almost anytime I see an O2 sensor code, I start my troubleshooting with a full, complete tune up.
An O2 code means either the engine is running lean or rich for too long, not that the O2 sensor has failed.
Do a tune up, including spark plug wires, rotor and distributor cap, and be sure to gap the plugs. Gap on spark plugs in an LPG engine is far more 'critical' than on older flat head motors running on Gasoline.
also be sure to drain the LPG regulator "honey" with the engine hot and fuel turned off and ran out.