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Mazdaspeed Archive => Mazdaspeed Miata Tech issues => Topic started by: PK1 on October 04, 2016, 05:47:46 PM



Title: Wideband O2 sensor w/ stock ECU at OBD-II time
Post by: PK1 on October 04, 2016, 05:47:46 PM
I wanted my yearly OBD-II work to be limited to switching the plugs back and forth b/w my newly acquired Hydra and the stock ECU so I opted to stay with the stock injectors. What I didn't know was that FM recommends physically removing the WB when switching back to the stock ECU. Apparently leaving it in place can damage it - even in the week or so a year that it will be disconnected from the Hydra - since the heater in the sensor won't work without Hydra.

Just curious: those of you who do switch to the stock ECU for occasionally, do you remove your WB or leave it in place? Any issues? If so, are there any workarounds to avoid removing it? Removing a stuck Sensor once a year in that tight space won't be much fun and I don't feel like having to take the exhaust down either.


Title: Re: Wideband O2 sensor w/ stock ECU at OBD-II time
Post by: SilverMiataRacer on October 04, 2016, 05:52:57 PM
First, you are putting the WB in a different bung from the stock O2 sensor, right?

Second, are you leaving the stock sensor in when you are running the Hydra?

Third and this is a what if as I have no knowledge of this working or ever having been done.  Could you rig a plug to plug into the WB when it's not in use and connect it to power for the heater to have the heater on when the car is on??  If the reason for them failing, and I don't know that this is it, is that the heater isn't on when the sensor isn't in use maybe you could just turn the heater on manually whenever you use the car in stock ECU mode.


Title: Re: Wideband O2 sensor w/ stock ECU at OBD-II time
Post by: PK1 on October 04, 2016, 06:05:00 PM
First, you are putting the WB in a different bung from the stock O2 sensor, right?

Second, are you leaving the stock sensor in when you are running the Hydra?

Third and this is a what if as I have no knowledge of this working or ever having been done.  Could you rig a plug to plug into the WB when it's not in use and connect it to power for the heater to have the heater on when the car is on??  If the reason for them failing, and I don't know that this is it, is that the heater isn't on when the sensor isn't in use maybe you could just turn the heater on manually whenever you use the car in stock ECU mode.


Yes to the first question, I installed it in the bung that goes before the cat. (Full FM exhaust system).

Also yes to the second question, the stock one is installed after the cat. Basic question: does Hydra still use the stock O2 sensor? If not (and assuming that one is heated too), then the same issue would exist wth the stock sensor!

And as for the third, this is the first question I asked Jeremy when he told me I needed to remove it. His response was that Hydra controls the heater, it's not on all the time!

In all the reading I've done I've never come across the need to remove the WB sensor so I'm hoping the chances of failure are remote and I can leave it in place.


Title: Re: Wideband O2 sensor w/ stock ECU at OBD-II time
Post by: rotorglow on October 04, 2016, 06:46:47 PM
I leave the WB in place for a week each year.  I probably wouldn't drive cross-country like that, or cold-start it 30 times, but because it's usually only for something like 25 miles or so over 3 or 4 trips, it's been no big deal.  

Also yes to the second question, the stock one is installed after the cat. Basic question: does Hydra still use the stock O2 sensor? If not (and assuming that one is heated too), then the same issue would exist wth the stock sensor!

Your answer is confusing.  There are TWO stock sensors.  One before the cat, one after.  Yes, the Hydra can indeed use the stock sensor before the cat, for AFR control wherever the map calls for stoich (14.7:1).  It does not use the rear one; some people pull it out to keep it from dying, but mine is still the original one from the factory.

Note that you can choose to use the wideband for stoich instead of the narrowband, but it probably doesn't react as quickly as the narrowband.  Regardless, it's helpful to leave the front narrowband in as a sanity check on what the wideband is doing, to say nothing of making a changeover to stock easier.


Title: Re: Wideband O2 sensor w/ stock ECU at OBD-II time
Post by: SilverMiataRacer on October 04, 2016, 06:53:11 PM
The O2 sensor after the cat (in the cat on a stock exhaust) is not the O2 sensor that control the stock ECU fueling.  It is to check the condition of the cat an d report that it is working.  They Hydra does not use that one at all.  It can use the stock NB which is up in the fitting by the turbo.


Title: Re: Wideband O2 sensor w/ stock ECU at OBD-II time
Post by: PK1 on October 04, 2016, 08:54:39 PM
Thanks guys. Yes, in this context I was only referring to the stock NB that is after the cat. Here's what I have:

- the OEM sensor by the turbo is still the original one, I haven't touched that one at all
- when i installed the FM exhaust a month or so ago, I changed the rear NB sensor with a new Bosch unit and spliced it in the original harness. I'm not overly worried about this one. If it fails over time when not used it's cheap and easy access with the FM setup
- I just installed the WB that came with my Hydra before the cat.  This one I'd rather not have to mess with every year. It's nowhere near as bad as changing the one by the turbo, but I'd still rather not have to mess with it every year

Sounds like keeping he WB and keeping the time and number of starts to a minimum may not be too bad an issue. I'll give it a shot in 9 months!