Land Rover Repair: O2 Sensor, mass air meter, circuit resistance

Question
Hi
I have a 2001 Land Rover disco 2 I replaced four O 2 sensor at 70.000 miles after getting this four codes
P0135   O2 Sensor Heater Circuit Malfunction (Bank 1 Sensor 1)
P0154   O2 Sensor Circuit No Activity Detected (Bank 2 Sensor 1)
P0155   O2 Sensor Heater Circuit Malfunction (Bank 2 Sensor 1)
P0734   Gear 4 Incorrect ratio
I deleted the codes and  50 miles later the same four codes returned .
please advice
Thank you

Answer
If you get those same codes with new sensors, I'd say the sensors are defective or there is a wiring problem in the heater circuit.  The engine control unit measures the heater circuit resistance, and that reading has nothing to do with how your engine is running.  If you get those faults there's a sensor problem, a wiring problem, or the ecu is faulty.

The Gear 4 fault may point to transmission trouble or a failing mass air meter