emmadc33

Hi everyone , I recently just replaced the 2 front cats on my 92 ls400 that were clogged and also replaced the downstream o2 sensors . It started to run again but noticed that there has been a loss of power . When I floor it , it takes longer than usual to shift and it also limps a tiny bit . There is also a hiss sound which sound like air.

some things I want to add on for context:
when I was putting the cats back together onto the headers , I didn’t line up either flange flush with the headers and noticed that there was air coming out where the cats meet the headers and gets really hot under the car . (Would that be the problem because they’re not lined up flush?)

I have done a full tune up just to mention.

Thank you!!!!

Superfast1

When you replaced any emission related components i.e. catalytic converter(s), it's fairly normal to notice & feel a slight loss of power & acceleration at first as the ECU needs time to recognize & make the adjustment. However, in your case, since you didn't line up the exhaust manifolds with the headers on both banks & can hear the hissing sound then you're having a sizeable exhaust leak. An exhaust leak will cause a lean condition. Initially, the ECU will try to compensate the lean condition by adding more gas to the air/fuel mixture but due to a sizeable leak, eventually it won't be able to when the LTFT's go up to ~ 18%, it'll set & throw the P0420 & P0430 codes. You'll need to fix the leak ASAP to cure the performance issue & also to prevent the new cats from failing prematurely.