Hey guys, for the past two weeks i've been getting a check engine light (once a week). I went to my trusty AutoZone and they hooked up their OBDII scanner each time. The scanner read "Main catalyst below minimum threshold". I told the guy i had a full aftermarket exhaust fron to back (SR421 header, carsound high-flow cat and 2.25" exhaust). He also said when running with a CEL, the car is in "open loop" and "the car is no longer using the O2 sensor when the CEL is on (open loop) and that my fuel milage will decrease as well as performance".....

So first off, is that true (about going open loop w/ CEL on)?

second, what exactly is "Main catalyst below minimum threshold"? I'd figure i don't have enough back pressure or something from my exhaust setup especially with the high-flow cat. (Both times CEL came on while driving 45 mph @ around 2000 rpm in 5th)

third, how do i fix this?!?

[ September 23, 2002, 06:01 AM: Message edited by: Koreandude ]

 

sounds like your new cat is not cleaning the exhaust well enough and causing it throw a cel. but I am not sure.

but yes, CEL will **** up performance.

 

basically, the cat you have is not working properly. It is not cleaning up the exhaust enough, or someone screwed up your o2 sensors, and the #2 o2 sensor is before the cat, and the #1 o2 sensor is AFTER the cat.

When a bad Cat CEL is triggered, that should NOT send the ECU into open loop. But yes, prolonged open loop operation will cause poor performance and poor fuel economy. It will also probably clog your cat with unburned fuel.

 

What happens in open loop is that the ECU stops taking input from the O2 sensors, as the ECU has determined that the information coming from them is faulty. The ECU begins using pre-programmed info to determine fuel delivery/fuel-air ratio.

This can cause the engine to run richer, which will waste fuel, and cause unburned fuel to enter the cat. Then, heat from the exhaust and the fuel combine to create excessive heat in the cat, which can cause the material inside to melt into a big exhaust-blocking mess.

You should look into the position of the O2 sensors, as Random said.

If that checks out, then find out if your Carsound cat is guaranteed to be emissions legal... if it is, it shouldn't trigger a CEL if everything else is okay.

 

Well there shouldn't be any reason why the o2 sensors have switched positions becuase the header was put on months before the carsound cat/2.25" exhaust was installed. So any time my exhaust was modified, only 1 o2 sensor was touched. I can't see that any of my O2 sensors would be bad since i haven't had any CELs for O2. I can drive my car for almost a week without any CELs but sooner or later it seems, i get the same problem code (so far). I guess it's the CAT.