P0420 code thrown in a 2000 forester 160k MT. The car runs really terrible, stutters and hesitates under acceleration almost all of the time in 1st and 2nd gear and a little bit in 3rd gear. I am looking into changing the front 02 sensor like the other threads suggest, but I was curious if the 02 sensor could make it run that bad? Also a possible exhaust leak (could that cause it to run this way?)

 

Another bit of interesting information, I ran my scangauge2 on the car and the fuel system loop status seemed to change from open to closed when it wasn't supposed to. (The information I saw said that it was usually a closed loop except when the engine was closed or at full throttle) Is that regulated by the front 02 sensor (or both?)

You should probably concentrate on why the engine is running so poorly, and not specifically on the P0420 code. If emissions are bad enough before the catalytic converter, things can't be cleaned up sufficiently in the cat, and you'll get a P0420.

 

A "lazy" or failing front oxygen sensor can cause all kinds of grief. However, if the system isn't remaining closed loop once the engine is warm (with the possible exception of WOT), something else is wrong. One possibility is a bad ECTS (engine coolant temperature sensor). If your scanner can monitor that, see if what it says makes sense.

Edited August 28, 201015 yr by OB99W
Corrected transposed words

What he said.

 

Lots of things can cause a P0420, but since you have the ability to monitor specific components while the engine is running you should check their values to be sure they make sense for the conditions present.

Frexample: If immediately after starting, your ECTS says the coolant temp is 30°F, but it's 75°F ambient outside temp, then the ECT isn't right. This throws off the fuel mixture which screws everything else up.

 

Also, the O2 sensor output does not influence fuel mixture until the sensor reaches operating temp of about 600°F. So if the car is running poorly right from a cold start the O2 sensor is probably not the culprit.

Edited August 28, 201015 yr by Fairtax4me

i keep forgetting to try this or if i asked before - on the old gen stuff you can disconnect the oxygen sensor and the car runs fine. can you do that with new gen stuff?

Thanks, I'll see if I can program the scanner to read the temp gauges. Is there a way to bench test the 02 sensor, for instance what should the voltage read on it when its running and its resistance readings?

No resistance testing on an O2 sensor. They create electric current, not restrict it. You can test resistance of the heater, but that's not going to change the output of the sensor.

This page has great info for how to bench test. http://mr2.com/TEXT/O2_Sensor.html

 

i keep forgetting to try this or if i asked before - on the old gen stuff you can disconnect the oxygen sensor and the car runs fine. can you do that with new gen stuff?

Sure. It'll set a code, but defaults to failsafe fuel setting that will at least allow it to run, just pig rich.

Edited August 30, 201015 yr by Fairtax4me