Contour Enthusiasts Group Archives Forums General Troubleshooting P0136, and P0156

Well, I have "fixed" my pre-cats, put in the MIL eliminators. When I drive on the highway, and the computer interrorgates the O2 sensors, I throw a CEL. The two codes I get are (definitions according to CEG, I have different definitions in the book that came with my scanner):
1) P0136 Downstream Heated Oxygen Sensor circuit fault - Bank No. 1. (My book says that P0136 means: Low voltage bank 1 sensor 1)

2) P0156 Downstream Heated Oxygen Sensor circuit fault - Bank No. 2. My book says that P0156 means: Low voltage bank 2 sensor 1)

I thought that the downstream O2 sensors should be putting out a low voltage, meaning that they are working right? What is the voltage supposed to be on a working downstream O2 sensor? Any ideas as to what I did wrong??? Thanks for the help! -Nick

Sounds like the MIL eliminators are not working. They are puttin out a lower voltage than the computer expect to see. Try clearing the computer and disconnecting the MIL eliminators. If the code is the same, the MIL eliminators are either not working or have some connection problems.

First off, your book's comments are wrong. Downstream sensors are sensor #2, not #1. I don't know what a 'MIL Eliminator' is but I assume it feeds a signal to the downstream sensors simulating a 'good' precat? Looks like yours isn't working and the sensor is detecting a lot of extra air from a 'fixed' cat brick?? So it's stuck reading lean, which is a low voltage. The HEGO voltage range is 0.1-1 volt, .2 being a lean reading, .9 being a rich reading.

That's what happens when you try to mess with the system. Have fun fixing it.

I had MIL code P0136 as well with my MSDS headers after about 5 months. Ford replaced the 02 sensor under warranty but did not reinstall my MIL eliminator. Surprisingly the CEL has not come back without the eliminator though, so I never questioned it

The book was right, sensor 1 was a typo on my part. I want to hook up a voltmeter up to the output side of my MIL eliminators when the car is warmed up so I can see the output voltage. This is why I need to know what the propper voltage range on an operating O2 sensor. I understand that the range is .1 and 1.0, but where should the voltage be on a working O2 sensor? I enjoy working on my car and 'messing with the system'. Nor do I mind fixing what I might have messed up, it's all a learning experiance for me. -Nick

TTT, still cant find the info I need, anyone??? Thanks... -Nick

You'll typically see normal O2 sensor voltage fluctuate through most of that 0.1-1 volt range, mabye more like .2-.8. It 'switches' back and forth across 0.5v, which is the optimum fuel-air mixture. For both good fuel control, and to keep the catalyst supplied with oxygen, the system is continually dithering between slightly rich and slightly lean. The response curve resembles AC voltage. The thing to remember is it should NOT be consistently below or above .5v. When it stops 'switching', something's not right.

Thanks. It was my understanding that the O2 sensors after the cats were supposed to stay in the low voltage range because there should be no unburnt gas after the O2's. I will check the O2 sensor outputs today to see if maybe that will give me a better idea of whats going on. -Nick

You're partially right. The downstream sensors do switch, but not nearly as much as the upstream, and not through as great a range. The precat brick absorbs excess O2, so your mixture coming out of there is slightly rich. If the DS sensors are switching too much, that's an indication of failing cat. But you can also set lack-of-switching codes in the DS sensors, too. You should see a .3-.6 voltage range.

UPDATE: I did some testing, and with a warm engine in idle, the DS sensors should be reading about .6-.65v. If you rev the engine and back out, at least one will read low for a while, but both will slowly work their way back to the .6v range, which is slightly rich.