Secondary O2 sensor needed for Cruise control?
 

Backstory first...

Removed my secondary O2 sensor from my Borla UEL and installed a wideband sensor in its place. I have an ECUTEK tune, so I figured that the secondary oxygen sensor checking is removed and I thought nothing of it.
The next day I start the car and begin to drive. The CEL comes on after a few minutes, and when I press the cruise control on/off switch the cruise control light comes on but will NOT turn off. The wideband is also reading a rich air/fuel ratio (13.2) at idle when the car should be in closed loop (fully warmed up etc.)
Its almost as if its stuck in open loop or some sort of catalyst self-test mode where it runs rich at idle and doesn't use primary O2 sensor to trim fuel

WOT shows normal A:F of low 13's so I believe the wideband is getting accurate readings.

Does the ECU need the secondary O2 hooked up for some reason?
I know there was a thread stating that you don't need the secondary (as is common on older OBD-II cars), but it is creating some strange problems for me.

Anyone with any input?

I'd just have an extra bung welded on for your wideband. I imagine any exhaust shop should be able to do it for you.

I would rather just use the available bungs on the header due to them being there and my not needing a secondary O2 sensor anyways....

I may just make a dummy plug to simulator the secondary sensor and have it plugged in so the ECU thinks its there.

I just find it hard to believe the ECU will disable cruise control and make the car run rich without the secondary O2 plugged in

Its probally a fail safe, the O2 sensors are heated and and the car will stay in open loop mode until warmed up. In this case it can't read the second 02 so it stays in fail safe not allowing cruise and running rich.

You may have to ask specifically for the downstream HEGO sensor check to be disabled in your tune, it may not be there by default. Check with your tuner.

Cruise control is more than likely disabled when the vehicle is in a limp mode, or has DTC's to be cleared, in the interest of safety.

I think a resistor will fix it, but I don't know how many ohm's of resistance it is.

13.2 isn't "rich" mid to low 12's or 11's or 10's is rich and getting richer.

13.2 is pretty spot on IMO

On the cars I have tuned, closed loop idle target AFR is 14.7 unless the FRS/BRZ is different

My main concern/question is that it looks like the ECU is running open loop with a missing secondary O2 sensor, to which there is debate about if the secondary O2 sensor is actually used for minor fuel trim adjustments on subaru engines.

I will plug in the seondary O2 tonight when I get home and see if it fixes the idle AFR readings

In terms of stoichiometry 14.7 is a lean mixture

14.7 is correct for target closed loop afr. Unless it has been specifically altered somehow.

For pure gasoline 14.7 is Stoic, but with ethanol added (common 10% ethanol fuel) the "stoic" ratio is less. However, I still believe that the ECU has a target AFR of 14.7:1 at idle/cruise in closed loop operation

As far as I know, all cars shoot for Lambda = 1 in closed loop... unless some are compensating for 10& ethanol in their fuel, which would be around 14.2:1 stoic which may explain my "rich" readings

That's not always the case these days, although generally in the past it was when cars were only fitted with narrow-band HEGO sensors.

A lot of modern vehicles have wide(r) band sensors fitted and closed loop targets away from lambda 1.0.

Any cel disables cruise control.

Lots of good info in this thread.

Does any of the tuners on here familiar with this platform specifically know what the target lambda/AFR is during closed loop operation?

Also this is my first drive-by-wire car, so I did not expect cruise control to be disabled if there is a DTC code present... but it makes sense in a way.

not at idle.. should be targeting 14.5 - 14.7

.. some of the 32bit wrx's have similar issues even with the 02 checks turned off in the rom... hope that is not the issue... resistor packs did not solve the problem for them.

E10 will still show 14.7 at stoich because stoich for any fuel is lambda 1.0 and the rom is calibrated to translate that to gasolilne afr so 1.0 - 14.7.

This car has a closed loop target afr table set that is as low as 14.2 in some places... the actual target (easily logged with ecutek or brzedit) varries from 14.2 to 14.7 depending on a number of things (like some emissions strategy based on cat temp etc). I posted the table in my openloop / fuel trims thread a couple days ago.

Should still be able to disable cels..... Mine and a friends 11 wrx's both ran catless and with multiple tuning solutions disabled the o2 cel.

Yes, that is 100% true. Did you or your friend notice any changes to your afr? I read a bunch of stuff about the rear 02 issues etc. I assume you could adjust for it once you disable the checks etc.

@DJCarbine did you pull the codes yet just to be sure they are not unrealted? Code numbers might also help the tuner know exactly what needs to be disabled.

My tune disables/modifies the secondary catalyst checks, it ran great with no CEL for around 2K miles.

I know there is a bunch of talk in the subaru community about removal/relocation of the rear O2 sensor causing strange fueling issues, but I saw a thread on here where a member disproved this. The subaru guys seem fairly convinced that newer Denso ECUs do NOT like the rear O2 disconnected/moved out of the exhaust stream.

The codes are for O2 sensor heater circuit and no sensor activity from what I remember....

I will double check and grab the exact DTCs in an hour or so when I get home.

If all else fails, I can just run the Innovate narrowband output to the ECU, but I wanted to avoid splicing into the factory harness

You're going to need to talk to your tuner either way.

Disabling the CEL for low catalyst efficiency (which most tunes do in order to work with aftermarket headers/cat removal) does not disable the codes that will be thrown when the sensor is missing all together.

I can always just plug the sensor back in and keep it out of the exhaust stream (ziptied out of sight), but there might be fueling issues according to the other subaru guys....

However, the secondary is just a switching O2 sensor, so as long as it reads leaner than the primary it should be fine.

I will do testing tonight and report back

this seems on point with my thoughts.

So with an ECUtek tune and no secondary O2, the car is actually in closed loop.
The ONLY DTC code was for an open O2 heater circuit, nothing else.

I have my O2 sensor plugged in and ziptied out of the way for now to get rid of the CEL. I am going to look into fooling the ECU with a high watt resistor for the heater circuit so I can get rid of the second sensor completely.

AFR target is 14.7 at idle, and I was hitting that.... I need to recalibrate my wideband, as it is reading off from the factory wideband

you should be able to turn off P0030, 31, 32, 37, 38 with ecutek, I have them in brzedit so i assume ecutek does as well.

We left the sensors plugged in just deleted the cel.

Just ordered a 6ohm 50watt resistor. Its got an aluminum body with small heat fins. I am going to try that.

Do you have a catted midpipe? maybe buy longer cables and go around that cat vice the header one??

No cats at all, and catalyst efficiency DTC is removed in my tune. I would just rather not weld in another bung when I have a perfectly good one that is taken up by a useless sensor lol

The resistor should do the trick, service manual says the heater circuit resistance is between 5 and 7 ohms or thereabouts. 6 ohms should trick the ECU

I'm having the same issue, running catless UEL header, Ecutek tune, my CEL light came on with cruise control being used. CEL does disable the cruise control. My code was P0137, low voltage secondary o2.

You need to have the secondary O2 disabled through Ecutek. The only one that should be used is the primary.

Just an update, ecutek tune can disable the dtc if you have no secondary o2 sensor, just ask your tuner

I had the same issue
 

I had the same issue i had some dc sports headers installed and nothing happened i drove for like a month and then i was on the highway and ia had my cruise control on and it shut down and the check engine light came on.I took the car to the dealer and they said it was bc of the o2 sensor so i heard that by installing a spacer on it you can fix the problem.