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Discussion Starter · #1 ·
2002 4.3 V6
I started getting intermittent/random misfires when really pushing it (1000lbs in the truck going 70mph in third gear up a 5-6% grade) that would disappear as soon as I pulled back.
Considering most (if not all) of both ignition and fuel side was replaced, I started looking for other possible causes.
Someone suggested that primary O2 sensors might not be up to task (after 300k+ miles) so I decided to change them.
Soon after changing them, I got a CEL. P0420 (Catalyst System Efficiency Below Threshold Bank 1)
I looked up the O2 voltages and, just as expected, Bank 2 was perfect (primary going all over the place, secondary varying very little) while Bank 1 was dead (secondary sensor voltage followed primary's almost perfectly with a short delay)
I checked the temperatures and, once again, as expected, Bank 2 was working fine (catalyst temperature higher that the pipe going in) while Bank 1 was doing nothing (catalyst temperature was at least 100F lower than both in and out pipes)

So, it turns out, Bank 1 cat REALLY doesn't work but now I have three questions:

1: How is it possible that I never had that code before? Was the primary sensor THAT bad that it hid the problem?
2: Is there any way to "clean/renew" that catalytic converter or do I need a new one? I read about trying an "Italian tune-up" while using premium gas (I can do that), using some varnish cleaner (I don't want to do that) or some snake oil-looking "catalytic converter cleaners" (not sure about that)
3: Should I consider replacing the secondary sensors too (now or after fixing the cat)?

Help?
 

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Hello,

Did the O2 sensors fix the misfire? Most likely, it was the misfire that took out the cat. So, before you install a new cat, you'll want to be sure the misfire issue is fixed.

The ECM will not run the catalyst monitor if there are other DTCs (misfire, O2 sensor, etc.). Also, the ECM may refuse to run the catalyst monitor if there are performance problems preventing the enabling criteria from being reached.

In other words: Yes, it's possible that the other problems the vehicle was having were masking the bad catalytic converter. It's also very likely that the other problems caused the cat to fail.
 

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Discussion Starter · #3 ·
The misfire was obvious only when I was pushing it hard and I've been trying to do that lately but pre-pandemic traffic is back so I didn't have the chance (yet)
Whatever you said makes sense (more than the O2 sensors "hiding" the condition by them selves.
I ordered some "catalytic converter cleaners" I'll post back in a week or so with results
 

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Are you sure the misfire isn't being caused by the clogged catalytic. Your engine requires a certain amount air to enter the engine to function. If the catalytic is clogged it restricts the removal of exhaust which in turn prevents oxygen to enter the engine.
 

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Discussion Starter · #6 ·
I doubt that the cat is clogged (both exhausts entering the cats have a similar temperature)
The truck is using/losing about 1 qt every 2000 miles but, judging by the the spots on my driveway, most of that is leaks. When I changed the oxygen sensors, I disconnected the pipes from the exhaust manifolds and both were dry inside (no signs of oil, just soot)
I already used some "catalytic converter cleaner" and the O2 sensor voltages are just a little bit better but still not good.
Most likely I'll have to replace it. I'm very curious to see how it looks inside (after replacement).
I'll post pictures if and when I get it replaced.
 
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