If you look at the FULL list of what COULD cause this, you will see that there is still alot of work to do.
Consider getting a propane based vacuum leak detector. A quick seach of Amazon for an example of an affordable unit.
If high mileage, consider replacing the front O2 sensors. This would at least give you known good ones to work with.
Note, fuel pressure is listed as a possible cause. Is the fuel pressure vacuum reference line connected and leak free?
This could also be a clogged fuel injector. Consider an injector cleaning flow test service such as injectorrx.com.
If asked to handicap this thread, since the lean conditon is being reported on a single bank (left), IMO an exhaust leak on the left side of the engine should be high on the list. Check for leaks especially around the EGR system and DPFE sensor. Exhaust gases are hard on rubber parts. Any leak in the lines to/from the DPFE sensor will be hard to see because the sensor is slow close to the firewall.
A MAF problem would affect both banks. As well as a vacuum leak early in the induction system would affect both banks. If this is a vacuum leak, look for something that would affect only a single side. Valve covers, oil dip stick, or fuel injector O-rings for example.
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P1151 - Lack of HO2S-21 Switch, Sensor Indicates Lean
Electrical:
Short to VPWR in harness or HO2S
Water in harness connector
Open/Shorted HO2S circuit
Corrosion or poor mating terminals and wiring
Damaged HO2S
Damaged PCM
Fuel System:
Excessive fuel pressure
Leaking/contaminated fuel injectors
Leaking fuel pressure regulator
Low fuel pressure or running out of fuel
Vapor recovery system
Induction System:
Air leaks after the MAF
Vacuum Leaks
PCV system
Improperly seated engine oil dipstick
EGR System:
Leaking gasket
Stuck EGR valve
Leaking diaphragm or EVR
Base Engine: Oil overfill
Cam timing
Cylinder compression
Exhaust leaks before or near the HO2S(s)