No Secondary O2 sensor

[sallyspeed3]

Okay so I know with the AP you can disable certain ecu codes from popping up, like for those running catless. My question is, could you not have a secondary o2 sensor at all and still be able to drive around without codes popping up? Only reason I ask is because I was not able to get my secondary o2 sensor out of my stock dp and I like to save money where possible. What do you guys think considering its not really doing anything anyway?

[Tokay444]

The secondary needs to communicate with the primary for proper ol/cl operation

[Spoofmastafly]

You can disable the CEL with your ap. But if you have any type of vehicle inspection that reads your OBII port your gonna get a not ready signal from that secondary o2 sensor. You're in luck though the secondary o2 sensor is the cheaper of the two.. And you can usually pick it up at autozone, napa, or checkers. I wouldn't risk not running that sensor personally.. Gotta pay to play , would you rather spend $$ on a new engine or on a pretty cheap
o2 sensor?

[speedms6]

first, if you dont have an inspection that will fail bc of a not redy code just disable the cel for it.

second if anybody tells you different here do not listen. the secondary o2 is necessary for closed loop fueling. and be careful buying the bosch or denso, i got the bosch and the cord was two short and also had a female plug on it so i was left with two female plugs. the best way to go about it is get an oem mazda one, you can prob find one cheap online i got one from a member off of here, but yes u need the secondary.

[sallyspeed3]

Okay thanks guys. I had no idea it was involved with closed loop fueling. I'm due for an oil change this week so I'll have a new one put in unless the shop can get my stock one out of there. But I have a feeling even if they do get it out that the threads will be jacked up.

[nofear72784]

Just buy an 02 simulator or make one... there just capacitors and resistors. I would like to see in writing a secondary o2 effecting fuel trims negatively and not allowing closed loop operation. The secondary o2 is only there to monitor the efficiency of the catalyst. It monitors the voltage to keep the cat at over 500 degrees.. when it cools it adds fuel and vice versa. It compares the pre cat signal to the post cat. If it never went to closed loop it would load the cat with excess hydrocarbons and destroy it. If anything vehicles would get better mileage without cats because the constant adding of fuel to keep the cat hot is a waste. Without a sensor it will see the lack of a heater circuit and defualt back to pre-programmed logic. I've removed several post cat sensors and never had an issue going into closed loop. The simulator simply makes it look like everything's good.

[thatsmrgimp2u]

Its been proved on this vehicle that it does infact affect your trims....


Correct me if im wrong but even if you clear the o2 in ap, you get another code for it not being there at all.

I was troubleshooting a lean at idle issue I had some time back... this was my first troubleshooting procedure (completely removing it).

Pretty sure the code that you clear in atr only disables the fact that it may be reading improperly, not the fact that it is absent.

[speedms6]

Originally Posted by nofear72784 Just buy an 02 simulator or make one... there just capacitors and resistors. I would like to see in writing a secondary o2 effecting fuel trims negatively and not allowing closed loop operation. The secondary o2 is only there to monitor the efficiency of the catalyst. It monitors the voltage to keep the cat at over 500 degrees.. when it cools it adds fuel and vice versa. It compares the pre cat signal to the post cat. If it never went to closed loop it would load the cat with excess hydrocarbons and destroy it. If anything vehicles would get better mileage without cats because the constant adding of fuel to keep the cat hot is a waste. Without a sensor it will see the lack of a heater circuit and defualt back to pre-programmed logic. I've removed several post cat sensors and never had an issue going into closed loop. The simulator simply makes it look like everything's good.

that doesnt mean this car doesnt use it. i never said it doesnt allow it, it will make ti inaccurate. i'll let the guys who are known on here chime in when the feel necessary.

but for example i had an o2 extender on, in closed loop it would go almost a full point rich for a while until enough exhaust gas reached the sensor, thus it was seeing so little exhaust gas it was compensating by adding fuel because it thought it was running lean, then as soon as enough gas reached the extender it would go lean for a second before settling, took the extender off and this completely stopped. now tell me it doesnt effect closed loop operation. open loop ran perfectly with or without the extender.

[nofear72784]

It does affect it to a point.. it thought you still had a cat and was attempting to add fuel to heat it up. If he kept his disconnected completely that wouldn't happen. It would see an open circuit and not add the fuel. Now it would cause other issues like the cel and failed emissions test. But, if that was the case an o2 simulator which blindly just cycles rich and lean would mess up trims and guys wouldn't be able to tune properly yet they have been used since obd2 came out in the later half of 1995.

