850 Exhaust leak and cat. efficiency

1995 854T; 258,000 miles

Can an exhaust leak downstream of the catalytic converter cause a 4-4-3 (catalytic efficiency) code?

There is a small leak at the front pipe /rear pipe junction and also a hole on the top of the muffler. Front O2 sensor was changed at 216K and the rear sensor at 250K.

I was told that an exhaust leak downstream of the converter could cause the 4-4-3 due to a "lack of vacuum" in the exhaust system after the cat. That just sounds unlikely to me.

Not likely that an exhaust leak after the cat will cause any codes.

I tried to find a cross reference for you but couldn't.

So, take this with a grain of salt.

4-4-3 is the same as a p0420 OBDII code if I remember correctly.

Here is some info on p0420

Hopefully someone else can chime in and confirm my recollection.

Rear 02 measures the catalyst efficiency. With a scan tool, I graph the front and rear o2 sensor voltages. If the rear one mirrors the front one=bad cat. Doesn't always show up that way when there is a code.

No, I have never heard of that claim. (Edit-coworker says he's seen it. "Depends on how close the leak is to the sensor") I haven't seen it yet. However, exhaust leaks before the front o2 sensor can throw your mixture way off, to the point of kitty damage.

If the leak is after the rear sensor, it won't have any effect on it. There isn't vacuum in the exhaust, there is slight pressure and flow. Depending on the leak, you can wind up with a venturi effect pulling some fresh air in, but that wouldn't cause this code.
Basically, the computer compares the switching of the two sensors, and if they get too close it assumes the cat isn't working right. The easy fix is to get an 18mm spark plugs non fouler, more it out to fit the nose of the rear O2, and put it in between the sensor and pipe. It's still there, your cat is still there, it just numbs the overly sensitive computer a bit.

The downstream exhaust leak can actually mask the low efficiency cat by affecting the rear O2 Sensor to read leaner post-cat - not cause the code. Basically the opposite from what you were told.

Darkness said:

I tried to find a cross reference for you but couldn't.
4-4-3 is the same as a p0420 OBDII code if I remember correctly.
Here is some info on p0420

Click to expand...

That's what I thought too. I was using the black-box code readout. The link confirms what I've learned about the cat. efficiency codes.

ZVOLV said:

Rear 02 measures the catalyst efficiency. With a scan tool, I graph the front and rear o2 sensor voltages. If the rear one mirrors the front one=bad cat. Doesn't always show up that way when there is a code.
No, I have never heard of that claim. (Edit-coworker says he's seen it. "Depends on how close the leak is to the sensor") I haven't seen it yet. However, exhaust leaks before the front o2 sensor can throw your mixture way off, to the point of kitty damage.

Click to expand...

I have a scan tool that will display voltages of the front and rear sensor. Since the car isn't registered right now, it's tough to get them up to temp in the driveway. I may have to just leave it idling for a half hour or so. I should also check the inlet and outlet temps of the cat. Wish I could drive it around to get it properly up to temp.

The "vacuum" explanation doesn't make sense to me either.

4x4_Welder said:

If the leak is after the rear sensor, it won't have any effect on it. There isn't vacuum in the exhaust, there is slight pressure and flow. Depending on the leak, you can wind up with a venturi effect pulling some fresh air in, but that wouldn't cause this code.
The easy fix is to get an 18mm spark plugs non fouler, more it out to fit the nose of the rear O2, and put it in between the sensor and pipe. It's still there, your cat is still there, it just numbs the overly sensitive computer a bit.

Click to expand...

I've seen the anti-fouler fix before, just never had to try it. I've read that it works, but don't know anyone that could confirm in real life. I'm going to try it. It's coincidental that I've been dealing with the same code (P0420) on my '01 S40 for the past year and was getting ready to try an anti-fouler. Maybe I can get a volume discount!

Thanks all for the help.