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Old 08-05-2019, 04:15 PM   #1
Jaakko
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Join Date: Aug 2019
Default O2 sensor heater circuit problem

Hi all, just registered here. I hope someone can help me with my lambda problem, please!

Impreza STI 08 (EJ207 JDM, twin scroll)

The o2 sensor heater works randomly. (I use android torque app.) Sometimes it will fire up instantly and sometimes (usually) you have to drive 15 mins before o2/afr will be activated. Car runs fine when itís working. Normal driving AFR 14,7 and WOT below 12. But when itís not working, it will hesitate and do other evil things...

I checked the heater circuit and it was fine (2-3ohms). I got +12 volts from harness but if i plug the sensor back in it shuts down power. Even when the engine is cold. I just bought new sensor (oem) but itís the same.

O2 fuse is good, relays SBF 5 and 7 looks good shape also.

No trouble codes. The tuner said that ecuís and o2 codes are fine. I have also done full ecu reset many times.

I have done few mods but the problem was there allready when the car arrived from Japan in February.

Garrett GT3076r
Perrin inlet
Cosworth CAI
Walbro 255
850 injectors
3Ē DP + exhaust
Custom pro tune

Sorry for bad language, iím finnish
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Last edited by Jaakko; 08-05-2019 at 04:29 PM. Reason: Missing details
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Old 08-06-2019, 07:10 AM   #2
Cougar4
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2001 LL Bean Outback
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Since the heater circuit has 12 volts on it while the sensor is removed from the circuit it means that there is a bad connection to the power supply for the heater. When the load is applied to the circuit then the voltage drop occurs across the bad connection, where the high resistance is at. Power for the O2 heater passes through the Main Relay on a yel/red wire to the sensor.

I gotta say- your Engish is excellent.
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Old 08-06-2019, 04:31 PM   #3
Jaakko
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Thanks. Where can i find the main relay? 08í doesnít have that brown relay under the dash... iím confused.
I have read that the main relay and SBF#5 is related to o2. And o2 fuse of course (15amps).

Btw, when i start the car, torque shows info:
Lambda heating: ready
(Lambda sensor: incomplete)

But it isnít warming. Or is that data from secondary lambda?

I have searched hours and hours information of this and tested countless things... but nothing seemed to work. Iím gettin frustrated... someone help me, please
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Old 08-06-2019, 07:12 PM   #4
Cougar4
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Look for the Main Relay behind the kick panel on the passenger side of the car. What you stated about the relay and SBF-5 is correct. After we get the heater circuit working we'll see what else needs to be checked.

Last edited by Cougar4; 08-07-2019 at 06:43 AM.
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Old 08-07-2019, 02:42 AM   #5
Jaakko
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I have searched and iím pretty sure that thereís no main relay and power comes straight from ECU. So there must be bad connection or component in ecu...? I have to borrow new ecu somewhere and test it. Lot of work but thatís how it goes. Motorsport
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Old 08-07-2019, 07:02 AM   #6
Cougar4
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The ECU doesn't provide power to the O2 heaters, it comes from fuse SBF-5 and the Main Relay. If your model doesn't have that relay that would be the first time I have heard of that and it would have to have been modified. The ground side of the heaters is controlled by the ECU. If there was a problem there you would have seen 12 volts present at the heater, but since the voltage goes low when you plug in the sensor it means the trouble is on the power delivery side of the circuit. You need to find the relay. If you haven't checked behind the passenger kick panel yet, or above that area, you should do that. Check pin1 of the OBD-2 connector (blk/red) for 12 volts. That wire is tied to the same power going to the heaters also and if the voltage is low there then connector B61 needs to be checked for a problem.
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Old 08-07-2019, 03:16 PM   #7
Jaakko
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Okay, i found three relay panels inside. One from the left side of car (letís not talk about driver/passenger because itís RHD). Thereís four black relays and one white. And fuse for o2 heater.

Other two panels are on the right side of the car. Thereís nine smaller relays and one grey medium sized and a big brownish one (flasher). Which one i should look and test?

Thereís no blk/red wire in OBD-harness and i really donít know whatís connector B61. I have a very little experience with electrics... but I will gladly receive all teaching I have all my skills in mechanics and bodywork.
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Old 08-08-2019, 06:24 AM   #8
Cougar4
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The brown unit should be the Main Relay. I assume that it is on the passenger side of the car. Check the yel/red wire going to it for power when the )2 heater should be ON.

Pin 1 of the OBD-2 connector should have power on it that goes to the heater. My info shows it should be a blk/red wire. Your vehicle may be different possibly.

There is a connection between fuse SBF-5 and the Main Relay. One side of the connection is connector B61. There is a blk/red wire that provides power to the input side of the Main Relay and connects to the yel/red wire going to the heater. That connector may have a problem if there is no power on the wire going to the relay. If pin 1 of the OBD-2 connector has power on it then that connector is good. If there is no power there then that connector needs to checked out. It is under the dash area.
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Old 08-16-2019, 12:19 PM   #9
Jaakko
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I couldnít find the problem yet. Relays are fine, i think. Thereís own relay (#11) for o2 heater and main relay (#3) under the dash on the left/passenger side of the car. Both black relays.

Today i run few voltage tests from heaters pins (two black wires). Same result always:

Starting car, 0,0V, sensor not working
After 3 kilometers, 0,5V
After 8 kilometers, 5,0V, lambda is about to wake up
After 10 kilometers, 5,0V, lambda is working properly
Then i shut off the car and immediately started it, 13,0V for two secs and then it dropped to 5,0v, sensor still working.
Another shut off and start, 0,00V and sensor NOT working... wtf?
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Old 08-16-2019, 06:52 PM   #10
Cougar4
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The Main Relay should be the only relay used for the heater circuit. If you are showing 5 volts across the heater while it is supposed to be turned OFF then it seems to me that the ECU isn't shutting off the circuit like it should. It is allowing some current to pass through the heater and so there is a small voltage drop across the heater.
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Old 08-17-2019, 03:02 AM   #11
Jaakko
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I got relay diagrams from local subaru dealer and thereís main relay #3 and af / oxygen sensor relay #11 in the same box. I have no idea anymore and iím going to take car to the workshop soon, if they have better knowledge. My tuner has a new ecu but itís madness to buy another one if the problem is something else...
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