[nofear72784]

Ok i reread the posts and I though I read in there someone saying that his car would never enter closed loop if the secondary o2 was removed. On all the vehicles I've ever monitored without cats and with simulators it never prevented it from entering CL. If your point was simply that it affects the fuel trims in CL then I agree because it is trying to attain the correct catalyst temp, I worded the incorrectly. But.. his question was removing it entirely.

[kmac]

Secondary o2 sensors never did or will effect fueling. It does not trim anything it just wants to make sure u have a cat and It's doing its job. Some manufactures got smart and if u have a secondary o2 code or detects no cat it pulls timing so u keep it, but u can over ride that with an o2 simulator.

[sallyspeed3]

Hmm...well shit, now I'm not quite sure what to do. Someone above mentioned it affected their CL fueling, but others are saying it doesn't, whats the consensus?

I don't want to remove the extender because I know I won't be able to weld in a cat before the sensor. I was thinking of having one welded in the test pipe section just for less smell and noise.

How much are O2 simulators and where can you get them? I'll do some googling as well and see what comes up.

[thatsmrgimp2u]

@djuosnteisn can you clear this up? I may have misunderstood our conversation about this when we were working on my car.

[Spirry]

Just have a new bung welded in and cap off the extender. Try googling how many not ready codes you can have in your county, and pass emissions.

[kmac]

It does not effect any fueling , only primary o2 sensors do. That's only there because of the federal mandate to have it for emissions. Other wise it wouldn't exist.

[ABSpeed3]

Originally Posted by kmac It does not effect any fueling , only primary o2 sensors do. That's only there because of the federal mandate to have it for emissions. Other wise it wouldn't exist.

It had an effect on my car. I ran without it for about 3 or 4 days and the car was running like overall shit and very rich. My trims were all over the place and the exhaust sounded fat. As soon as I got the new sensor and threw it in, my car was instantly hitting my AFRs and the exhaust note had a noticeable difference to it.

[SSinstaller]

Originally Posted by sallyspeed3 Hmm...well shit, now I'm not quite sure what to do. Someone above mentioned it affected their CL fueling, but others are saying it doesn't, whats the consensus? I don't want to remove the extender because I know I won't be able to weld in a cat before the sensor. I was thinking of having one welded in the test pipe section just for less smell and noise. How much are O2 simulators and where can you get them? I'll do some googling as well and see what comes up.

The secondary O2 on the DISI motors has two purposes.

1) To check for proper operation of the primary cat.
2) To correct calibration drift of the primary wideband O2.

All wideband O2's need to be calibrated for time to time(our primary sensor is one), but narrow band sensors are very very accurate at stoich and do not drift, Our ecm compares the signal from the primary and secondary O2 sensor and continually corrects it calibration of the primary..

I have seen the effect first hand. I had a diode installed on the signal of the rear O2 to prevent a MIL for no cat, and within a very short amount of time the ECM reported AFR as read by my scantool showed .4pts richer than the actual AFR as reported by my innovate wideband(installed in my DP and calibrated monthly). When I removed the diode the two wideband outputs matched again (within about 50 miles of driving)...

[karl-hungus]

sigh.

[thatsmrgimp2u]

Originally Posted by karl-hungus sigh.

His issue isnt it reading improperly...

Its stuck in his stock downpipe so hes checking to see if he can get away with out it.

[kmac]

Originally Posted by ABSpeed3 It had an effect on my car. I ran without it for about 3 or 4 days and the car was running like overall shit and very rich. My trims were all over the place and the exhaust sounded fat. As soon as I got the new sensor and threw it in, my car was instantly hitting my AFRs and the exhaust note had a noticeable difference to it.

If u have a shorted heater ckt it will effect the running of the veh . That's not because It's using it for trim .

[karl-hungus]

take the dp off, pb blast it, use a 7/8 inch or 22mm closed wrench and tap it with a dead blow hammer. problem solved.

[thatsmrgimp2u]

Id let the PB blast sit on there for a bit, maybe heat up the bung if you can. When i did my install i took them off when they were hot and they came out pretty easy.

[sallyspeed3]

Originally Posted by karl-hungus take the dp off, pb blast it, use a 7/8 inch or 22mm closed wrench and tap it with a dead blow hammer. problem solved.

