Long story short.... (I had a previous problem posted and that has been fixed for about 1-2 years).. now it's back...

1) P0420 code comes up, but ONLY when I am driving the car over 50 MPH. If I drive around in the city, it never comes on. If I drive on the highway, P0420 comes on about 30% of the time.

When I bought the car 3 years ago, I saw that the O2 sensor on exhaust manifold was replaced (it looked new). I also replaced a few things on the Evap system due to fuel smell (and this improved my gas mileage by about 20%).

SO... did all this excess fuel before I fixed the problem damage the CAT? Should I try to replace O2 sensor in the CAT first? If that doesn't work, then replace the CAT ?


My take is that I think the CAT is bad, due to slowly being damaged slowly over the years and things have finally caught up with it.

Can anyone tell me WHY the code ONLY comes on when I drive on the highway and not in the city? could it be due to a slight misfire when on the highway ( like timing is off a bit at high speeds? or maybe a slightly inefficient ignition coil?)

many thanks for any opinions !

 

Can we assume it's a 4-cyl?

If you think the rear O2 sensor is original, 170k miles, it might not be a bad idea to replace it. (as much as I don't like to suggest throwing parts at something without testing)

Fuel smell from EVAP problems is probably leaking from the EVAP canister, purge valve, little hoses, etc. More likely that leaking fuel is not actually in the combustion chamber, so it won't do anything to the cat.

Anything like an exhaust leak going on? O2 sensors actually measure the oxygen DIFFERENCE inside vs. outside, so exhaust blowing over the outside of the O2 sensor will throw off the reading. Or oily crud dripping nearby?

 

(yes it is a 4 cyl auto-transmission V-TEC) I fixed the fuel smell problem over a year ago ( replaced the EVAP solenids, valves that are connected to the EVAP system, etc...) I believed back then alot of the fuel vapor from the EVAP was getting into the vacuum lines and going into the intake manifold (therefore increasing the fuel amount going into the engine- even though it was a slight increase). After fixing this, the gas mileage increased by about 20%.

now the P0420 is back ( after about a year), and trying to decide what to do. I guess i have nothing to lose by changing the O2 sensor at about $60 for a Bosch brand. But I am also wondering if the CAT was already damaged by the extra fuel over the years ( before i bought it and fixed the problem) and now the CAT is just starting to fail. I guess the main thing that perplexes me is why the P0420 is the only code and that it ONLY comes on when I drive on the highway.

As for oily crud... are you saying I can try spraying (cleaning) the outside of the sensors to remove any oil/contamination that sprayed on the sensor? just thought of something else, maybe I can check the connections and wires going to the O2 sensors?

AS for throwing money... I dont mind as long as I am zeroing in on the problem. better to shotgun 2 O2 sensors, sparkplugs, maybe the ignition coil, rotor/distrib cap for about $200-$300 knowing they will be good for a few years... then deal with the CAT last.

 

Check out Rock Auto: $23/downstream & $39/upstream Denso.

Even a minor exhaust leak can cause a P0420; something to check for.

 

Not saying to spray off the outside of the sensor. Just wondering whether the outside of the sensor IS dirty with oily crud. If that's the case, I'm not sure what's the best way to clean it - or whether it's been damaged.

Absolutely, check wiring connections. Anywhere. The car can only throw error codes based on it's own measurements, so it can't tell the difference between a bad sensor vs. bad wiring.

Throwing money at sparkplugs & normal maintenance items isn't bad. New O2 sensors might not be so expensive either. I wouldn't throw money at a new catalytic converter without some level of confidence that it's worthwhile.