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My daughter’s 2006 Mitsubishi Colt CZ1 failed its MOT check due to bad emissions.  The exhaust lambda reading was higher than permitted. The tester said that it was likely due to a leak in the exhaust.

Close examination revealed that the front joint that connects the down pipe to the exhaust was leaking.  This joint is prior to the catalytic converter and the 1st oxygen sensor.  The joint is fastened by two spring loaded studs and nuts.

To remove this section a ‘U’ clamp needs to be removed.

A couple of rubber mounts support the front of the catalytic converter.

The front 1st oxygen sensor needs to be disconnected, so that the catalytic converter can be removed.  There is a connector under the carpet on the right hand side of the vehicle.

A view of the down pipe with the catalytic converter removed.

The face of the catalytic converter pipe was a bit rough, so i cleaned it up with a small grinder.

The metal seal that fits in this joint was damaged, so a new one was purchased.

The new seal installed.

After refitting the exhaust and checking it for obvious leaks I took the car back for a re-test.  Unfortunately while the readings were better, it still failed on emissions.  I wondered if the new seal wasn’t sealing properly and I could hear a leak when the engine was first started.  I thought that the exhaust joint springs may not be clamping the seal firmly enough, so I tried to get hold of some new springs.  I ended up temporarily replacing the springs with some tube to ensure that the seal was compressed well.  I also separated the rear exhaust box joint and resealed that, just in case that was having an affect on the emissions.  However, the car failed the test again.

The only other place that could be causing the problem was the manifold, so I decided to take it off and examine it.

It turned out that all 3 manifold ports were cracked as can be seen in this photo.  I used my TIG welder to repair the cracks.

Finally the car passed its MOT.