We recommend making sure your Volvo's EGR system is functioning, the engine is not running lean, there aren't any vacuum leaks, engine compression is not high, and the engine is not running hot. NOx emissions are created when engine combustion temperatures rise over 2500F. Any of the conditions mentioned above can cause engine temps to increase, resulting in high NO, consequently causing your Volvo 240 to fail the smog inspection.
The first smog part to be checked however should be the EGR valve. Your Volvo's EGR valve re-circulates a small amount of exhaust (inert) gas back into the combustion chamber to help keep the chamber temperatures low; reducing NOx.
Next... a "fuel feedback test" will ensure your 240 is running at optimum air/fuel ratio. This includes making sure the oxygen sensor is working properly, the ECU (engine control unit) is receiving the O2 signal and sending the correct signals to the fuel injectors to either increase or decrease fuel delivery to the combustion chambers. At the end of the feedback test it will be know whether there is a fuel control problem causing high NOx, and whether it is electronic or mechanical. The technician can further diagnose the fault.
There will be several other emission sensors which will be inspected during the feedback test as well; including the MAF (mass air flow) sensor, ECT (engine coolant temperature) sensor.
