OXYGEN LO QTY
(Caution)

Oxygen pressure has fallen to a low level below 1,400 psi (75%), an oxygen bottle pressure switch is indicating < 45 psi (bottle discharge), a failed pressure switch, a shut-off valve has been left closed following maintenance action.

• Oxygen System Pressure has less than 1400 psi
• One of the four Oxygen Storage Cylinder has less than 45 psi
• Pressure Transducer or a Pressure Switch
• Data Acquisition Unit (DAU) 3
• Associated Wiring

Advisory Wire/Service Bulletin: None

Forum Articles/Infoservice/Newsletter: None

Notice To Operators (NTO): None

Flight Operation Notifications Manual (FONM): None

1. Service the oxygen system per AMM 12-13-01-614-801. Is the OXYGEN LO QTY (caution) CAS message still displayed?

   1. If YES, continue with next step.
   2. If NO, perform close out.

2. Perform a check of the pressure gauge on the oxygen ground-servicing-panel. Does the pressure gauge indicate below 1400 psi?

   1. If the pressure gauge indicates below 1400 psi, go to step 4.
   2. If the pressure gauge indicates above 1400 psi, continue with next step.

3. Perform a check of the pressure gauge on the oxygen storage cylinders. Does one of the pressure gauges indicate pressure below 45 psi?

   1. If the pressure gauges indicate pressure above 45 psi, go to step 5.
   2. If a pressure gauge indicates pressure below 45 psi, continue with next step.
      NOTE: The oxygen pressure in the cylinder must not be less than 50 psi (345 kpa) at any time. If this occurs, the oxygen cylinder must be removed and sent back to the manufacturer to be purged and hydrostatic tested.

4. Perform a leak test of the crew oxygen system.

   1. If a leak is identified, rectify as required.
   2. If no leaks are found, continue with next step.
      NOTE: Observed fault 351OBS004 provides guidelines and documentation for identifying the source of an oxygen leak.

5. Perform a wiring check between the pressure switches and DAU 3 using System Schematic Manual (SSM 35-11-00).

   1. If wiring defects are found, repair defective wiring as required and do close out.
   2. If no defects are found, continue with next step.

6. Disconnect the pressure switch electrical connectors S48P1, S49P1, S50P1 and S51P1. Perform a continuity check between each pressure switch as follows:

   From	To	Result
   S48P1-B	S48P1-C
   S49P1-B	S49P1-C
   S50P1-B	S50P1-C
   S51P1-B	S51P1-C

   1. If there is no continuity, replace the defective oxygen pressure switch and do close out.
   2. If there is continuity, continue with next step.

7. Replace the oxygen pressure transducer. Is the OXYGEN LO QTY (caution) CAS message still displayed?

   1. If YES, continue with next step.
   2. If NO, do close out.

8. Replace DAU 3. You may interchange DAU 3 with another DAU to confirm faulty unit before replacing.
9. Do close out.