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Old 05-25-2021, 11:51 AM   #1
guciodog11
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Default O2 sensor issues

Hi everyone,

Having some issues with my front o2 on my RSTi. No CEL regarding the front sensor however on cold or partially warm startup, the AFR's go wild however readout of sensor is a rock solid 14.7 even though true AFR jumps lean to rich cyclically per my wideband. It seems to activate after 2 or so minutes, and AFR stabilizes.

I could just grab a new one and test but I'd thought I'd ask for advice before spending money.

Is there anything I can test or do to get the sensor respond immediately?

Thank you!
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Old 05-25-2021, 12:17 PM   #2
GeoNV
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you can datalog AF sensor #1 current and resistance. It sounds like the O2 sensor isn't being heated properly, and so only comes up to temp via the exhaust gasses warming it. if you log the current and resistance and it is sending current then it is likely the o2 sensor's heating element has gone bad.
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Old 05-25-2021, 12:39 PM   #3
guciodog11
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Quote:
Originally Posted by GeoNV View Post
you can datalog AF sensor #1 current and resistance. It sounds like the O2 sensor isn't being heated properly, and so only comes up to temp via the exhaust gasses warming it. if you log the current and resistance and it is sending current then it is likely the o2 sensor's heating element has gone bad.
Didn't even know I could log that, thanks!
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Old 05-26-2021, 11:59 PM   #4
AliBenn
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Quote:
Originally Posted by GeoNV View Post
you can datalog AF sensor #1 current and resistance. It sounds like the O2 sensor isn't being heated properly, and so only comes up to temp via the exhaust gasses warming it. if you log the current and resistance and it is sending current then it is likely the o2 sensor's heating element has gone bad.
Wow
Awesome diagnosis
Sounds Golden

My Q
Is it possible for this issue to occur without OBD code?
Guessing that if not noticed early, CE would eventually come on?
Thanks man
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Old 05-27-2021, 10:50 AM   #5
GeoNV
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yeah, I'd imagine it would eventually trigger a CEL. not sure how bad it would have to get before it triggers the code.
https://www.scoobymods.com/showthrea...nt-12146.html?
has a bit more info.
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Old 05-27-2021, 10:51 AM   #6
GeoNV
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Also, I bought my O2 sensors from Rockauto.com as they were way cheaper than Subaru.
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Old 05-27-2021, 10:10 PM   #7
AliBenn
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^^^awesome thanks bud
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Old 05-29-2021, 08:14 PM   #8
guciodog11
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So I did some digging around with Romraider and a voltmeter today, here's what I found:

Cold start, few seconds after startup.
https://i.imgur.com/oMVa93n.jpg

After 4-5 mins idling:
https://i.imgur.com/1fULxJW.jpg

255 ohms i assume means open circuit, no idea why it drops to 30 when it's warm.

Tested the 2 black wires on the O2 sensor itself and got 2.2 ohms when cold, 2.7 after idling.

Disconnecting the O2 while running made zero difference to idle or AFR which is a bit suspicious. Also gave me low voltage CEL which was of course expected, but surprisingly nothing about the heater circuit.

Does anyone think it might be the sensor? I don't think theres a dedicated fuse or relay for the o2 heater circuit so not sure where to even go with this if the sensor is ok.

Thanks!

Last edited by guciodog11; 05-29-2021 at 08:24 PM.
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Old 05-31-2021, 01:07 AM   #9
SlammedSix
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Just out of curiosity what software are you using for your setup?
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Old 05-31-2021, 09:56 PM   #10
guciodog11
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Quote:
Originally Posted by SlammedSix View Post
Just out of curiosity what software are you using for your setup?
Nothing special, just Romraider Logger through a Tactrix cable.
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Old 05-31-2021, 10:15 PM   #11
SlammedSix
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I've had a couple of my buddies cars that are using Carberry and a speed density tune and they had this issue. I found the issue was related to the tune.

Edit....there cars AEM wideband went lean as well as the cars OEM wideband sensor. Sound like your sensor is bad.

Last edited by SlammedSix; 05-31-2021 at 10:21 PM.
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Old 05-31-2021, 10:34 PM   #12
guciodog11
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Quote:
Originally Posted by SlammedSix View Post
I've had a couple of my buddies cars that are using Carberry and a speed density tune and they had this issue. I found the issue was related to the tune.

Edit....there cars AEM wideband went lean as well as the cars OEM wideband sensor. Sound like your sensor is bad.
Looks like I'll have to pick up a new one! Tried to do what I could to not spend money buy I guess that's unavoidable.

Thanks!
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Old 06-17-2021, 12:23 PM   #13
guciodog11
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Bought a new sensor, no dice.

Heater is still out of commission. I checked the body harness and it's getting 12V from the main relay.

Curiously, my Oil and airbag light do not light up with ignition on.....looks like I have a grounding issue!
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Old 06-17-2021, 07:12 PM   #14
AliBenn
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Damn
Sorry man

Be sure to look for critters that chewed wires
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Old 07-23-2021, 09:29 AM   #15
guciodog11
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Oil light was my fault, turns out plugging in the connector makes sensors work!

and 95 imprezas didn't have airbag lights so that's out of the picture.

Still having an issue with the ground loop of the o2 heater circuit :/
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Old 08-03-2021, 10:38 AM   #16
guciodog11
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Came up with this ****ty diagram:
https://i.imgur.com/TfbQMDj.png

1. 12V from battery and SBF-5 is solid, and can pull 5+ amps without a problem.

2. sensor ground to ECU is solid (0 ohms)

3. signal + and - are solid (0 ohms), ecu recognizes correct AFR after 2-3 minute warmup time.

4. pin 7 to engine ground is solid (0 ohms)

5. Resistance from "Ground to ECU" to battery neg terminal is 37-40k ohms, regardless of key position.

So now I have absolutely no idea where to go forward with this issue.

Can anyone help me out and get this thing up and running properly? Thanks!
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Old 08-03-2021, 11:08 PM   #17
AliBenn
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Sorry Gdog
Imgur does not work here anymore

I hate electrical
Did u test O2 real time bench test ? Like with lighter/blow torch?
See the ohms actually go up/down?
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