When the P2A00 OBDII diagnostic code is set, it means that the PCM (Powertrain Control Module) has found a voltage reading on the bank’s Oxygen (O2) Sensor that is outside the range set by the manufacturer. The number 1 O2 Sensor is located before the Catalytic Converter (TWC).
The Oxygen Sensor circuit usually has an 8-ohm resistor. That voltage should match the battery voltage. Therefore, if a variation of at least 10% more or less than the above resistance occurs, this code will be illuminated next to the Check Engine light.