Not necessarily. The PCM only cares if the cat is working or not, and is pretty lackadaisical about what it considers 'working'....... It could be totally plugged up, and the PCM would still be perfectly happy with it.

 

 

You could probably get away with it, and if your engine is in decent condition, it won't even be that much dirtier. (exhaust emissions-wise.)

 

 

In years past, bad lifters can also cause the "knocking" problem on start up. These are hydraulic lifters designed for maintenance free usage, but...they can and do fail. Also, are you using a good quality oil filter? Cheapo brand filters without check valves to trap oil in the filter for startup can let the oil drain from the filter causing a temporary reprime that'll cause something like this as well.

 

If your cat is clogged it won't rev up past a certain rpm. Did you do what i suggested in my last post? This will confirm if it's an oil pressure issue on start up or possibly worn bearings.

 

 

Nope. Didn't post.

 

 

When I had a noisy lifter, it was more of a pfft pfft pfft right at the initial rev following ignition. A "knock" should be more metallic sounding and deeper.

Valvoline maxlife cured my lifter noise over 100k ago.