EMB ERJ 170/175 ER/LR
Flight Crew Air Transport Pilot (ATP)
Human-Machine Interface; Confusion
Aircraft Equipment Problem Critical; Deviation / Discrepancy - Procedural Published Material / Policy
Flight Crew Diverted; General Declared Emergency
Aircraft; Human Factors
During cruise flight at FL310 the Captain and I noticed an abrupt fluctuation in cabin pressure. This was first identified by ear discomfort; and then confirmed by an abnormally high rate of climb on the EICAS. The cabin began to climb at approximately 2;000 FPM until a pilot advisory message appeared PRES AUTO FAULT.  We donned our O2 masks and verified that I was still flying the aircraft and 'working the radios' while he was to run through the QRH and advise the passengers and flight attendants. I informed ATC that we were experiencing a pressurization issue and would like 'lower.' ATC authorized a descent to FL200. The Captain continued to follow the QRH for guidance. Once level at FL200 the pressurization seemed to stabilize. The Captain made an announcement to the passengers and flight attendants. After approximately 10 min at FL200 the pressurization anomaly returned. The Captain advised me to tell ATC that we would like to land in ZZZ. I was recently informed that the Captain followed the incorrect QRH procedure; which resulted in the second pressurization anomaly. Instead of following the PRES AUTO FAULT QRH procedure he used the PRES AUTO FAIL procedure. This procedure instructs the pilot to cycle the mode controller from MAN to AUTO. By doing this; the system logic was overridden and the aircraft pressurization fault returned.
When an EMB-170 flight crew utilized the incorrect QRH procedure in response to a PRES AUTO FAULT warning accompanied by a rapid rise in cabin altitude they were ultimately unable to control pressurization and diverted to a nearby airport.