Found mods and other info about tricking PCM that requires to thread sensor into exhaust pipe. Everything past exhaust flang and upstream sensors has been modified and no threaded hole for sensor 2. Is there any other way to bypass this sensor with my setup? Getting 1131 after I replaced bank 1 sensor 1. Just found small leak at weld left by muffler guy. I found that exhaust leak may cause this code but wanted info on what can happen running truck with no sensor 2s in harness

 

You ask an involved question that requires understanding of system operation because it's not so simple.
The code 1131 is a Lean code indicating either the intake tract is allowing excess air or the flange is allowing air ingestion ahead of the front Ox sensor it is detecting.
We need to know the extent of the modifications made.
For instance, are the cats removed?
Was Hedders installed?
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The front sensors both sides control fuel injection cylinder by cylinder in real time as the motor runs. If they are missing or not operational, the computer substitutes a fixed fuel table value that will be constantly on the rich side, by intent.
The rear sensors being close to the last cats, monitors the oxygen output. The computer monitors their reading and compares to the front sensors as a way to tell the health of the whole cat system both sides by using a Ratio calculation. The rear sensors do not control any fuel injection, only monitors the cats.
If this system become non operational, expect to see codes 420/421 at some point. You can look up those codes that will support the failure by description.
Also, the Ox sensor codes will be confirmed by looking at the Long Term Fuel Tables with a Scanner to see their shift in values from a Zero point in percentage rich (+) for a lean code and Lean (-) for a rich code. This is because the computer program is built to allow the system to counter the 'fault' up to a limit. That limit when a code is set. The ability to shift over a short shift is the way the system lives with short term changes drive cycle to drive cycle.
I'm sorry, but things are not as simple as you might have thought.
From this you see it's not so simple to defeat the engineered operation and have everything normal.
For example, with long tube Hedders you cannot sample all cylinders for each bank close to the heads where the system needs the sensor signals to originate because there it a time element 'window' for that signal so control of the next cylinder can be accomplished.
Putting the sensor in a Hedder collector so far down stream lengthens the time outside the window in addition the sensors operate too 'cool'.
Those who install long tubes most often don't know these things and the mfger/vendors do not tell or know.
The results are what you get from modifying outside hardware the computer senses and very often does not expect.
The computer program does not change anything but shift some tables from their normal operating ranges.
This often alters drivability reactions, set codes, violates Emission laws etc where applicable, causing some lost investment in time and money if the vehicle does not pass inspection.
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I'm not telling you you can't do what you want, only to take time to explain the facts of life about such changes.
You don't get this anywhere else or you would not need to ask such a good question here.
Good luck.

 

No headers.Just straight pipe after flange to factory y. Small leak at weld just after sensor 1. See what'll happen once that's fixed. Truck started hesitating on highway around 1600 rpm. Threw code bank 1 sensor1 lean. Changed Fuel filter, cleaned maf while I waited on o2 sensor to arrive. Terrible mpg. Threw in new sensor code fell off for about 30 miles. Truck increased mpg stopped hesitating and rough idle. Threw 1131. Made sure I tightened sensor down enough with truck running when I noticed exhaust leak. If sensor 2 doesnt effect fuel ratios what are the possible future problems at hand given no vacum or exhaust leaks? I understand now the computer shifts tables around but as far as performance what would be expected with no downstream? Thank you for your reply.

 

Code 1131; sensor is detecting excess Oxygen in the exhaust. Likely a leak close to the sensor. Try using a Stethoscope with the pin removed to find the leak at idle. The leak will result in a shift of the long term fuel table for that bank, reduced fuel mileage.
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Straight pipe in place of the cats that bank will result in code 420 at some point. The code may cancel the CEL at some later point because the failure is not useful to the system and does not affect drivability but is still set in program.
There is no cancelling the rear sensor from a missing Cat assembly on that bank.
The program was written so you can't get by it by any simple means.
US EPA dictates this beside being illegal to run without Cats or even for a Yard to sell them 'used'.
Good luck.