Hi,
I have a 1999 Mercury Mystique 6 cylinder 2.5 liter. I recently replaced an old fuel pump which had debree in it. With the old fuel filter the car would run for two days in which it would then conk out. I would relieve the pressure at the fuel filter and then it would run fine for two more days. I replaced the fuel filter, and the fuel pump. I flushed the lines right were it would enter the fuel injector, and at the fuel pressure regulator. After installing everything back together the car still wont start. I have checked it is pumping gas all the way to the engine now. It is getting spark and the car would run with the old fuel pump for a few days. Is it possible I did something wrong while connecting the fuel pressure regulator back up. I am also getting a new code number p0420. Any suggestions?

 

The P0420 could be a number of things. Bad O2 sensor, catalytic convertor, even an intake manifold leak. The code calls for catalyst system efficiency below threshold. That's just how the PCM is reading it though. Could be caused by too lean on one bank or another and is more often than not, an O2 sensor problem.

You can test an O2 sensor by attaching multimeter leads to the sensor but the car needs to be running to do so.

The O2 will not prevent the car from starting though. The no-start symptom does sound like a sensor though. My girlfriends car had the same issue and it was a bad cam-position sensor.