99 Contour 2.5L - DTC P0420

Guys,

I've recently had the 'check engine' (CE) light come on. Scanned it and it showed only code P0420. I've had no trouble with the car's performance, yet. The CE light did shut off by the 3rd time driving it after the light 1st came on.

I've read a few other threads on that code. Looks like is could be the CAT or Oxygen sensor (OS) on the rear side of the engine.

A few questions:
1) Should I erase the code and forget about it until the light pops up again (like maybe it was just a fluke)?
2) If it comes on again, is there a reliable method to test the CAT and/or the OS(s) to see which is bad?
3) If the problem is the OS(s) does this code mean it could be either the pre- or post- CAT (or both)?
4) If I do begin to have performance issues, can I pretty much count on it being the CAT?

Thanks for any input you can provide.

I see my thread has had 79 views, yet not one reply. Did I not provide enough info? Can anyone help answer my questions?

Welcome to the forum!

A P0 420 DTC is set when the OBDII system sees a drop in converter efficiency. It measures the exhaust gas upstream to the exhaust gas downstream. If the catalyst of emissions exceeds 150% the light comes on.

You can erase the code and forget about it, but the light will probably come back on. If the light went out again the third time you started the vehicle it is indicative that it was a soft code (happening only once) as compared to a hard code (an ongoing problem).

If the light comes back on with a 02 sensor DTC, the downstream 02 sensor is bad. (It could be the heated circuit not working, loose or corroded wiring, contaminated sensor element) The same is true for an upstream element. In either case you would have an 02 sensor code rather than a catalyst below efficiency code. A sure way to check if a cat is plugged is to put a vacuum gauge on the intake. A reading of 17 inches or less indicates a plugged cat or a restricted exhaust muffler.

A good sensor should show between .3 and .8 volts. A flat line sensor is a bad sign that it is on the way out. However after warm up a downsteam sensor should slow down and go flat once the cat is working.

Bottom line, your cat is probably on the way out and needs to be replaced. Any performance issues are a definate yes that the cat needs to be replaced. You're very welcome. ~Dave~

Thanks for the info Dave. I read you input long ago but never responded. I did solve the problem and thought I'd post it.

As I was looking over the engine. I finally noticed the air intake's connection with the air filter assembly duct was loose. Very loose. Too loose to have happened on it's own. (I had a new battery installed a couple of weeks before. I guess he felt like he needed to disconnect the entire filter assembly to do the work. I was PO'd.) In any case, I cleaned up the internals of the duct and throttle plate, reconnected, and it's been fine ever since with no code repeats.

Thanks for the input anyway.

Respectfully,
Jerrold

Thank you for coming back and letting us know what the problem was. Hopefully this will help others with the same thing.