Did that actually, also used a breaker bar and an airgun, but by then the nut had rounded off a bit so it wasn't as effective.

[nofear72784]

Well like I said it won't hurt anything if you need to drive the car without the rear o2. If it richens it up a little so what.. you can always get a universal o2 sensor... just make sure you get one with the same amount of wires. Use the included diagram and match up the signal/ ground and 2 heater wires to the factory harness, make sure you solder them. If my memory is any good the 2 blacks are heater circuit, white is ground the other is the signal.

[SSinstaller]

Originally Posted by nofear72784 Well like I said it won't hurt anything if you need to drive the car without the rear o2. If it richens it up a little so what.. you can always get a universal o2 sensor... just make sure you get one with the same amount of wires. Use the included diagram and match up the signal/ ground and 2 heater wires to the factory harness, make sure you solder them. If my memory is any good the 2 blacks are heater circuit, white is ground the other is the signal.

So, poor gas mileage, less torque, and high egt's are ok as well??

[timjs]

The rear O2 does affect fuel trims.

There is even a PID in IDS labeled "HO2SB12 fuel trim" or something similar.

I can call the tech line tomorrow to ask them if you guys want. To settle it once and for all.

Here is a snip from the service highlights manual.


Rear HO2S feedback
correction:

Purpose: Corrects feedback amount according to deterioration of
front HO2S and catalytic converter
Correction condition
• Engine coolant temperature is at set value or more
• Engine speed is 500—4,250 rpm
• Charging efficiency is 10—80%
Correction amount
• According to rear HO2S electromotive force→correction

[nofear72784]

Your missing the point. He wanted to remove it completely. The extender pulls it out of the exhaust path to try and trick the sensor signal to imitating a cat.. that will cause more fuel to be added since the ecu thinks the cat is too cold. It will not add fuel blindly.. it only adds fuel to maintain the cat temp. It will see an open heater circuit, know the sensor is not connected and not do anything besides turn on the CEL.
Please someone show me in writing from Mazda that I'm wrong. I was taught this 9 years ago from GM when they started widely using wideband o2's. I'm only trying to help this guy, I'm not just making this up off the top of my head. I was a Subaru tech, GM Master Tech certified for all platforms and a GM field engineer for 3 years after. I was an ASE L1 master tech at that time but now I work for the government due to a back injury so if things have changed in the past few years I would like to know.

[SSinstaller]

Originally Posted by nofear72784 Your missing the point. He wanted to remove it completely. The extender pulls it out of the exhaust path to try and trick the sensor signal to imitating a cat.. that will cause more fuel to be added since the ecu thinks the cat is too cold. It will not add fuel blindly.. it only adds fuel to maintain the cat temp. It will see an open heater circuit, know the sensor is not connected and not do anything besides turn on the CEL. Please someone show me in writing from Mazda that I'm wrong. I was taught this 9 years ago from GM when they started widely using wideband o2's. I'm only trying to help this guy, I'm not just making this up off the top of my head. I was a Subaru tech, GM Master Tech certified for all platforms and a GM field engineer for 3 years after. I was an ASE L1 master tech at that time but now I work for the government due to a back injury so if things have changed in the past few years I would like to know.

I think you're the one missing the point, the post directly above yours clearly shows that the rear O2 plays a role in maintaining proper calibration of the primary O2... It does not only monitor the cat. You keep preaching what you remember from your subaru/gm days, and we'll all move along based on the Mazda FSM, and our actual experiences with the Mazda we're all driving...


OP, If you have to drive without the rear O2 for a short while until you can get a replacement, just take it easy and expect poor mileage, but your most likely not going to grenade your motor.. Long term it's a very bad idea.

[nofear72784]

If you could call techline and post what they say I'm interested. I was unaware of any current system that allowed for on the fly ecu correction of the wideband from a narrowband secondary. I would like to know what they say would happen to the correction table if it was removed.

[timjs]

I don't see where the argument is.
We know that the rear O2 sensor does influence the fuel system.

However, if it were removed, I'd expect the PCM to ignore it's input entirely, why? Because there isn't an input to take into account anymore.

The MIL would illuminate.
Codes will be set.

And the PCM will continue to operate normally.
There might be a little variance over time from the wideband "drifting" with age..but nothing short term that should mess anything up.

