VersaTune User Forum

Masking the secondary O2 sensor DTC does not stop the ECU from reading it and using it for trims. It just stops the DTC from appearing and lighting the light on the dash.

The solution to this is PHYSICALLY remove the secondary O2 sensor wiring (just unplug the pins where it enters the ECU).
Then do not forget to disable CEL for that (low voltage, etc, etc).

The reason is that ECU tries to lean out to min desired lambda when it reads something which makes it think like you need to add fuel for emissions. By removing secondary O2 completely this logic is blocked an no longer presented.

For whatever the MS6 doesn't like to follow commanded AFR above 1 lambda. My guess is there are tables that have not been discovered, or we don't have access too, that affect the overall trim. I still command 1.12 lambda at cursing, and consistently only get 1.08 or so actual AFR. This happened on my Cobb Accessport as well, and I have replaced both O2 sensors over the years and it never had an affect on the end AFR. I know of several MS3 that are able to run 1.12 without issues, so it is definitely an issue with the MS6 platform.

ShakesReaper wrote:Update on the tune.

When the tune was still fresh, the car was commanding 1.021, but as soon as everything settled, commanded AFR went back to 1

Make sure that all your tables for closed loop are commanding the same lambda. 1.021 isn't that significant of a change TBH. I'd say anything less than 1.08 and your not really gaining anything in terms of gas savings. I've ran as lean as 1.15 lambda without major issue, although I had low speed cold surging. Once the car fully warms up it ran like a champ. I often wondered if the lack of VTCS contributed to the cold low rpm surging as well.

All my tables are 100% the same. closed loop as well as partial throttle. I do realize that 1.021 isn't the biggest change, but I would like to take it up in small steps.
I first want to get my car to follow what I tell him to before going more lean.

I'm commanding 1.021 lambda in the low end and my car follows it. What I did was remove the secondary O2 sensor and remove some DTC errors related to it to avoid the ECU thinking that it needs to heat the catalytic converter.

@mituc; 1.021 vs. 1 lambda means that in a 125 km/h. trip I'm seeing 9.8 l/100 km. vs. 10.6 l/100 km. which is not bad at all. Going as low as 1.088 lambda allows you to save fuel up to 8.6 l/100 km. but your NOx will be high and your catalytic converter won't last much.

Thanx.

Is there no way of disabling the second o2 sensor without having to disconnect it?
I am not too worried about my Cat, because I have none.

What bothers me is that why will the ecu try and achieve 1 lambda when in the flashed values it should target 1.021 (according to my map)? what other maps are being overturned by something else that we do not know of and why only the 6MPS?

No, there's no way of disabling it otherwise. There's no way to make the ECU logic to ignore it because we only have access to the calibration tables (or a good amount of them but not all), not to the actual logic.

I think the ECU logic is trying to mess with the AFRs in such a manner that in closed loop it expects a certain voltage from the secondary O2, not a specific AFR which we command. So if we command an AFR which the secondary O2 doesn't like... we end up having the AFRs at that level where the secondary O2 voltage feels good.

Hi all. Been a while since I tuned my car and is in the middel of doing a complete remap.

Getting back to the second o2 sensor: I have no CATs, so no worries there. What does the second o2 sensor do and what does the first one do? I know they measure the exhaust stream to calculate the afr's, but why 2?

What are the disadvantages of disconnecting the second o2 sensor?

Thanx guys

The first O2 sensor is the one responsible for measuring the AFRs, this is the reason why it is placed before any cat. Based on its input the ECU logic determines the fuel trims and that means a pretty important job of continuously adjusting the intake and fuel injection stuff.

The second O2 sensor is there only for emissions control and will lit a light when there is too much fuel and carbs coming off of the exhaust in closed loop operations. Its input is also taken into consideration when the idle or near idle AFRs are established.

So if you have no cats then the secondary O2 sensor will only be a pain in the butt, it will whine all the time about something. Of course you can fit it on a higher bung but that is not a solution. You can simply disconnect it and turn that alarm off. With it disconnected you will also be able to run leaner AFRs in closed loop/low load situations to improve the cruising fuel economy (on long trips where you stay mostly with the cruise control on on the highways the fuel economy can be improved visibly if you lean the low load AFRs to about 15.5-16).