Trouble code 420

[02legacygt]

Here is what I got

2002 Legacy L 2.5 motor with 156k

P0420 - Catalyst System Efficiency Below Threshold (Bank 1)

I need to get the car inspected and here in NY, there can be no check engine light on or it wont pass inspection

HOW DO IT FIX THIS PLEASE

the tech at Auto Zone was clueless.

[LunchBox04V]

Well, correct me if I am wrong, but my gen 2 EJ22 occasionally throws a P0420. I can clear it without issue using a code reader. I believe that the ECU will inhibit the P0420 for something like 120 miles or three driving cycles before allowing the code to show again.... I think...... My P0420 has not come back in a while..... Been spending most of my time on the FZ6.
So......clear it before going to the inspection. Get checked and cleared, and then find the solution..... Then again, you could do the legal thing and check the O2 sensor, check for leaks (this might actually be the problem) and reset the computer..... Hope this helps....

[rougeben83]

rear oxygen sensor.

[02legacygt]

so it goes on for about 200 miles, then goes off for 200 miles???

ANYONE

[httrdd]

so it goes on for about 200 miles, then goes off for 200 miles???

ANYONE

My Xterra for the longest time would have the o2 sensor come on and I would just clear it and it would come back a some miles later. Chances are if you get an continious o2 failure you are going to have to replace it. Ask me how I know! :roll:

[porcupine73]

That's not at all uncommon for that era soob at that mileage. I get that periodically on a 2000 Outback too. It's quite possible the catalysts are marginal at this point - after all that is what the code is monitoring for.

It can be front or rear o2 sensor needing replacement. It can be exhaust leaks, even tiny pinhole leaks. Sometimes it seems to happen on really windy days. Sometimes it happens with fuel from a certain station.

I have seen what some people do is put a spark plug anti-fouler on the rear o2 sensor and supposedly that keeps the code away.

After you clear the codes, make sure you check all the I/M monitors with a scantool before returning for inspection. They have to all say 'ready' or it can't pass. After resetting the codes it can take like a good 3-7 trips for all those to go ready. The evap usually takes the longest.

[ouch1011]

^^ Yeah there is no way you'll pass emissions by just clearing the code because it will show some of the I/M monitors as not ready.

Subarus are picky about cats. Sometimes you can get by with replacing the O2 sensors, but most of the time, the cats need replaced.

[Huffer]

Rear o2 sensor...