I'm not sure, but does the T190 use a separate ECU to the Ford PCM like the T230 ?
That used a Pectel T2 ECU to control the engine management. The Ford PCM was retained though to control the security and ancillaries like fans, petrol pump, alternator, a/c, etc.
Therefore, as the EML, controlled by the PCM would be permanently on......if the bulb in the instrument cluster wasn't removed !! So not surprising it never comes on for any engine fault.

On bad idling the first things to check are the same as a N/A engine. All the PCV & vacuum hosing, then ICV. Boost (unless vacuum leak) has little/no bearing, same as MAF and/or MAP, not relevant to the idle management system.

Martin