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Sendero
04-25-2005, 01:12 PM
Why does the car trip a Check Engine Light with the rear O2's disconnected even though they are turned OFF in the programming?

-and-

Why do the Wideband installation instructions tell you to put the O2 sensor in upstream of the cats because it will give inaccurate readings if the sensor is placed downstream of the cats when all major dyno's use a Wideband sensor that sticks into the tailpipe?



(I really don't know these answers, I am just asking)

Italian LX
04-25-2005, 01:20 PM
Why does the car trip a Check Engine Light with the rear O2's disconnected even though they are turned OFF in the programming?
What rear O2 sensors? :shrug:

Why do the Wideband installation instructions tell you to put the O2 sensor in upstream of the cats because it will give inaccurate readings if the sensor is placed downstream of the cats when all major dyno's use a Wideband sensor that sticks into the tailpipe?
What's a cat? :shrug:






:jester:

QWKSNKE
04-25-2005, 02:07 PM
Why do the Wideband installation instructions tell you to put the O2 sensor in upstream of the cats because it will give inaccurate readings if the sensor is placed downstream of the cats when all major dyno's use a Wideband sensor that sticks into the tailpipe?



Because the cats do remove part of the fuel that is coming out (which is part of their job)of the exhaust.

I believe we have already discussed issue between 'tailpipe' widebands and real widebands. The tail pipe ones are not as accurate but at the same time I do believe they sample a little different.

93Cobra#2771
04-25-2005, 02:21 PM
Yes, cat's burn even more of the unburnt fuel left over from the combustion process. And in the case of 93 and older fox bodies, the air tubes actually pump oxygen into the cats to keep them nice and hot.

However, at WOT, the cats don't work nearly as well. Don't know how, if any, differnent tailpipe o2's sample the air behind the cats.

Sendero
04-25-2005, 03:05 PM
I can live with that.

How bout an answer for #1? :D

Ponygirl
04-25-2005, 03:16 PM
What's a cat? :shrug:

:jester:

You know furry, purring awesome creatures! Like this:

QWKSNKE
04-25-2005, 03:22 PM
You know furry, purring awesome creatures! Like this:

:lol:

Italian LX
04-25-2005, 03:29 PM
You know furry, purring awesome creatures! Like this:
Wow, I can't belive it took 2 hours before someone finally came back with that. . . however, I knew Shelly wouldn't pass it up. :D

Italian LX
04-25-2005, 03:30 PM
:lol:
:slap:

What are you laughing at. . . don't you have some Chevy 350 you need to be working on somewhere. :P

QWKSNKE
04-25-2005, 03:30 PM
about to head over there now

Sendero
04-25-2005, 04:34 PM
Wow, look... a :hijacked: thread. :wave: :jester:

Wicked
04-25-2005, 05:03 PM
Why does the car trip a Check Engine Light with the rear O2's disconnected even though they are turned OFF in the programming?

-and-

Why do the Wideband installation instructions tell you to put the O2 sensor in upstream of the cats because it will give inaccurate readings if the sensor is placed downstream of the cats when all major dyno's use a Wideband sensor that sticks into the tailpipe?


The car probably gives a check engine light as the sensor itself completes a circuit. Even if the programming does not use the input, it still needs a dummy resistance there. Just a guess based on other experience.

At WOT, the cats are not going to have enough time to do their job and therefore will make little difference in the A/F ratio. At cruising when the exhaust gases are moving much slower is when you would see the most error because of the longer time the gases spend in the cat.

86GT
04-25-2005, 05:07 PM
All of the EEC-IV and EEC-V's were designed for O2's being present. The scalar that you are setting to turn it off may accept a value of zero, but the EEC may see that and know that it is wrong and reverts to some know value.

I am just guessing, but I don't think '0' is a valid number.

93Cobra#2771
04-26-2005, 07:08 AM
Yeah, seems like I recall a discussion about it not working on the tweecer board, but I generally don't read 94+ threads....

Sendero
04-26-2005, 09:51 AM
I am just guessing, but I don't think '0' is a valid number.

If you are refering to the odbii_tst-switch scalar, I have a value of 250 which is off (251 is on). This turns off the rear O2's (supposedly) because I have a Dr. Gas off-road X-pipe. This worked because I never did install my MIL eliminators and I've had the X-pipe for over a year.

Now, I pulled the codes yesterday and they are:

P0156 Downstream Heated Oxygen Sensor circuit fault - Bank No. 2.
P0161 Downstream Heated Oxygen Sensor Heater circuit fault - Bank No. 2.

Obviously the switch I turned off doesn't turn off the circuit continuity check. This brings me to my next question.

If I just plug the O2 back in, but leave it hanging out of the pipe, will the heat circuit continously run? What about when the ceramics crack because of moisture, will it throw the light again?

86GT
04-26-2005, 10:10 AM
The A9L has a scarl that says something like "Num Of HEGO". That is the one that I was refering to.

93Cobra#2771
04-26-2005, 02:58 PM
I wouldn't leave them hanging. Throw a post on the tweecer board and see what they say...

Doug904
05-01-2005, 08:18 PM
Using the SCT software you have to turn both the rear o2 sensor switch and the rear 02 sensor heaters for both 02's. If you just turn rear 02 sensor off it will still code just as you've described.

later Doug.