V40 Air Pump.....

Karl you are slightly out on the info. The O2 sensors do have a heating element to assist in rappid warm up but no fan. The air pump is used to inject air in the exhaust manifold to help reduce nito- oxides (one of the harmful components of the exhaust). During start up with a cold engine the engine is in open loop control, that is to say the ECU is not responding to the O2 sensor. Since at this stage emissions will be high the system uses the air injection system to help combat this. Once full operating temprature is reached the engine goes into closed loop control and now responds to the o2 sensor and air injection is stopped. A cold engine could not work on the fine Lambda mixture as a lot of the fuel condences on the manifold wall and the engine would be very rough. The ECU will use a set program of injection duration based on air and coolant temprature. Once operating temprature is reached the ECU will now respond to the 02 sensor reading and will adjust the injection duration to keep within Lambda.