The operation of a diffusion type gas detectors used in fixed, machine mounted and handheld applications is reliant on the natural equalisation of dissimilar gas concentrations driven by partial pressures inside and outside the detector. Typically, this equalisation is inhibited (to a greater or lesser degree) by protective filters and barriers surrounding the fragile sensing elements from the typically harsh ambient environments. The accumulation of dust and other foreign matter on the protective filters can further inhibit the diffusion of gas into a detector. The usual calibration process for a gas detector – typically by a ‘bump’ or ‘challenge’ test – will often fail to detect when a detector is blocked, or partially blocked. This can lead to the ‘calibrated’ detector reading high or low, but with no way to determine if that is the case. Retrospective examination of records and equipment from Pike River Mine lead to the conclusion that critical detectors were affected by filter blockages, resulting in methane detectors reading approximately one-half of the true concentration. This presentation explores how a blocked detector can give an erroneous reading, and what steps can be taken to avoid replicating previous mistakes.