Jim Grant's Tech Tips
'96 Mercury Sable, OBD II Code P0420
you tell me what OBD Code P0420 Catalyst system efficiency below
threshold (bank 1) refers to? Is this an O2 sensor problem or does
the catalytic converter need to be replaced? This is on my 96 Mercury
Sable with a 3.0 liter V-6 engine.
computer in your vehicle uses an oxygen sensor in front of and behind
each converter. The converter processes and reduces exhaust emissions.
The computer program knows that there will be a difference in the information
being reported from the front O2 sensor and the rear O2 sensor as the
converter processes the exhaust emissions. As a converter ages it becomes
less efficient. This loss of efficiency is known by the computer because
of the information that the O2 sensors mounted before and after the converter
report. Just because you have a converter efficiency code doesnt
always mean the converter needs to be replaced. A defective O2 sensor
can cause this code to be set by the computer as well. Your technician
has to check the O2 sensors to be sure that they are operating correctly
and that other related systems are doing their job. If all checks good
then a replacement converter is needed.