How do you replace a transmission thermostat on a 2004 BMW 325i?

Very simple...

Remove the Expansion Tank (there are many DIY on the E46 Expansion Tank removal with very good pictures, just google it) and the transmission thermostat will be on the base that holds the tank in place.

Be careful not to hit the top part of the thermostat when removing the tank (make sure you pull it 100% upwards, not sideways like I did.. since it brakes very easily).

In case you do break the thermostat in half, it will be difficult to pull it up, since the two O-rings will hold it in place and you do not have any firm way to grab it...

Solution? Carefully drill a hole and screw a metal screw into the left over piece and pull it with a set of pliers. It does come easy then.

Good Luck. Robinson Fonseca