p0420 and 02 Sensors

[Rob Brown]

2008 Pontiac G6, 140,000 miles
Hey there,
I’m trying to diagnose a P0420 code, Catalyst Efficiency Below Threshold.
To do so, I hooked up a scanner with live data, got the engine up to temp, held the RPM’s just above 2K and looked at the live data for 02 sensors. My pre-cat sensor wasn’t jumping around like I had assumed it would. It would give me sometimes 13 consecutive readings at around .75V, then jump down to .1 or .3V, but it was spending noticably more time up around the .7V mark. I revved the engine and watched it drop to near 0…so it’s definitely not stuck high. And again, it would go low on it’s own…just nowhere near the frequency you’d expect.

The downstream sensor was holding steady at around .65V…sometimes a bit higher – .68V, but definitely not jumping around.

So my questions.
1. Is this behavior indicative of a bad 02? I DON’T have an 02 code, nor a code indicating a prolonged rich condition. I have a cat code (P0420) and a primary 02 that seems to be acting abnormally.
2. Is that downstream sensor reading (.65-.68V) indicative of a destructively rich condition? I know it’s supposed to be ideally at about .5V. But at the very least it’s holding steady.
3. With the downstream sensor holding steady, I’m leaning toward the conclusion that the problem is NOT the cat…especially when I see the primary O2 acting wacky.

Any thoughts at all appreciated!

Topic

[jason]

i would look and c if maybe you have a vacuum leak and etcg has a how to test on checking the converter with a digital thermometer that might help.

[EricTheCarGuy]

You might also check it’s temperature to see if it’s working to confirm. Usually however, P0420 codes are bad cats. Everyone hopes for a bad O2 but that usually sets a different code.

Good luck and keep us posted.

[Author]

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