S/V/C '01 S40 P0420 code

216,000 miles
I'm getting a recurring P0420 code (Catalyst System Efficiency Below Threshold (Bank 1)) on my 2001 S40.

Sometimes it resets itself after 120 miles +/-. Sometimes I reset the CEL with my scangauge 2.
I've checked for broken vacuum lines or leaks but don't see any.
The car seems to run well excepting a misfire at WOT (rarely have it at WOT).

One of the things I've noticed is that usually the CEL comes on in the same spot of my daily commute under the following conditions:
I'll be going downhill about 65 mph;
I will have just crested a not terribly steep hill;
I will be barely on the throttle.

For some reason, I think that last issue is key as the CEL has come on once or twice under similar (barely on the throttle) condition.

I'm in the process of trying to get the scangauge2 to read oxygen sensor voltage and will check the inlet and outlet temps of the front cat. too.

If you've got any other input, I'd like to hear it.

420 code ? who ever came up with the 420 phrase should send Volvo a cease & desist order!

Sorry I'm no help with WWD.

A P0420 code is set when the downstream oxygen sensor mimics the upstream one. When the cat is working properly, the downstream sensor will not oscillate like the upstream one. Typically, this indicates a failing cat but can also be an indication that the cat is unable to catalyze an excess of unburned fuel which could be caused by a misfire.

Address the misfire and any other engine related codes first. Clear the codes and complete the drive cycle to reset the readiness monitors and see if the catalyst code comes back. If it does, be prepared to buy a new one.

I have seen the rear o2 completely mirror the front sensor on a bad cat. I have also seen the code thrown where the rear o2 isn't mirroring the front one at idle.

Install a new cat, but fix the misfires first. I would use a factory level scan tool to monitor the misfire data and then swap could around and see if the miss follows the coil.

I should clarify.
There are no other codes showing. The misfire at WOT is detected by the seat of my pants. The car sees WOT maybe once or twice a year. I just don't drive it that hard.
I guess what I'm saying is that when the P0420 code shows up, it's not right after a WOT.

Curious why you guys think the misfire at WOT could be causing a cat. efficiency code.
Maybe it's misfiring during normal driving conditions but is not detectable or bad enough to store a code?

Misfires bad enough to blink a CEL mean enough to possibly damage a cat. The raw fuel ignites in the kitty and can burn it up. If there are no misfire codes, throw a cat at it. I did one on my 2001 v40 and it was only $900.

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ZVOLV said:

throw a cat at it. I did one on my 2001 v40 and it was only $900.

Click to expand...

Pretty sure you're not serious given your proclivity to diagnose before throwing parts.
Pretty funny though.

Low efficiency of the cat. Not unusual at this mileage.

It needs a cat. The p0420 code has always been a true code in my experience.

I'm an idiot.

I forgot that I asked about this issue last year because the issue went away. I started a new thread last week - https://forums.tbforums.com/showthread.php?t=349004.
Mod., delete the new thread if desired.

Still, would anyone know the maximum voltage specification for the following two oxygen sensors?
Upstream is Bosch part # 30617335;
Downstream Bosch part # 30617336.
I have a ScanGaugeII reader. It reads out a percentage of the sensor's maximum voltage.
So that's why I'm trying to find the max.

Strange that I haven't had any CEL (including P0420) codes since last fall - when ambient temperatures were generally less than 60 F. Fast forward to this week where spring ambient temps. have just started touching 70F and the P0420 CEL code shows up again.

Could it be that colder winter ambient temps. may have had something to do with the P0420 code showing up?

I could just throw a new front catalytic converter at it, but not crazy about "crossing my fingers" that will solve the issue at $200 for a new aftermarket converter (Rockauto).

Posting for posterity.

I ended up replacing the entire exhaust system from front cat. (Magnaflow) to the lousy Bosal tailpipe tip.
$850 later, it's all back together. Damn, those exhaust parts are pricey.

It may be crazy to spend that kind of dough on a 18-year old S40 with 235,000 miles, but aside from some rusty spots, everything still works on this car and I consider it a reliable dd.

Edit: I should add that it's been 250 miles since the cat. replacement and there hasn't been a P0420 code...yet.