[glenncof]

I always go to the GM Shop Manual at local library when diagnosing codes but I 'suspect' a lean code is thrown by an O2 sensor reading ( O2 does a trim +/- 128 ticks, when out of range I believe rich/lean code is triggered).

If the O2 sensors have not been change in 130K miles it may not be accurate as life expectancy is only 100K for the type in our vehicle 5.7l. If much over the 130K miles then the O2 sensor is probably not working perhaps not at all. In this case I don't know how a lean situation would be detected.

Vehicles can run fine and often pass smog without any O2 sensor, I ran a porsche 911 for ~100K miles without one working in California. If no O2 signal then the ECU will default to a preset table with no trim from O2. I had a 1991 Range Rover that ran ~80K miles with dead O2 sensor and no codes passing smog.

The exhaust manifold bolt has been broken awhile <I suspect> with no code. The change was K&N filter. Over oiling can contaminate MAF sensor wire which would tell ECU that less air coming in than actual and to reduce fuel causing a lean condition. There could be air leak(s) down stream of the MAF that resulted during your modification.

So, verify what is causing code (GM service book is great on diagnostics). The error could be a sensor (O2 or MAF) but start with the error detect sensor (O2). Replacement is not absolutely necessary but requires measuring the electrical signals at the O2 sensors needing an adapter to get to the connections OR you can measure the resistance and get some idea if they are working at all.

There are 4; bank 1/2, up/down stream of cat. They are heated also.