2000 Cadillac Deville CATALYST EFFICENCY BELOW THRESHOLD BA

BIGEC4000

MEMBER
2000 CADILLAC DEVILLE
V8
2WD
AUTOMATIC
55,000 MILES

MY CAR WHILE AT IDLE IS ROUGH, CHECK ENGINE LIGHT SITS ON. TRY TO DRIVE IT CHECK ENGINE LIGHT FLASHES AND DRIVES ROUGH. MY CAR SMOKES AT IDLE AND AT DRIVING. GET A TEST CODE READER AND IT SAYS MISFIRE AT 1 CYLINDER. GO IN SPARK PLUG COVERED IN HARD BURNT OIL. IS THIS DUE TO VALVE STEM SEALS WORN? AND WILL THIS CAUSE CATALYST EFFICENCY BELOW THRESHOLD CODE? CAUSE WHEN I PUT IN A NEW SPARK PLUG IN NUMBER 1. IT RAN FINE. NO CODES UNTIL THE NEXT DAY. SAME CODES. SPARK PLUG HAD RESIDUE OF OIL. HELP!

1 Reply

RASMATAZ

MECHANIC
75,992 POSTS

Misfire at no.1 also check the compression and injector on it-

A code P0420 may mean that one or more of the following has happened:

Leaded fuel was used where unleaded was called for
An oxygen sensor is not reading (functioning) properly
The engine coolant temperature sensor is not working properly
Damaged or leaking exhaust manifold / catalytic converter / exhaust pipe
Retarded spark timing
The oxygen sensors in front and behind the converter are reporting too similar of readings

Some suggested steps for troubleshooting a P0420 code include:

Check for exhaust leaks at the manifold, pipes, catalytic converter. Repair as required.
Use a scope to diagnose the oxygen sensor operation (Tip: The oxygen sensor in front of the catalytic converter normally has a fluctuating waveform. The waveform of the sensor behind the converter should be more steady).
Inspect the downstream heated oxygen sensor (HO2), replace if necessary
Replace the catalytic converter