P0420 Code

The ECU is probably expecting a greater difference. I'd

No answers, but some thoughts: Observing your data, the issue comes down to 3 possibilities: The front O2 is bad The rear O2 is bad The catalyst is bad

The front O2 may be bad, but the ECU is not throwing a code . . . . and this is a normal occurence. I have the same car, 1999 Sentra GA16DE motor, and I recently replaced the front O2 even though I had no check engine light. Look at my thread here;

That cat may also be bad. Since it is the most expensive item to replace, I would start with the O2s first. Usually, the front O2 goes bad first. It's in a more hostile environment sitting right up there in the exhaust manifold.

You can diagnose further by looking for symptoms. I was able to find out that my front O2 went bad because the car lost power. Since the car is new to you, you may not notice what's missing. Does your car feel peppy from idle to about 2500-2700 rpms, then sort of lose its ability to accelerate? If so, then I'd go with the front O2 sensor, because it tells the ECU when\how to fire the injectors, hence play with the car's power curve. The rear O2s reason for being is just to make sure that the cat is working.

Hope this helps.

CD

That's a nice site, but it's not perfect. For example, it says that all OBDII cars have a front and rear O2 sensor. My car is OBDII and it only has a front O2 Sensor.

CD

Thanks for the helpful ideas. Yes it's a California set-up (two O2 sensors and two cats). It is new to me so I can only compare it to my

Have you tried connecting and disconnecting the O2 sensors, just to make sure the connections are alright? Or try driving it with the revs up to get the exhaust real hot, as to burn any deposits that may be accumulating on the sensors or even on the cat converter.

By the way, in the autoshop place, when you take the online tests, Scan tool diagnosis, there is a bunch of problem scenarios with scan readings and the right answers. It is worth your while.

Good luck.

al wrote:

check if someone put the front sensor to the rear(interchanged)

"al" wrote: >

Just got a 1999 1.6L Sentra with 99000 miles. The car runs > great. > The engine is smooth and quiet, it idles beautifully and all > four plugs > look fine. Gas mileage is fine too. However, the check > engine light > comes on setting a P0420 code (catalyst effeciency below > threshold, > bank 1). If I turn the light off, it comes back on in 2 or 3 > drive > cycles, so the problem is persistent. I know that P0420 is > set when > the ECM sees the rear O2 sensor tracking too closely with the > front O2 > sensor. So after driving for an hour and getting everything > to normal > operating temperature I put it on a scanner and ran the engine > at about > 2100 rpm for a minute or two to watch the O2 sensors. I did > the test > four times. Between tests I let the engine idle for a minute. > All 4 times the front O2 sensor switched about every > second or two > from about 200mV LEAN to about 800mV RICH. That seems normal. > The > rear O2 sensor behavior was puzzling though. The first two > times the > rear O2 sensor never switched but rather stayed very constant > at about > 400mV LEAN and very slowly rose to about 500mV LEAN. The O2 > heater was > always OFF. The next two times the rear O2 sensor again never > switched > but was now at 700mV RICH and rose very slowly to about 800mV > RICH. > The O2 heater was now ON. I'm not sure how to interpret these > results. > Is there a problem with the rear O2 sensor? Also, why is the > light > coming on if the two O2 sensors are operating so differently? > I'd > appreciate any thoughts as to what's going on and what the > problem is. > Many thanks.