The engine trouble light comes on after 200 kilometers of driving. The mechanics reset the Car computer. The engine trouble light stays off for 200 kilo then comes on at deceleration. This is intermittent.

The mechanic keeps hooking the scanner to my 2000 Mazda Millenia S . Been hooking up the scanner for the last three weeks. Code P0431 comes up. He clears the car computer or resets it so the light dosen't come on.

What should be checked. Before spending hundreds of dollars. Any good suggestions? Anyone with similar symtoms?

What is your S odometer reading ? It can be a oxygen sensor (Mazda says it over sensitive) or air leak at the exhaust manifold or the bank 2 cat convertor. In the US, cat convertor is warranty up to 8 year/80000 miles.

The odometer is 169,000 kilometer. Is the leak in the exhaust system leak?? Or vacuum leak??. The catalytic was removed because it was fried. The oxygen sensor were not changed...is there a way to check them to find out what type to get so I can replace them? Thanks

I think it is out of emission warranty. Remove either front cat converter (the one mounted on exhaust maninfold) will set converter efficiency code. You have P0431 because you remove the bank 2 cat converter (the one at radiator). Before replacing with a new cat, make sure all other systems are working ok (Ignition, fuel, air ....).

The reason that the engine light comes on after about 200km is that the emissions test has to go through a number of specific cycles before the test is completed. The cats in the manifold are fairly pricey. I had a similar problem. A custom muffler shop removed the catalyst from both manifolds put in a large standard cat (for a V8) just downstream and moved the O2 sensors after it. There is just enough room to do this. The one cat now takes place of the 2 in the manifold. There is another cat further downstream ... emmissions should not be a problem.