Contour Enthusiasts Group Archives Forums General General Why does the bank 1 pre-cat fail?

For those with the V6...

It seems that the bank 1 (aka firewall-side) pre-cat seems to be a weak point on the car. Many people seem to get the dreaded P0420 (Catalyst System Efficiency below Limit - Bank No 1). But the bank 2 (aka radiator-side) pre-cat doesn't seem to be as predominant.

NOTE: If you search for "420" in the Troubleshooting Forum, there are ~ 275 hits, whereas if you search for "430" there are ~ 89 hits.

Does anyone have a theory on why the bank 1 pre-cats are failing much more (as opposed to the bank 2 pre-cats)?

Is this a chemical failure? That is, is the catalyst loosing its inherent ability to "catalyze" over time (possibly because of the higher temperatures that build on on the firewall). My understanding of a catalyst is that "theoretically" it should last forever.

Or is this a mechanical failure? That is, is the pre-cat crumbling or falling apart? If so, what is causing this and why only the bank 1 pre-cat?

Or is there some other reason that the bank 1 pre-cat is failing (e.g., bank 1 design vs. bank 2 design)?

Thanks ahead of time for any and all responses.

The only thing I can think of directly off the top of my head is it's because the Bank 1 exhaust manifold has a much higher flow rate due to it's better design.

The front bank manifold design is big pile of crap! (Not saying the rear is great, just much better than the front)


You see the "common" code is the sensor is "below threshold" That doesn't mean the sensor is bad it's just that the deviation from sensor 1 to sensor 2 is not outside the minimum limits set in the PCM.

Perhaps...
The "threshold" should be been set differently for bank 1 due to it's better flow.
The higher flow is not letting the pre-cat scrub the exhaust enough. Especially as the precat ages.
The O2 sensor see's more unburnt fuel and just loads up faster and starts reacting slower.


I'll stop there. It's just a thought off the top of my head...

I would bet on a couple of things

1. Failure is generally mechanical or excessive heat. The firewall converter does not get as much external airflow as the front cat. So it probably runs hotter.

2. Heat is generated by misfires. The ignition system on our car may be more prone to misfire on the rear bank than the front. This could be due to the waste spark style of ignition. I have read that many people notice greater wear on the rear bank plugs than the front bank.

3. Since the firewall side is much more difficult to access, Murphy's law comes into to play such that the likelyhood of failure is directly and inversely proportional to the ease of access of the device.

Not trying to start an argument, but I doubt the rear bank gets a measureable amount of greater exhaust flow than the front.

Catalytic converters fail from meltdown (Usually from excessive fuel in the exhaust caused by a misfire or other engine problem causing a rich running engine), physical breakdown of the matrix and rust. Theoretically, though, the catalyst material itself can last "forever".

If volume is the same, and pipe size the same, the velocity would be about the same. However, I was talking about the Precat converter, rather than the O2 sensor. I thought the question related to pre-cat failure rather than sensor failure.

...and I agree with you on that.


I was just addressing the issue he was thinking every 420 code meant a bad bank 1 precat. It doesn't really have to mean anything is "bad" at all.

I agree with you on your point too.