sensors attached to the OEM Midpipes / X-Pipes are the O2 sensors.




Joe Z

Fsport2UR

My car constantly smells like sulfur/rotten eggs regardless if I get on it or not. I was thinking my cats might be on their way out, but then again, I am starting to think it may be my 02 sensors are getting worn out. ODO is at 80k miles. I just may change mine out to see what happens. Best to try that first then do a costly cat swap. I live in Cali and don't plan to do any cat deleting.

MileHIFcar

Thanks Joe, so the front sensors are the ones that read the A/F mixture in the exhaust gases, so when people say that the F has 4 O2 sensors then they're meaning that 2 of them are A/F sensors and the back sensors are O2 per se

Jamakan

https://www.clublexus.com/forums/is-...-a-little.html

I getting magnaflow metal cats to help with this. Hopefully that's the solution.

lobuxracer

iTrader: (2)

Not only does it make sense, but having an O2 sensor of any kind for feedback on AFR after a cat is just dumb. It will never work.

abohler

Darbs, This is what I have been going through and it is driving me crazy. I literally just installed headers and new exhaust and since then the 02 heater and monitor sensor have never come back on. I have tested 2 sims and 2 sets of 02's and still nothing. So with yours the replacing the two header 02's worked and now you can pass inspection?

darbs242

iTrader: (5)

Yea. Just replaced both fronts with oem and within a drive cycle they set. There's a test on updated snapon scan tools and front o2s on our car. You can lean the car out and Richie it up and see if the sensors bottom out and are reading a certain range apart. Mine were out of spec. Talked with the local Toyota foreman cause I was gonna replace just one and he said they always do them in pairs or the customers come back Ina week or two for the other anyway.

It's pretty common to have to replace fronts when headers are installed. Je put it in my original quote although they were fine for the last 45,000 miles. With all the nitrous I'm not surprised they were on there way out.