New X-pipe, P0156 and P0174 codes - MustangForums.com


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Old 03-16-2012, 07:14 AM   #1  
bbayne
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Default New X-pipe, P0156 and P0174 codes

I have a 2000 GT. I installed a Bassani catback exhaust and Bassani X-pipe about 10 years ago along with some MIL eliminators soldered into the rear O2 sensor harnesses. I needed to replace the X-pipe due to a rattling sound possibly caused by a small slit in the heat shield area. I replaced it this weekend with a new Bassani Catted X-pipe. I replaced the original front O2 sensors as well. I reinstalled the old rear O2 sensors since they have the MIL eliminators. After a few days of driving, I saw a P0156 code (O2 Sensor Circuit malfunction, Bank 2, Sensor 2). I rechecked the rear driver side O2 sensor, reconnected it and cleared the code. After a few drives, the light came back on with P0156 and P0174 (Too Lean Bank 2) codes.

I was originally thinking that I may have damaged the rear O2 sensor during uninstall from the old x-pipe and re-install into the new. So, I was going to replace both rears, but the P0174 code is making me wonder if something else is going on.

My old x-pipe was an "off-road" version. Is it possible that the MIL eliminators are causing a problem now that I have cats? Since I am only getting codes for bank 2, it is possible that an exhaust leak (at the header to x-pipe joint) is causing both codes? Just to verify, is bank 2 the driver's side?

Please advise. Thanks!
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Old 03-16-2012, 07:28 AM   #2  
cliffyk
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You first need to remove the MIL eliminators, they work by filtering the signal from the rear sensors to reduce the signal amplitude--and trick the PCM into thinking cats are there and working. However with cats actually installed (which by themselves reduce the amplitude of the rear O2 sensor output) the MIL eliminators will further reduce the signal, possibly to a level less than the PCM likes.

An exhaust leak before or near the front sensor could cause the lean code. Bank 2 is the driver's side.

You can bench test an O2 sensor with a propane torch and multimeter--here's how...
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Old 03-16-2012, 11:53 AM   #3  
bbayne
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I had the same thought process about the MIL eliminators. However, if that were true, I should get the equivalent code from the Bank 1 sensor. I did log data on my Predator on my 30 minute drive home from work yesterday. The rear driver side O2 sensor seemed very low. It ranged from 0 to .09 V with an average of about .05 V. The rear passenger side sensor ranged from 0 to 0.55 V with an average of about .15 V. Based on these values, do you believe the MIL eliminators are reducing the voltage too much? Why would Bank 2 be lower?

I agree with you that there is probably an exhaust leak where the header meets the x-pipe on the driver's side. I will tighten those bolts down some more.
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Old 03-16-2012, 12:54 PM   #4  
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Because bank 2 is running lean...

The bank 1 rear sensor is reading a bit lower than would be normal, but as the RC filter (MIL eliminator) is a low pass filter and as the cat is reducing the frequency of the O2 output signal (this is one of the newer methods of monitoring the cats--read more about that here) the filter is not attenuating the amplitude as much as it would with no cats).
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