Just my thoughts, I haven't actually removed a rear O2 and observed its effect.

[sallyspeed3]

Regardless of who is correct, I appreciate all the input. Tomorrow I'm getting an oil change and my snow tires put on and hopefully they'll be able to get the stock sensor out of my dp that way I don't have to pay for a new one. But if its ruined I'll be getting it replaced.

[xtasy]

Be happy that it wasn't your primary o2 that got stuck. Costed me 230$ for a new one

[sallyspeed3]

Originally Posted by xtasy Be happy that it wasn't your primary o2 that got stuck. Costed me 230$ for a new one

Oh I was very happy about that. Not to rub it in for everyone who had lots of problems doing this install, but I was almost in shock at how easy it was (except for the stupid bracket attached to the dp) Maybe I'm just lucky.

[mrQQ]

guys, sorry to bring back up the old thread, but I have a related question:

how does the ECU use the second o2 sensor, if the cat is removed? Wouldn't it throw off the calibration stuff?

I want to add a high flow cat to my downpipe, but there is no place for it except after both o2 sensors.

[timjs]

Originally Posted by mrQQ guys, sorry to bring back up the old thread, but I have a related question: how does the ECU use the second o2 sensor, if the cat is removed? Wouldn't it throw off the calibration stuff? I want to add a high flow cat to my downpipe, but there is no place for it except after both o2 sensors.

Not sure what the true effects are but I can tell you this, my cats have been gone quite a while now and I left the rear o2 installed and even plugged in. The only DTC set is for catalyst efficiency (naturally) so the code is disabled in ATR.
Nothing has changed since my tune, and if the primary 02 sensor was out of any type of calibration or anything of that nature, it would show in the fuel trims over time. They haven't moved, pretty much at all. All slightly positive of 0 most of the time just like always.

[skiptowncat]

not had a secondary o2 in for about 3k miles now with no issues

[Tokay444]

@Jasper_3MPS;

[SSinstaller]

Originally Posted by timjs Not sure what the true effects are but I can tell you this, my cats have been gone quite a while now and I left the rear o2 installed and even plugged in. The only DTC set is for catalyst efficiency (naturally) so the code is disabled in ATR. Nothing has changed since my tune, and if the primary 02 sensor was out of any type of calibration or anything of that nature, it would show in the fuel trims over time. They haven't moved, pretty much at all. All slightly positive of 0 most of the time just like always.

Your primary sensor is what monitors your engine AFR to set the fuel trims.

Why would you expect the trims change if it's calibration was off?

The only way to see the problem is with a calibrated aftermarket wideband installed to compare to the OEM sensor readings. What you will eventually see is the actual AFR drift rich while the OEM sensor continues to show your targeted AFR..

[timjs]

Originally Posted by SSinstaller Your primary sensor is what monitors your engine AFR to set the fuel trims. Why would you expect the trims change if it's calibration was off? The only way to see the problem is with a calibrated aftermarket wideband installed to compare to the OEM sensor readings. What you will eventually see is the actual AFR drift rich while the OEM sensor continues to show your targeted AFR..

See post 26.

If the primary sensor swayed from actual AFR the fuel trims would show it.
Let's say you're burning 14.7 with perfect 0 trims. Fast forward 6 months and you have some level of "deterioration" of the sensor.
Now the true 14.7 is read by the sensor as something, let's say leaner.
The PCM would correct by richening the mixture until the sensor tells it that it's burning 14.7 again. In reality, you would have a richer than target mixture, and positive fuel trims to show it.

It would probably have to be an excessive amount of deterioration to cause this I would imagine, and at that point you would probably have already had other driveability issues and replaced the sensor.

[SSinstaller]

Originally Posted by timjs See post 26. If the primary sensor swayed from actual AFR the fuel trims would show it. Let's say you're burning 14.7 with perfect 0 trims. Fast forward 6 months and you have some level of "deterioration" of the sensor. Now the true 14.7 is read by the sensor as something, let's say leaner. The PCM would correct by richening the mixture until the sensor tells it that it's burning 14.7 again. In reality, you would have a richer than target mixture, and positive fuel trims to show it. It would probably have to be an excessive amount of deterioration to cause this I would imagine, and at that point you would probably have already had other driveability issues and replaced the sensor.

I understand what you're saying, but it just doesn't fit what I've seen on my own vehicle.