p0240 Random problem at low rpm
 

Hi everyone, this is my first post on this forum.
I own a honda civic Dx 1998 150 000 miles 1.6
Since 4 months I had a p0240 error on my ecu (P0240 Catalyst System Efficiency Below Threshold (bank 1))
Honestly since the car was running well I though that I would only take care od it when it is inspection time again.
But recently I encountered a very strange problem, one out of four times the car is ineficient at low rpm I have a manual so I can go to higher rpm before clutching but still it is allmost undrivable. but the 3 other times the car runs fine, that is very strange. Seing the randomness of the event I first though it would be the 1st oxygen sensor sending misleading signal so I unplugged it to run in closed loop and see if the problem was still happening. It ran fine 3 times but the 4th I had the same problem.
I have been advised a tune up but I changed spark plugs and wire less than one year ago. Do you thing that a plugged cat can have this strange random effect, also I didn't checked the distributor last tune up.
Thanks you in advance for your help, feel free to ask me if you need anymore informations.

that would be a p0420, a clogged cat could cause a symptom like you are describing. will seem to run fine, with no missfire. but have no power at all when stepping on the gas, and possibly not shift properly. may be random at first. have you had it rechecked for any new DTC's?

Does it run smooth, but slow, or does it misfire / sputter / stumble? If the latter did you use aftermarket wires? Some of them are of very poor quality and might not last a year.