what is the main cause why my car failed the emission test

Here is an article on why you could have failed your emission test: http://autos.yahoo.com/maintain/repairqa/exhaust_muffler/ques077_0.html

a longshot but an o2 sensor possibly? Also make sure that you have a cat and not running on a straight pipe.....also make sure your car isn't leaking oil and running rich into the exhaust causing blue smoke. For that it could be a head gasket, or maybe you have a faulty cap on your oil, dipstick could be leaking or a break in your lines somewhere. Honestly it could be a whole smack of things but fixing those would improve your chances for sure!

I had a '92 Integra GS Hatchback and it failed because of fancy Spark plugs that is listed below. Acura integra engines don't like fancy spark plugs, like Bosch Platinum+4 or Bosch Platinum+2 spark plugs. The engine rejects the spark plugs by having premature ware like corrosion on them. This premature corrosion causes the spark plug to misfire. Use NGK, Champion or any spark plug that is gaped properly for your cars make and model.

Cadalic converter almost guarantee it