Pre-cat heater blower systems were introduced on all post-2003 smarts in order to tackle more stringent emissions criteria. It works by blasting warm air into the exhaust system via the turbo manifold upon engine startup.

When you start the car cold, you will hear the engine rev around 1500rpm. This will drop to around 1000rpm after 30s (or so). However, you will still hear a high pitched squeal last for another minute. This will then drop out with a noticeable relay click and the revs should settle to around 950rpm. This squeal is the pre-cat heater blower. 

We have made the decision to remove the pre-cat heater system on a number of our cars with no adverse effects and no MOT failures.

When the heater is not running, exhaust gases travel up the pipe in the direction shown. This image shows a car on which the owner insisted that blower system was restored when their turbo was replaced.

The simplest way to remove the system is to 'cap' the outlet pipe from the turbo's manifold. This pipe is to cut and welded to stop any gasses escaping under boost. (Ensure the the weld is strong as it needs to handle exhaust boost pressure).

Now that you have cut off the main supply, you need to stop the rest of the blower system from running. Unplug the cable connection from the pump in the right hand side of the engine bay.

From the main valve, there is a small pipe which links to a switching valve between coil packs 2 and 3. This pipe is shown dotted below. Remove it from the car:

Remove the Y-section. Ensure that the one way valve stays on the pipe leading to the inlet manifold.

Join the manifold pipe to the one heading down to the turbo switching valve. Remove the Y-section.

Remove the remaining pipe to the switching valve completing the pipe arrangment.

The blower valve and the pump are now redundant. These can now be removed from the car. Remember, when you remove the main valve, you will need to replace the bolts with shorter ones. Don't try to use the same ones as you will damage the rocker cover.