Quick question. I've posted my Civic's problem a while back, thowing P0420 code, and have done the following:

Tune-up, New OEM CAT, upper/lower O2 sensors.... still no change.

Original problem was intermittant stalling, along with the code, seemed to run hot on occasion. Stalling was solved by putting in an new ignition module.

I've noticed a dark, dirty, oil-like residue in my radiator reservior, and when the radiator's been topped off, the radiator fluid seems to leave the radiator for the reservior (and not siphoning back...), leaving the radiator fluid level low (even when I carefully followed the filling procedures).

Q: can backpressure caused by a cooling system side headgasket leak cause a condition of lowered radiator fluid levels in the radiator and the inability for the reservior to back siphon fluid back to the radiator when the system is heated????

Q:... an ulitimately... can a condition like this somehow affect what the car is sensing as an exhaust problem tripping a P0420 code?

(No milky motor oil, no white smokey exhaust. I need to check for moist exhaust).

I haven't driven the car since May. 165K on a 99 Civic. Any thoughts? Thanks for the assistance in advance.

Best Regards... Rob

 

No idea...my 03 Civic has same code