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Hey guys,

I have an issue that i hope someone can provide some input on. For the past month my 95 tbird has been having lean codes for both banks. I would clear the codes and they would reaper after 200 miles or so. One day the CEL came back on and about two minutes later the fuel pump failed. I replaced the stock pump with a 190 lph pump along with a new fuel pressure regulator and new filter. Shortly after the CEL came back on only this time I had a Bank 2 lean and a Bank one rich. I swapped the upstream o2 sensors with each other to see if the problem would follow and it did. Noting this, I replaced both upstream o2 sensors with bosch replacements and I changed the spark plugs as well to motorcreft copper. All seemed well until a little after 200 miles the CEL came back on again, only this time it had codes for bank 1 lean and rich (P0171 and P0172).

I feel like this is just a computer issue as there is no way that one bank could lean and rich at the same time. I'm just not sure if the computer is sophisticated enough to track engine going back and forth between two contradicting conditions and spit out both codes.

I have contacted my tuner and he has stated that could be a bad ground, frayed o2 sensor wiring or possibly the pcm. I would like to know if there are any other possibilities to consider.

The car still has the stock injectors on it. The coil packs are fairly new and the wires are ford racing with about 15000 miles on them.

Also the car runs great. I don't have any complaints in the performance department.

Please let me know what you all think.

Thank you,
 

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Hmm, that code pops up sometimes when there is a vacuum leak, happened to me as well. Same codes but I didn't have any fuel issues. It was the vacuum line on the back of the plenum that was cracked, this can however be caused by any vacuum line that may be damaged. Maybe during the process of installing the fuel pressure regulator the vacuum line wasn't properly put on or another vacuum line got pulled off in the process. Let us know.
 

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Hmm, that code pops up sometimes when there is a vacuum leak, happened to me as well. Same codes but I didn't have any fuel issues. It was the vacuum line on the back of the plenum that was cracked, this can however be caused by any vacuum line that may be damaged. Maybe during the process of installing the fuel pressure regulator the vacuum line wasn't properly put on or another vacuum line got pulled off in the process. Let us know.
Thanks for your reply. I have checked for leaks but have found none. I'm receiving the lean and rich code both on one bank only. Also there is an exhaust leak on this bank. could this cause it to read both lean and rich?

Thank you,
 

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Thanks for your reply. I have checked for leaks but have found none. I'm receiving the lean and rich code both on one bank only. Also there is an exhaust leak on this bank. could this cause it to read both lean and rich?

Thank you,
Of course that could cause it! That probably is the issue. Exhaust leak will definitely cause an imbalance in the reading that the O2 sensors put out. Your exhaust leak, IF it's before the O2's is most definitely the cause. If you want to try something easier first to see if it fixes your issue: clean the MAF sensor. The MAF is the cause of a lot of the issues related to fuel and air in these cars.

The readings from one bank can always affect the other bank. It's all designed to work together, one thing fails, the computer will detect an anomaly in the other. Makes it harder to diagnose. That's why a P0171 can be caused by vacuum leaks, fuel pressure issues, injectors, O2 sensors, exhaust leaks, intake manifold leak, MAF and many other factors. But since you are aware of an exhaust leak, I would definitely fix that before trying anything else.
 
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