It's (almost) never the instrument.
"It's instrumentation. It must be instrumentation." said the subtitle. As a technician responsible for calibrating and troubleshooting various instruments in manufacturing, I can tell you from personal experience that, if you're getting a reading that is theoretically possible, it's probably about right. (Most instrument failures produce impossible readings, like -25 °C or 200 °C on a room temperature.) The number of times I've seen someone call for testing of a "faulty" instrument, and it turns out the readings were real, is beyond counting. Unless there is VERY strong evidence to the contrary, assume the reading is real and there's a serious problem!