[Dunwood]

Hi all,

My son has a '96 Trooper with the 3.2 V6. Mileage is poor about 10 mpg in around our hills, around town, zero Freeway. He is a new driver and could also be treating the acclerator pedal a little aggressive.

When we got the vehicle, I found a bad ignition coil on the drivers side. It was pulling a mis-fire CEL code and I traced it down. That is fixed. After I fixed the mis-fire code, I was able to clear all the codes and the vehicle passsed our CA smog test. My friend who gave us the vehicle said the CEL light had been on for close to a year and a half.

The P0430 bad cat code has come back. I think the mis-fire probably ruined the cat, but it did pass smog. Engine runs smoothly and has about 120K on the odometer. As far as I can tell, none of the O2 sensors have been replaced.

My approach would be to replace both upstream O2 sensors, and the rear stream O2 sensor behind the bad CAT. Clear codes and see what happens.

If the cat was "ruined" (showing below efficiency) would the mileage be effected? Or is mileage more likely effected by worn out O2 sensors.

No other CEL codes have been thrown, other than the mis-fire when I got the vehicle and the P0430 bad cat code.

How would you approach this repair?

Thank you in advance.

 

[SurferJoe]

You've got twin cats - right? That code is telling you that (as far as your P0430 code goes) that the ECM thinks the passenger side cat isn't working properly.

I'd check the chatter from the Post Cat O2 sensor first. Is it talking? Is it making up false numbers?

You need a decently good DIGITAL meter to read it (Hi-Z reasons) so you don't load down the ECM's miniscule voltage input from the Post Cat sensor.

First though - make sure that no ambient air can enter the exhaust stream anywhere before the post cat sensor. A cracked weld or a busted joint will do it.

And - you MAY have to have the code cleared with a bi-directional scanner.

 

[Dunwood]

Well thats interesting. Somewhere I read that the drivers side was Bank 2. If the passenger side is Bank 2, that's the opposite side of the failed-and-fixed ignition coil.

I'll start with some leak detection.

Also the vehicle has close to 120K on the odo. How likely are the O2 sensors worn out?

(I can replace them very reasonably, have a close friend that works at Denso).

I own a 20 year old Fluke digital VOM.

 

[SurferJoe]

Actually - #2 is counted from the #2 cylinder, whereas the #1 bank is counted from the #1 cylinder - so I may be off by a cylinder or two. I was just responding to the codes as called out in my tech-mans.

Oxygen sensors are SUPPOSED to be changed fairly regularly - it helps with your mileage and performance, beside the obvious positive beni's for air that I can actually breathe.

The post cat sensors are supposed to arrive at a very neutral position and just sit there once everything's warmed up if the cat is decently working. Any wild oscillations or chatter is just wrong.

If the values sit in the wrong side of the neutral zone, then that too is bad.

Some sensors are higher voltage than others, and I'm not totally sure the correct voltages that you need - suffice it that they should be somewhat solid voltage, performance-wise, and not swing all over the place. That's a good start for testing purposes.