O2 sensor questions

[pennzoil_guru]

I have a 2000 Civic with 16DY7 engine. I had been getting DTC P0420 a few times (Catalyst efficiency below threshold bank 1). There were no other codes. I graphed both the pre and post O2 sensors on my scan tool. The pre cat sensor switches rapidly in closed loop and shows the pseudo sine wave shape and cycles rapidly between < 200 mV and > 800 mV. The post cat is fairly flat with occasional spikes. I back probed both sensors with a DMM and recorded the min/max/avg values at 2000 rpm. The pre cat had readings of 0.890 max, 0.02 min and and average of 561 mV. Everything I have read says it should have and average of 0.450 V. The post cat sensor max 0.911, min 0.144, avg 0.722. I recorded both for about 2 minutes. I know the scan tool graphing function and the DMM are not fast enough to catch glitches or drop outs. I can't afford a scope right now.

Question: is the pre cat average of .561 too high (too rich)? Is the post cat average of 0.722 too high? I appears the post cat is indicating either the cat is not functioning or else I have an ongoing rich condition. My short term FT is between 1 and 11% and the long term is pretty constant at 5%.

Also, my IAT and ECT readings with the engine cold (not run for > 24 hours) are always 15 F apart. The cat is an aftermarket with about 60000 km on it. The O2 sensors are NGK and also have 60000 km on them. Cat was replaced over 2 years ago for the same code by my wife while I was away at school.

 

[JHZR2]

aftermarket cats stink bigtime, super-low metals loading that is only enough to get it to pass for a time. A new aftermarket can be as bad as a marginally functional OE cat. I went through that before... made that mistake.

60k km is not a lot for an o2 sensor, but it can bring out the worst in a cat.

 

[mechtech2]

The average should be .5V. But .561 is virtually the same - normal.
The cat or rear O2 is bad.
But is your front O2 sluggish or off a bit? Maybe, but is it not bad enough to throw a code.

 

[pennzoil_guru]

Without a scope, is there anyway to tell if the front sensor is bad. The
scantool graph doesn't have a time division marked on the bottom

 

[Vikas]

Given that you have the scanner, do this.

Hook it up to show both the sensor voltage on it and then take it on the highway. Keep it at highway speed for a minute or so and then take your feet off the gas pedal. Have your helper monitor the voltage now. If both sensor display similar switching, you know why you are getting the P0420 code. If the switching frequency is high for the rear sensor, you converter is no good.

Given it is an aftermarket converter, you are lucky to get 6 month out of it. Was it replaced to fix the P0420 code? Your choices are either OEM converter or O2 sensor extender. Google it.

- Vikas

 

[The_Jeff]

Too bad you're in canada. IIRC US aftermarket catcos have a mandatory, lengthy warranty. (5 years?)

Your temp senders should be closer together (2 degrees or so) when cold. With the parabolic ohm "ramp" resistance-vs-temp table a small error at room temp can grow larger at running temp.