An electronic controller having at least one microprocessor controls the solenoid operated valve adding oxygen to the breathing loop of a closed circuit mixed gas rebreather with carbon dioxide scrubbing in maintenance of an oxygen set point in the rage of 0.13-0.50 without any need for monitoring or interpretation of signal data by the operator. The controller receives signals from at least one oxygen sensor located in the breathing loop and sends signals to an indicator: visual, aural, or tactile; during operation providing only intuitively understood normal functioning, limited time remaining, and bail out indications. Automatic diagnosis including oxygen sensor calibration, indication of actions required such as scrubber replacement, and confirmation of an action taken with signals from an action sensor are provided.