I have a 1997 discovery with 145K on it. I recently changed all the 4 oxygen sensors.
Occasionally my engine light would still turn on, and this is the error code that it would produce..
P0138 - H20S Failure to switch to lean. Bank A.

What does this mean? Is it one of the oxygen sensors is malfunctioning?
If it is, which one is Bank A?

 

are you using a generic OBDII reader?

if I recall correctly, the 0138 code is related to the O2 sensors after the cat. generally either bad O2 sensor or the wiring going to it.

Check it out further, though.

 

I agree with demonlarry....P0138 " o2 sensor circuit high voltage ( bank 1 sensor 2 )
that's drivers side sensor behind the cat.....