My '96 Saab 900S has multiple symptoms, and I suspect multiple problems. For one thing, it gets the error code: P0422 Main Catalyst Efficiency Below Threshold (Bank 1)

I made a video of my vacuum test results. The idle speed and vacuum pressure tend to fluctuate mildly, and if the A/C is on, the idle speed oscillates 200-300 RPM and the vacuum drops below normal. Intake leak maybe? Or exhaust blockage?

After holding the throttle at a higher RPM for a few seconds, the needle goes crazy! Bad slip rings, I suspect. But there's also this ticking sound, which doesn't change with the speed.

So my best theory is a combination of bad piston rings and either an intake leak or exhaust blockage (bad cat?). I'd appreciate any advice as to what to check next, or tips from folks who have seen this sort of problem before.

  • Where are you getting your vacuum port from? Is it close to the throttle body? – Pᴀᴜʟsᴛᴇʀ2 Aug 23 '15 at 0:09
  • No, I tapped from the EGR valve outlet. Ah. Shouldn't have done that, huh? – Keith Aug 23 '15 at 16:50
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    Try taking the reading at the crackcase ventilation hose. It usually goes from the crackcase to the air intake, sometimes before and sometimes after the MAF. – Robert S. Barnes Aug 23 '15 at 17:27

From your description, I doubt anything you've stated is at fault.

I would bet the vacuum port you used for testing is associated with another device which is where you heard the "ticking" sound. As this alternate device is turned on/off, it is causing the wild vacuum readings you are seeing. If I were to guess, I would think you are probably reading fluctuations due to an Exhaust Gas Recovery (EGR) valve which is electronically controlled via solenoid (the solenoid making the ticking sound).

If you move your test apparatus to a different, non-shared direct vacuum port, you'll probably find a truer vacuum reading and the crazy vacuum readings will go away.

As for the P0422 code, it can be one of two things:

  • Bad cat (obviously)
  • Bad O2 sensor

To test which it might be, the most accurate method would be to test the inlet/outlet temps on the cat with a laser thermometer. Run the engine up to operating temperature before you make your readings. Take your readings as close to the cat as you can, but still on the inlet/outlet exhaust pipes. If the readings show the front temperature to be close to or above the temp of the outlet, the cat is bad (or going bad). If your test shows that the outlet temp is much higher (100°F+), the O2 sensor is most likely bad.

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    Yeah, I took the reading from the EGR valve outlet. (Facepalm) Thanks for the answer! I will look at the cat. I still think the idle reading is a little low, and the idle speed tends to fluctuate, so I will look around for leaks too. But it's those oscillations I was really curious about. – Keith Aug 23 '15 at 17:01

That is a leaking intake valve. The needle swing coupled with the "tic" sound are classic symptoms. Off comes the head. After repair of the engine problem clear the codes and see if the CAT fail code resets. Poor combustions can fool the catalyst test.

source: 30 years automotive diagnostics.

  • I'm just wondering how it can be an intake valve when the tic sound doesn't increase with engine speed? – Pᴀᴜʟsᴛᴇʀ2 Aug 23 '15 at 13:27
  • In the video I thought the tic kept in time with the needle swing. If that is not the case the only other cause is the gauge seeing a EVAP purge valve. Normal engine vacuum pressure pulse change is under 1/2 in/hg when measured with a pressure transducer. – Fred Wilson Aug 25 '15 at 15:12

I thought I should come back and reanswer this, for future googlers, because I found the cause of the P0422 error, and it wasn't the cat or O2 sensor at all!

A while later, when I got another diagnosis, the computer was throwing a P0411: Secondary Air Injection System Incorrect Flow Detected also. As it turns out, a vacuum hose connecting to the SAI shutoff valve had come disconnected. The hose was part #7 in this diagram.

I reconnected it with a proper-sized hose and cleared the codes. A day later, the check engine light went off, and I passed the emissions test! Not to mention, I suppose I found the suspected vacuum leak.

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