Hi guys, i just finished putting my 240 together, and got it all certified…but i failed my e-test terribly. It has all new plugs, wires, map sensor, and tps. It seems to like bog and/or cut out when i’m driving sometimes, and sometimes its hard to start. Also, when it was really cold yesterday my speedo didnt work for like 10 minutes (until it warmed up). I’m thinking maybe the speedo was just frozen, but what could be the problem with the car running kinda rough? Could it be the speed control sensor? If so, where is it located?
If your emissions system failed the nitrogen oxides (Nx) and hydrocarbons (HC) tests you should change the O2 sensor as well as the cat. This is because when an O2 sensor gets gummed up and starts to wear at around 30,000 KM it sends a weaker signal (lower voltage) to the ECU. That weaker signal translates to a demand for more fuel to the ECU (engine computer) and richens the engine’s fuel/air mixture. The combustion process can’t burn all the fuel and this results in a higher amount of Nx and HC emissions. An O2 sensor runs about $40-$50 and is located on the downpipe between the header and the cat. It is very easy to change on a 240SX.
Disclaimer: the increased Nx and HC emissions could be caused by a number of other factors such as faulty wiring between the ecu and the O2 sensor, bad fuel injectors and fuel injector wiring, even a faulty ecu to name a few. If the car fails emissions after changing the cat and O2 sensor have a qualified shop diagnose the problem.