O2 sensor heater circuits have a relay in them because of the high current required for the heater. The problem could be the relay, the wiring from the ECM to the relay, wiring from the relay to the O2 sensor. There's a short (most likely) in this wiring somewhere (or the relay).

Regards,
paul...

 

I am having a similar issue. I ran a good obd2 reader on the car while in live data mode and had it reading .6-1.1 Volts as I had the engine reving at different rates. I used the Denso brand sensor. The previous sensor cable broke riding tight to the subframe. Ill try and figure out how to post photos. Looking for advice specs or maybe even solutions others have found. My foresters got the EJ251 sohc N/A 4speed auto

Woah old thread

 

It would be helpful if you update your profile with your Forester's details, year, trim, etc. 2009?

Mike

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www.subaruforester.org

 

@subatron Yea I updated my status but I'll add that here too. I got an '04 Forester X trim with 225k miles recently did timing and headgasket job and now it's throwing different o2 codes rn

 

@Jakester09 what CEL codes do you have?

Bobby...

['07 FSXT Member Journal] ['03 X Member Journal]

 

Back with a update. Bought a multimeter to test resistance and voltage at the wire harnesses. I have a good voltage at the in car harness. Very high olms /resistance. Up around 70 out of reading the 200 setting on the meter. Still have yet to test actual new sensor. Will start uploading code photos and possibly videos now.

First code

Second code which replaced first with the install of fresh sensor

Broken old sensor

Also the car runs fine just a a slightly high ram when the O2 sensor is unplugged. I just don't feel like going around town with big ol hole

Going to watch how to test the sensor itself now and update soon. I'll try to post a link to the video I'm watching aswell, it's well explained, better than I can do

 

@Jakester09 I see 3 wires on that O2 sensor. Does it have 3 or 4 wires?

Bobby...

['07 FSXT Member Journal] ['03 X Member Journal]

 

@2.5x_sleeper 4 wires. It's a heated sensor. Preheats to supposedly upwards of 750 degree F when starting colder than that. This give the sensor an accurate reading very fast which is better for emission control and fuel economy at startup. It's much more common in newer makes from what I understand. My 92 chevys are not heated

 

This is the orientation I'm referring to.1.2 ohm readout on the bottom two

Good youtube link

 

Flipping the ground and the positive the other way result is about the same. Jumps in at 1.5 the quickly drops to 1.3 the 1.1 slowly. This occurred on the first test of the bottom two lines.

The top shows no reading. I believe that's the portion run by the ecm/ecu. Which is it own circuit from the bottom for the heat, as a continuing circuit to always be heated. Not sure but I think so...

 

@Jakester09 below is the troubleshooting procedure from the Subaru Technical Service documents for the '03-'04 non-turbo.

Bobby...

['07 FSXT Member Journal] ['03 X Member Journal]

 

That's all the testing I can manage to squeeze in tonight. I may not have the right idea of how all this works but the sensor itself looks to be just fine. And the same circuit in the hood or firewall/ dash has a short creating more resistance. Which we see with the high andout of spec ohm readout or 70 when maximum is 50. If you have an idea of where I can check for broke wires? Know what gauge wire this may be? Or have any tips on using a multimeter? I'd love to hear them. Thanks for the help so far!

 

Today I'm going to unhook the battery and let it drain to do a more hard reset on the ecu then try and see if the wiring is shorted anywhere in the engine bay

Battery is drained with a brand new warrantied sensor installed. We got rain so I'm waiting for it to pass before I go back out

Still not keeping itself running. No cel or pending codes. I'm stumped unless I have a short. Recommendations?

 

O2 sensor codes are now gone. Had a short in the wiring from what I found. But now it is throwing codes for the Idle Air Control. I replaced the unit with the Autozone brand part. Now I still have iacv code and car will not run with out throttle.

 

Here are a list of codes I now have. No breaks in cable on this for the first foot or so. I'll also add a photo of the broken iacv

 

Broken IACV