keeping in Trim

Decoding fuel trims can be a key part of engine diagnosis.
But what is a fuel trim?
The fuel trim is the ECU's adjustment of the fuelling based upon information received from the oxygen sensors.
A positive trim is the ECU adding fuel to the normal amount injected for the conditions reported by various sensors.
A negative trim is the ECU removing fuel.
They can be a used to diagnose various running problems, from air leaks to fuel pumps.

One such problem had baffled a number of garages, a Fiesta with a running fault.
On the road performance was much reduced, and it posted an air mass meter fault code.
The component had been replaced (twice), and wiring checks proved that no wiring faults existed. Live data showed only 2 volts from the AMM during acceleration tests.
However fuel trims showed -18% at tickover.

So despite low outputs from the AMM the ECU was removing fuel, this seemed to stump the previous garages. New O2 sensors were fitted before the ECU was sent away for testing.
The ECU returned with a clean bill of health.

So the question remains what fault would result in low AMM voltages and minus fuel trims?
Restricted engine breathing, so the usual suspects were checked. Air filter, intake trunking, and exhaust inc CAT condition.
A blocked CAT was found to be the cause.