checking for a plugged cat using load and rpm

I was wondering if watching the load and rpms during a test drive would give any indication of a partially plugged converter. We normally do back pressure test at different locations in the exhaust to determine which cats are plugged but sometimes the tech does not do a thorough check and and skips the pressure checks completely, relying solely on the check engine light codes for determining which cat to replace. This misses the third cat flow after the rear O2 sensors completely. So I've begun some test driving for the exhaust repairs and was thinking about what could be watched for a possible plugged converter. Thoughts?

Not that it's any of my business, but did you get burned by a Ford Duratec engine? :silly: The upstream cats always crumble and plug the third cat, usually after coil failure.

If you haven't seen it already, ScannerDanner has a couple great series on restricted exhaust testing:



www.scannerdanner.com/scannerdanner-prem...ght=WyJleGhhdXN0Il0=

I don't know that there's a scan data test that will 100% condemn a plugged cat, but it can definitely put you on the right track. Getting comfy with known good vehicles is key. As Paul says in the videos, if you have good fuel at WOT but poor airflow/volumetric efficiency, then you can be confident you're chasing a breathing problem.