The heater in the oxygen sensor has failed. Its a good idea to replace BOTH oxygen sensors at the same time. If one has failed the other won't be lasting very long.
mercury villager heater core 2001
Trouble code P0135 means:O2 sensor heater circuit malfunction (Bank 1 Sensor 1)
Trouble code P0161 means:O2 sensor heater circuit malfunction (Bank 2 Sensor 2)
Inside the dashboard at the right.
I would not recommend that you try. A block heater is preferred.
If it is defective it will need to be replaced.
Heated oxygen sensor heater circuit malfunction (bank 1, sensor 1): Open or short in heated oxygen sensor heater circuit Heated oxygen sensor heater malfunction (Haynes Repair Manual)
There would only be a rear heater core if there was a rear heater, which I don't believe was an option.
The oxygen sensor is "done". It should be replaced.
O2 sensor heater circuit malfunction, Bank 1 Sensor 1.
Near the firewall behind the engine, or under the dash near the heater core.
In the lower part of the dashboard on the passenger side.