After driving around some i have the following codes:
P0171: System Too Lean Bank 1
P0174: System Too Lean Bank 2
P0136: O2 Sensor Circuit Malfunction Bank 1 Sensor 2
P1131: Lack Of HO2S Switch - Sensor Indicates Lean
P1151: Lack Of HO2S21 Switch - Sensor Indicates Lean
Still waiting for that P0356 to confirm a bad coil
Hmm .. so how many codes did you have before the exhaust fell off ???
Okay .. lets sort this out. The P0356 is a bad coil on cylinder #6 - a misfire .. this condition may or may not turn off the fuel injector to that cylinder. If it turned off the injector, you get the misfire plus it would run a little leaner on that side of the engine. If the injector is still pulsing, you get a rich reading which leans out the other cylinders - I think your injector is turned off .. "false lean". But this would typically one affect the bank that #6 is on which would be P0174 and P0151.
The P0136 is a post cat sensor, its a circuit malfunction would could be the sensor itself, or its just reading lean all the time .. these circuit malfunctions tend to be vague and can be any number of things from a sensor being unplugged, bad, the wiring, or the PCM itself.
Since you have P0171 and P0174 + the P1131 and P1151, these all indicate a lean condition which would be the result of an intake leak after the MAF sensor , up to the central point before the go between the two banks (ie. not an inake gasket leak at the heads, upstream at a central point that feeds both banks ) .. an exhaust leak would have to be on BOTH sides of the engine - its more likely an intake leak, but it can ALSO be a MAF sensor that is dirty / failed, giving your engine a false reading on the actual air entering the engine which affects the fuel trims .. your fuel trims are above 30% which results in the 4 codes, is trying to compensate with extra fuel but no change on the o2 sensors.
So either you have an intake leak, bad MAF sensor, or your primary o2 sensors both arent reading on the PCM ( wiring disconnected, or fried .. hard to say since your exhaust did fall off recently )