The smart fortwo 451 has 2 different types of exhaust for the petrol engine.
The removal of both of these types are covered in this guide.
The Euro 4 exhaust was a far more simple design. Both the turbo and non turbo exhaust were exactly the same.
The main body of the exhaust contained a CAT and the silencer. It made it easy to change but was more expensive.
The Euro 5 exhaust had a longer route for the gas to travel. Both the turbo and non turbo exhaust were exactly the
same. The main body of the exhaust only contained the silencer. The CAT was a separate part further upstream.
Smart used a separate CAT as it was cheaper and easier to produce. The upside of this design is that you
can change the silencer on its own for a silencer with Brabus tips. You aren't buying the CAT so it's cheaper.
Removing The Euro 4 Exhaust
Disconnect the wiring to the upstream lambda sensor, this is easiest to access by opening the engine flap in the boot.
The connection has a clip that is pushed in, the plug then pulls out.
Do the same on the downstream lambda sensor.
Use a 14mm spanner or socket to remove the sprung exhaust manifold bolts.
Spray both lambda sensors with PlusGas before attempting to remove with a 22mm spanner.
Taking out the sprung exhaust manifold bolts before this step ensures that none of the force
used to remove the sensors is passed up through the manifold which could damage the gasket.
The exhaust is held in with rubbers on each end. The arms that pass through these are
domed to stop them pulling out. Spray some lubricant on each dome, grab the rubber, put
your foot on the exhaust and pull like a madman. They actually come out fairly easily.
Remove the mesh reinforced fibre gasket, careful as these can damage quite easily.
Composite fibre seal - A0004921681 (if you damage it or want to replace it, take this number to smart).
Get your replacement exhaust, this one is a Smarts-R-Us Evil Twin twin centre exit. (it was shit).
If you want a decent aftermarket exhaust, buy an S-Mann exhaust.
Slide the metal stays back into the rubbers. Again, a squirt of WD40 will help.
Offer the manifold up, put the fibre gasket back and wiggle the exhaust into place.
Replace the sprung exhaust manifold bolts and do them up until they stop, they self torque.
On this particular aftermarket exhaust, the downstream lambda sensor wiring has to be extended.
This will not be the case for all aftermarket exhausts but may be necessary.
Cut the 4 wires and add a length of wire to each. Use straight crimp connections and heat shrink.
Replace both lambda sensors and do up tight (40Nm if you have the correct tools).
Reattach the wiring connections to both lamda sensors.
The exhaust is now fitted, your final job is to adapt the valance to suit the tail pipes.
This information can be found
Removing The Euro 5 Exhaust
Take off the rear panels to reveal the exhaust.
Remove the crash bar to get better access to the exhaust fittings.
Here is the over-long Euro 5 exhaust.
Remove the 2x E12 bolts from the CAT 1 header pipe.
Spray the rubber mounts with WD40 and pull the metal mount through.
The lower part of the exhaust will now come away from the car.
Remove the 2 bolts from the turbo using a 15mm socket.
Disconnect both lambda sensors and unscrew them with a 22mm spanner.
You will need a crows foot spanner or lambda socket for the 2nd sensor.
Remove the 2x E12 bolts in the bracket.
CAT 1 can now be removed from the car.
Refitting it the opposite of removal.
This information can be found