Any tips on Oxygen Sensor Heating circuit?

jmoryl · 03-04-2012, 08:57 AM

My SVT is throwing the P0135 and P0141 CEL codes, which is a fault in the O2 sensor heater for the front and rear sensors. Checking over the wiring, and the #18 fuse (under the hood), all looks good. Since I had a new front sensor (Ford) sitting around and the car has about 90k miles, I changed that sensor, expecting that would at least clear the P0135. No such luck. Any one here have a clue where to look next? Gotta get that light out long enough to pass my state inspection!

Thanks in advance.

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***1turbofocus*** · 03-04-2012, 09:16 AM

Need to see what the sensor is doing , see if a local member has a xcal and do a datalog , I will be glad to look it over for you

Tom

Grinder · 03-04-2012, 11:20 AM

If you have done a proper load test of the common power circuit and didn't see high resistance its likely a bad pcm. They rarely fail but the 02 heater circuits are tested by measuring the voltage drop across a resistor inside the pcm. This resistor bridges the control circuits for both heaters so when the pcm grounds one o2 current will flow from the opposite o2 and then through the resistor. Heat will often open this resistor and make the pcm believe both heater circuits have malfunctioned. Be careful if you ever buy a reman'd pcm - this is something that seems to miss their "checks" and I've seen them have this issue out of the box.

chada1208 · 03-04-2012, 04:10 PM

I had the same thing happen to my last 02 svtf! It happened when I bottomed out and hit the exhausted! Now I have the same problem with my recent 02 svtf so what can I do to fix it? I am planning on putting an orp and full exhaust on it soon so I will not b running an 02 Senor!

Mike_Mac · 03-04-2012, 04:20 PM

^^^ you NEED the front O2.

jmoryl · 03-04-2012, 05:02 PM

Tom - thanks for the offer, but I won't be anywhere near NC soon.

Grinder - is there any simple way to check if the heaters are drawing the correct current (e.g. checking the voltage being supplied, measuring the resistance of the heater)? If one of the sensors is bad could it indicate a fault in the heater circuit for both - that seems like a possibility given what you say above. Cause I think the rear sensor might be dinged.

Grinder · 03-05-2012, 03:08 PM

Quote:

Originally Posted by jmoryl

Tom - thanks for the offer, but I won't be anywhere near NC soon.

Grinder - is there any simple way to check if the heaters are drawing the correct current (e.g. checking the voltage being supplied, measuring the resistance of the heater)? If one of the sensors is bad could it indicate a fault in the heater circuit for both - that seems like a possibility given what you say above. Cause I think the rear sensor might be dinged.

It's possible to meausure the current flow through the heaters but it's a tricky measurement to set up and the heaters are duty cycled controlled which will mess with your readings.

If both heater circuit codes are set, something common between the two sensors is your problem. The only thing common is the power circuit and the PCM. I agree, it's not likely the PCM but it is possible and this type of scenario is one of the common causes of their uncommon failure....

Are you SURE the fuse is good?

jmoryl · 03-06-2012, 01:29 PM

Quote:

Originally Posted by Grinder

It's possible to meausure the current flow through the heaters but it's a tricky measurement to set up and the heaters are duty cycled controlled which will mess with your readings.

If both heater circuit codes are set, something common between the two sensors is your problem. The only thing common is the power circuit and the PCM. I agree, it's not likely the PCM but it is possible and this type of scenario is one of the common causes of their uncommon failure....

Are you SURE the fuse is good?

I've made some progress. After installing a new front sensor and getting the same codes, I'm down to one code, P0141, after several read/clear/CEL cycles. Funny how these things work sometime. So it looks like I'll just replace the rear sensor now. The SOB is bent, so it looks like it will be a hassle to remove - anyone have any experience to share?

Grinder · 03-06-2012, 01:55 PM

Quote:

Originally Posted by jmoryl

I've made some progress. After installing a new front sensor and getting the same codes, I'm down to one code, P0141, after several read/clear/CEL cycles. Funny how these things work sometime. So it looks like I'll just replace the rear sensor now. The SOB is bent, so it looks like it will be a hassle to remove - anyone have any experience to share?

This is common.

When you bottom the header out it flexes enough to smash the sensors against the engine/halfshaft. This bends the sensors and shorts the heating element to the body of the sensor. This will blow the fuse that is common between both sensor's power circuit and you end up with heater circuit codes.

When the PCM is bad it sets codes for both sensors all the time. So... if you're only setting the code for the rear - the heater element/circuity inside the sensor is toast. Replace the rear sensor and you should be good to go.

Make sure you bring your old sensor into the parts store to match it up. All parts stores categorize 02 sensors based on the element design and pigtail harness length. Autozone, Orielly, Napa, CarQuest and Pep Boys all show the incorrect length of pigtail for the rear sensor (at least for my SVT). I had to ziptie a crapload of extra harness because I couldn't find a direct replacement...