cat or 02 sensor
 

Greetings:

I’m get an error code on a ’98 328is, “Catalyst efficiency below threshold Cyl #1-3”. (it's table 11, code E9) Could I get some opinions on whether this means the cat itself is bad or if it could be an O2 sensor issue? Anyone else had to troubleshoot this code? SHould I start with the 02 sensors and hope or go straight for the car? TIA - Dan

It usually means the cat is bad. Reset see if it comes back then swap the post cat sensors and see if the side changes. Use PBBlaster or the like.

thanks Jeron, I'll give it a try
 

Will give it a shot and report back. thanks...

cat or o2 sensor
 

On both my '97 328s, the "catalyst below efficiency" code was the first indication of an oxygen sensor that was getting slow to respond. BMW says that they last only 90k miles, and that is about how long the front ones last. I replace all four at once.

I have 400k combined miles on my '97s, so I have replaced several oxygen sensors. You don't need any special tools. A combination wrench works the best.