Diagnosing bad cat?

[linust]

2004 A4 1.8tq tip, AMB
170k miles
Fuel mileage is poor--in-town driving averages 17-19 mpg.

Trying to confirm the P0420 (low cat efficiency) code I have is truly a bad cat and not the result of another ailment...
Are there particular measurements I can log in VCDS to confirm?

I don't want to just replace parts chasing codes. And if I'm replacing the cat, I'm leaning toward the 034 HFC. The existing cat is not original (Bosal, date code is 0611--not sure if that's 2006 or 2011)

Related: if I go with the 034 HFC, is the O2 sensor spacer required? I live in an emission-test area.

History:
When I got the car (heavily molested in the past by POs), I had P0420 along with SAI related codes and intermittent misfires on cylinders 3 & 4 (interestingly, I never had any Evap codes which bedevils me on my C5 S6!).

Since buying the car last summer:
- new plugs
- new injector o-rings
- new PCV hoses (034)
- replaced vacuum check valves, confirmed SJP and vacuum hoses are intact
- replaced Carbonio CAI with stock airbox
- timing belt, water pump, coolant flange, j-plug, etc replaced

Now, only the cat efficiency code remains.

[tHatOne guY]

Couple ways to confirm a failing cat.

Infrared thermometer temp, pre-cat should be lower than post cat temp.

More accurate way here... Check post cat O2 voltages, should be very stable and not fluctuate at all if the cat is good, as the mixture has passed through the catalyst neutralizing the AF ratio as opposed to pre cat voltage which will fluctuate regularly.

[jesse500]

I have a good stock Cat, a fairly new 3" HFC & a 3" Catless downpipe if your in need of one!

[diagnosticator]

The OBDII cat efficiency monitor implemented in the ECU firmware is specifically designed to evaluate the conversion efficiency of the cat converter. The monitor evaluating the cat efficiency can only proceed with the test after confirming correct function and accurate information from the O2 sensors and anything else that could effect the monitor accuracy. Everything is checked and confirmed correct before the monitor is allowed to run. Consequently, the DTC describing the "low cat efficiency" would not be set if something else was actually causing conditions duplicating a tired cat converter. If that was the case then the off spec operating parameters of the other dependant hardware would set other fault codes instead. This is a fundamental technical requirement for OBDII emissions monitors, established by Federal and California emissions regulations.

[ECS Tuning-Audi]

The OBDII cat efficiency monitor implemented in the ECU firmware is specifically designed to evaluate the conversion efficiency of the cat converter. The monitor evaluating the cat efficiency can only proceed with the test after confirming correct function and accurate information from the O2 sensors and anything else that could effect the monitor accuracy. Everything is checked and confirmed correct before the monitor is allowed to run. Consequently, the DTC describing the "low cat efficiency" would not be set if something else was actually causing conditions duplicating a tired cat converter. If that was the case then the off spec operating parameters of the other dependant hardware would set other fault codes instead. This is a fundamental technical requirement for OBDII emissions monitors, established by Federal and California emissions regulations.

Awesome info here. So cliff notes: If you have a low cat efficiency code you have a bad converter.

Jason

[SJorge3442]

Awesome info here. So cliff notes: If you have a low cat efficiency code you have a bad converter.

Jason

I feel its tough to say the cat is "bad" if you get a 420 code. With our cars being ULEV, it just means the cats aren't up to par with the ULEV standards, not necessarily completely bad. A cat on our cars throwing a 420, may still be cleaner than a none ULEV car with a brand new cat. Just something to think about.

[ECS Tuning-Audi]

I feel its tough to say the cat is "bad" if you get a 420 code. With our cars being ULEV, it just means the cats aren't up to par with the ULEV standards, not necessarily completely bad. A cat on our cars throwing a 420, may still be cleaner than a none ULEV car with a brand new cat. Just something to think about.

True. But does that mean it will or will not pass emissions? Technically if it wont pass emissions you'll have to replace it anyways I would imagine. I think that would be the biggest concern.

Jason

[SJorge3442]

True. But does that mean it will or will not pass emissions? Technically if it wont pass emissions you'll have to replace it anyways I would imagine. I think that would be the biggest concern.

Jason

Depends on the state and their inspection rules I guess. I'm not one to beat the system when it comes to this stuff, but I hear a o2 sensor spacer will keep the 420 code away and pending a smog test with a somewhat decent cat, you should still pass . I don't drive more than 5k miles a year anymore so I don't have to worry about emissions testing in PA.

[ECS Tuning-Audi]

Depends on the state and their inspection rules I guess. I'm not one to beat the system when it comes to this stuff, but I hear a o2 sensor spacer will keep the 420 code away and pending a smog test with a somewhat decent cat, you should still pass . I don't drive more than 5k miles a year anymore so I don't have to worry about emissions testing in PA.

Anyone know if you can still pass a smog test with a spacer and a test pipe? Haha.

Jason

[old guy]

Awesome info here. So cliff notes: If you have a low cat efficiency code you have a bad converter.

Jason

Lots of info in this thread: Clicky click®

[Luxus Panzer]

I have a good stock Cat, a fairly new 3" HFC & a 3" Catless downpipe if your in need of one!

What kind of HFC? 034?

I may need one ;)

[linust]

Thank you diagnosticator and old guy--just the people I was hoping would respond!
Logged B1S1 and B1S2...and as described in old guy's link, I have a fairly dynamic B1S2 :(

My work replacing vacuum check valves and PCV lines must have dislodged a few other items...I have introduced an EVAP code and I now have an N75 problem (0.00% WG frequency logged)...both of these may be related to the use of a pre-'03 silicone TIP (from APR) which has some convoluted routing of extra hoses...need to find an '04 TIP with all the right hose connections.

The good is I have until August to get this sorted and pass emissions! The bad is it's gonna be effing hot in my garage in a month.

[16vCorey]

Thank you diagnosticator and old guy--just the people I was hoping would respond!
Logged B1S1 and B1S2...and as described in old guy's link, I have a fairly dynamic B1S2 :(

My work replacing vacuum check valves and PCV lines must have dislodged a few other items...I have introduced an EVAP code and I now have an N75 problem (0.00% WG frequency logged)...both of these may be related to the use of a pre-'03 silicone TIP (from APR) which has some convoluted routing of extra hoses...need to find an '04 TIP with all the right hose connections.

The good is I have until August to get this sorted and pass emissions! The bad is it's gonna be effing hot in my garage in a month.

I may have a late APR TIP that I'd trade for an early, if you're interested. I'll have to double check that extra one I have is a late.

[linust]

I may have a late APR TIP that I'd trade for an early, if you're interested. I'll have to double check that extra one I have is a late.

Yes, interested! LMK...as I said, this car was heavily molested by a previous owner, and mostly (but often incorrectly) restored to stock. Thanks

[jesse500]

What kind of HFC? 034?

I may need one ;)

Local Canadian Made. Emico