Two factors cause this. A failed catalytic converter won't reduce what CO is generated to the required level. The CO level can also be too high due to poor combustion. This relates to the air/fuel ratio as controlled by the ECU. The level in the exhaust gas before entering the catalyst is typically1% or so, which is converted to carbon dioxide within the catalytic converter. A T4 session may identify poor fuelling. The need for a catalytic converter replacement is quite likely I'd say.
TC
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