i replaced all 3 o2 sensors and the engine light is still on and failed emission on 1996 Chevrolet Tahoe

code p0140 still come up...why and how can i fix the problem????? please help

Asked by for the 1996 Chevrolet Tahoe
Needs to be scanned with a proper scanner that has live data to monitor rear 02 sensor for correct readings.
Could be lots of causes, poor connections, fuel press. vacuum leak, exhsust leak, defective after market sensor, which is not unusual.