60 replies to this topic

[bstone]

CEL came on while driving this afternoon. Scanned and it came up as P0136 Oxygen O2 Sensor Circuit Low Voltage (Bank 1, Sensor 2). I have a new OE O2 sensor that I can swap but it appears this is a CIRCUIT issue, not a cat below efficiency or a O2 sensor failure, per se.

So, folks, talk to me about what to do. Is this the first or second cat? Should I check the wires going from that cat up to the wiring harness? Should I pull out the O2 sensor and clean it off? Your ideas are welcome.

[Cougar]

Bank 1 refers to the side of the engine with cylinder #1 so that would be the right side. Sensor 2 is after the first CAT I believe. If the signal wire from the sensor to the ECU doesn't have a short to ground on it and the resistance of the wire connection is within specs then the sensor is most likely the trouble.

[Fairtax4me]

Sensor 2 is post cat. What year and how many miles ya got?

[bstone]

The primary cat was replaced over the summer from a cheap muffler shop and the O2 sensor was reused.
Got 235k miles. Have another cat to put on if necessary.

[Fairtax4me]

235k on the original sensor?
Probably should change it anyway.

[bstone]

235k on the original sensor?
Probably should change it anyway.

I am unsure as I bought the car in May 2010 with 220k on it.

So this is the SECOND cat's O2 sensor? I'll swap it out and hope that helps.

[Fairtax4me]

Yes, this is the sensor behind the cats.
Check the wiring for the sensor before changing it. It runs up the side of the transmission. Make sure it isn't touching the pipes or rubbing against something it shouldn't be.

[bstone]

It's been in the negative digits and snowing a lot, so replacement of the O2 sensor will have to wait for a while. In the meantime I cleared the code and it hasn't yet come back.

[bstone]

Alright. I replaced the O2 sensor with OE sensor last week and cleared the code. Gas mileage came back. All seemed to be well until I filled up again and the CEL came back on, complaining of the same code.

Any ideas?

[SubaruFred]

Alright. I replaced the O2 sensor with OE sensor last week and cleared the code. Gas mileage came back. All seemed to be well until I filled up again and the CEL came back on, complaining of the same code.

Any ideas?

Disconnect the neg batt lead and press the brake pedal to drain any residual current, then re-connect the neg batt lead or skip these steps and clear the code with your scantool and see how long before the CEL re-appears. If it re-appears quickly, check the circuit wiring for shorts. If no shorts are found, check for exhaust leaks before the secondary bank 1 O2 sensor. A leak before the O2 sensor could cause the sensor to read too lean which could trigger a "low voltage" code.

My best guess is that you have an exhaust leak before the CEL throwing sensor.

[Cougar]

I suppose another possibility is a plugged CAT since you have so many miles on the car.

[bstone]

I think you are right about the exhaust leak. I've had some work done down there and I am pretty sure I still hear hissing. I guess I need to find a good muffler shop.

Quick question- how many O2 sensors are there per cat?

[johnceggleston]

what car?

typically in the late 90s there is one o2 before the cats and one after.

[bstone]

97 OBW AT.

I had a muffler shop replace this cat over the summer with a brand new Wagner cat.

[Cougar]

That eliminates a bad CAT then. Did you verify the wiring between the sensor and the ECU is ok before replacing the sensor?

[bstone]

That eliminates a bad CAT then. Did you verify the wiring between the sensor and the ECU is ok before replacing the sensor?

How might I do that? Checking voltage?

[Cougar]

You check the wiring by using the resistance test mode of a meter. You make sure the sensor lead has a good low resistance connection to the ECU pin and also make sure that the lead is isolated from ground and voltage.

[bstone]

I am pretty sure this is due to ongoing exhaust leaks. I am headed over to a muffler shop to go get it checked out.

[Cougar]

Ok. I hope they find the trouble.

[bstone]

So I went to the independent muffler shop across the road and they found a leak coming from one of the flanged on the passenger's side right before the converter. They cut out the flange and put on a metal pipe and welded it on, for $50. The hiss is gone and the car actually seems more responsive now.

[Cougar]

Good deal. Hopefully things be ok now.

[bstone]

I think the best deal was finding this exhaust shop that actually will do things cheap and good vs the dealer who wanted $2200 to replace the pipes. Yup.

[Olnick]

How about the code--is it gone now? Sure hope you've solved the problem.

[SubaruAlliance]

another test you could would be a voltage drop test. that requires the circuit be powered in example the engine running. you then put the volt meter on voltage dc and put one of the leads either neg or pos on the wire as close to the sensor or the ecu (depending where you start, the o2, or the ecu) and then a few feet away on the same wire you put the other lead on it. you should not get more than a half volt at any spot and if you do then you know that is the spot causing the voltage drop.

example if this is your wire:
step 1


o2 sensor here:X--x----------x-------------[clip]-------------------------------------X < ecu
one of the lowercase x's = one of your voltmeter leads

step 2

02:X-----------x--------------x-[clip]--------------------------------------X < ecu

then keep moving down the wire and when you get to a plug in or clip be sure to check it with one lead on each side of the clip and if you get more than a half volt then replace that section of the wire or that particular clip. the goal for this test is .5v or less. say for example you test the clip and you get a 2v reading on your voltmeter that means your sensor would only be receiving 10v instead of 12v. keep in mind that the voltage drops are cumulative so if you have two or three spots that average 1.5v then that is 3 or 4.5v less for the circuit to receive. remember THE CIRCUIT HAS TO BE POWERED for this test to work. LOL you wouldn't believe how many times i would start a voltage drop test with the power turned off only to realize it and then start over.

not trying to insult anyones intelligence here, it's just confusing (at least for me) without a visual reference of some sort.

if anyone knows this technique better or if i forgot something correct me so i can learn it too.

Edited by SubaruAlliance, 21 February 2011 - 03:02 PM.

[bstone]

Just drove 120 miles from New Haven, CT to Boston, MA. No codes and I think I am getting better gas mileage than I ever have.