Hi,

Relative has a 5 y/o Focus, ECU reports P0420 - Catalyst system
efficiency below threshold. I read the code over 6 months ago and said
it's likely the cat but could also be the O2 sensors or something
else. I advised he take it to a garage and tell them the code, however
what he did was take it to a garage and get a new cat fitted which
hasn't sorted it! Groan (and yes they did drive it for over 6 months
with the check engine light on).

Anyway, to avoid costly further mistakes can someone tell me how to
determine if the O2 sensors are working right and if there's any other
likely problems to check for? I have got something that lets me see in
real-time the various values the ECU reports and among them are the O2
sensor outputs, should make things easier if the Focus ECU actually
sends the values. I don't know what to look for though!

Something that might be related is that I have to keep pointing out to
them (and they take no notice) is that they never exceed 2000rpm
unless in top gear, often I see them try to accelerate from well under
1k and feel the engine vibrate through the chassis, often as a result
of thinking that you can't possibly use 1st for more than 5 seconds
under any circumstances.

My limited understanding of this is that it means that it'll be
running rich most of the time, could that have a bearing on this?

Thanks again,

Peter.

Peter Spikings wrote:
Hi,

Relative has a 5 y/o Focus, ECU reports P0420 - Catalyst system
efficiency below threshold. I read the code over 6 months ago and said
it's likely the cat but could also be the O2 sensors or something
else. I advised he take it to a garage and tell them the code, however
what he did was take it to a garage and get a new cat fitted which
hasn't sorted it! Groan (and yes they did drive it for over 6 months
with the check engine light on).

Anyway, to avoid costly further mistakes can someone tell me how to
determine if the O2 sensors are working right and if there's any other
likely problems to check for? I have got something that lets me see in
real-time the various values the ECU reports and among them are the O2
sensor outputs, should make things easier if the Focus ECU actually
sends the values. I don't know what to look for though!

Something that might be related is that I have to keep pointing out to
them (and they take no notice) is that they never exceed 2000rpm
unless in top gear, often I see them try to accelerate from well under
1k and feel the engine vibrate through the chassis, often as a result
of thinking that you can't possibly use 1st for more than 5 seconds
under any circumstances.

My limited understanding of this is that it means that it'll be
running rich most of the time, could that have a bearing on this?

Thanks again,

Peter.

My first thought is a service, followed by decent fuel and an italian tune
up.

if there is still a problem then look for air leaks.
heated oxygen sensors do fail and can give the code you describe, with it
running you would expect the voltage from it (or them) to be fluctuating
below 1 volt