The P2188 OBDII diagnostic code relates to the value delivered by the Oxygen (O2) sensor located in the exhaust to the PCM (Powertrain Control Module). This information is relevant to control the amount of fuel injected into the engine.
The PCM uses the O2 Sensors to determine the amount of fuel needed for efficient engine operation. In case the required air/fuel mixture balance cannot be achieved, this code will be set.