155 Q4 has a CO potentiometer for carbon adjust when idle
its adjust air-fuel mixture at idle

if its faulty or removed, ecu read zero value
range is -64 to +64

if your fuel pressure is 2.5 bar (std)
zero value is correct and not need adjust

but if you need it ?
when fuel pressure 3bar ?


generally need re-soldering for repair
CO trimmer has one 1k resistor and one 10k potentiometer