1997 850NA - P0422 code but no engine light

[dejongmc]

I brought my car for a State Inspection today and the engine had a P0422 code (Main Catalyst Efficiency Below Threshold).
The engine light is off so I was wondering if this could be an old code or something.
Shouldn't this code trigger the Check Engine light?

From what I found out so far, big chance my catalytic converter is up for replacement, to fix this problem.

[dosbricks]

Sounds like you had an "event" (like backing off the throttle for an exit ramp) but not enough events yet to trigger the light. Bad O2 sensor can sometimes set this code. I've had it sporadically for several years on my wagon. I first replaced front and then rear O2 sensors and then the cat. It continued throwing the code about every 3 months. I just drive for a few months, wait for it to come back, and clear it off with my reader. We happen to live where the air is clean and there is no problem getting inspection. These cats are more durable than the stuff that monitors them.

[dejongmc]

I checked my repair records last night and I replaced the cat when the car had 155,000 miles on it in 2007 with an aftermarket FlowMaster cat. Total repair back then was about $150. Worked fine for all these miles and years. The warranty was only for 25,000 miles but I drove almost 60,000 miles with this aftermarket cat. That is pretty good value for money to me.

I think the O2 sensors are doing their job and I will get the cat replaced today.

[dejongmc]

Update: Last week I reset all the OBD codes and when finally all the OBD monitors came back on (took a few days) I now have two codes: P0172 (system too rich) and still P0422 (Main Catalyst Efficiency Below Threshold).
After reading about P0172 I checked all my vacuum lines (which looked good) and I changed my MAF, drove 2x 30 miles, but both codes are still there.

Question: Will these OBD codes disappear if the problem was fixed? Or do I have to reset each time to see if they come back?

[rspi]

How is the PCV system?

If properly repaired, the codes will stay off.

[MrGruffles]

If you fix the problem but don’t clear the codes, they will clear themselves automatically from the OBDII system after the car has gone through a certain number of 'complete reach-operating-temperature-after-cold-start' drive cycles without the problem recurring. If the codes never disappear or come back after being cleared, then – of course – something is still unresolved.

I personally would try a couple things before turning to a new cat:

1) I know you checked the vacuum lines but they’re the first thing I’d re-check, including condition of all rubber elbows, connectors and missing nipples on the vacuum tree. On models later than mine I understand that there’s a hidden hose which is often missed and known for creating a too-rich condition. The hose is part of the PCV system and attaches to the end of the intake manifold on the passenger side, hidden in a difficult to reach place behind the power steering pump. It or its connecting elbow often deteriorate. I had the same code on my 1994 850 Turbo and it was caused when a very small diameter hose became disconnected from the PTC valve. At other times, I found vacuum elbows that appeared OK but cracked when flexed.

2) At the risk of being called a parts-changer, I’d gamble by replacing the O2 sensor associated with the P0172 code – I believe it’s the upstream one before the converter. The ECM gauges the converter’s effectiveness by comparing the readings between the upstream and downstream O2 sensors, so a failing sensor can mimic a faulty cat.

[matthew1]

P0172 is almost always the "evil" vacuum elbow:

https://www.matthewsvolvosite.com/forums ... 00#p233700

https://www.matthewsvolvosite.com/forums ... =1&t=27706