You guys are all guessing?It's not about passing emmissions test it's about yor car running efficiently.The three main things on the engine emmission test are 1. Nitrous oxides
2. Hydrocarbons
3. Carbon monoxides.
Nitrous oxides... means your burning too hot in your cylinders,that's why they pump in exhaust gases via the EGR system to cool it down. Check EGR sytem.
Hydrocarbons...is basically unburned gas(or petrol to you limeys etc).Check rich mixture,Misfires,bad cat converter. Carbon monoxide...is basically what is left after the burning process ..Too high?...Rich mixture.
You should have a CEL light if O2 sensor is gone or mixture is too rich. All the things the guys suggested oi,air filter etc are ok but you should be doing all these things anyway.The o2 sensor checks for oxygen levels in exhaust gases and signals the ECU to adjust pulse width of injectors i.e.how much gas your getting. Remember all this computerization is about emmissions i.e. efficient mixture and burning of fuel(stoichiometric ratio) and cleaning up any that did not get burned.
Try to read trouble codes and if you can't do that take it to a good repair shop because if you try to do it by elimination it will cost you a fortune.
Gleschu
