Fault code PO420

DAVID SIPILA

MEMBER
1998 CHEVROLET ASTRO
6 CYL
2WD
AUTOMATIC

Caty. Conv. Efficiency low bank one.
The converter has been replaced and the code later appeared again. The oxygen sensor was then replaced and again the code appeared. It does not set the code imeadiatly but does so with no apparent opperating characteristic to cause it.I've run out of options?

2 Replies

RASMATAZ

MECHANIC
75,992 POSTS

The most common cause is contamination of the catalyst because the engine is burning oil or leaking coolant internally (leaky head gasket or a crack in a combustion chamber or cylinder). Converters can also be damaged if they overheat due to ignition misfiring that allows unburned fuel to pass through into the exhaust (check for a fouled spark plug or bad plug wire). The same thing can happen if the engine has a bad exhaust valve that leaks compression into the exhaust (check compression).