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This paper documents an investigation into the performance and thermal efficiency of an air-cycle Environmental Control System (ECS) artificially injected with common operational failure modes. A two-wheel bootstrap system is taken from an in-service military fast-jet and installed in a bespoke Ground Test Facility (GTF) at the ECS Research Facility, Loughborough University, UK. The failure modes investigated are bleed air blockages in the intercooler and in the low-pressure water extractor, as well as positional inaccuracy in cycle bypass control valves. The full range of degradation in each fault is considered, allowing the quantification of overall system performance degradation. The performance of the system is found to be insensitive to moderate bleed air blockages (up to 80% by pipe cross-section area), whilst blockages at low pressure are more detrimental to cycle performance than blockages at high pressure. The cycle and/or control system will self-regulate around most degrading-type faults. This particular system is most sensitive to a failure at one bypass valve, where the hardware allows partial redundancy of the valve but the control system does not.