hi akll, i have a 96 legacy just replaced the 2.5 motor cel is on cat code with the old motor was getting 23-25 mpg with same code, the new miotor im getting 16-17 mpg, any suggestions?

What’s the mileage on each engine? 

Have you done a compression check on the replacement engine? 

Why was the engine replaced? 

Cheers 

Bennie

1st engine blew hg lost oil pressure it ran, made a lot of noise, 171000 on replacement Thank You Bennie

Edited January 10, 20206 yr by djmark7

possible misfiring from non-oem plugs/wires

possible bad knock sensor

possible bad ECT sensor

double check all ground connections.

CEL is for tha cat, no other codes popped up

P0420 can be caused by other issues, some of which may not throw a code. OF COURSE an older car could need a cat. just that, vacuum leaks, exhaust leaks can also throw that code. many folks have had cracked/bad knock sensors that didn't throw a code. Plus, regular misfiring could explain poor mileage AND the excess unburned fuel could have destroyed the cat.

the rear O2 sensor is what 'monitors' the cat, it or its wiring may be bad.

Cats are pricey, never change one based ONLY on the 420 code, there are other tests that can help pin-down the problem.

Edited January 11, 20206 yr by 1 Lucky Texan

Thanks, Tex