We were approached by a local coach company and asked to investigate an engine management fault with their 2014 2.2 diesel Tourneo minibus. After driving the vehicle approx 5 miles the engine malfunction lamp would illuminate and default function deployed. The companies own mechanics had already fitted a new maf (mass air flow) sensor after retrieving an error code associated with that component (a mistake we see quite often) After running a diagnostic check there was indeed an error code logged P00BD (maf circuit range performance air flow too high) After monitoring live data from the maf sensor all appeared to be normal as expected with a new sensor i.e. air flow and air temp. It is important to note that the air temp sensor incorporated in the maf sensor is air temp 1, there is another sensor (air temp 2) located in the intercooler assembly. The next step was to check the signal from this sensor, after viewing the display via live data the temp displayed was clearly abnormal. The sensor supply and ground were checked and found to be ok. A new sensor was installed and the temp displayed was then as expected. An extended road test was carried out with no faults apparent. This example was a case of correlation between two variables and basic physics, cold air is denser than warm so the abnormal high temp signal from inlet temp 2 indicated the air was less dense than it actually was which did not correlate with the signal from air temp 1.