Check that the pressure in the MS is within machine specifications. If the pressure is too high or too low then the MS will not work and the ion source will automatically turn off. Issues with pressure are caused by a short within the diffusion pump heater, leak within the MS housing or pump line to the MS, broken or loose gaskets, issues with the power supply, dirt within the seals or a faulty vacuum pump.
Check that the oil in the pump is at the correct level and is not dirty. Make sure all electrical connections are secure and that the proper power is reaching the MS. Ensure that the filament in the ion source has not burned out or is beginning to sag; this does need to be replaced every few years depending on machine usage. Check that all gaskets are the proper size and that they are seated properly. Check that all hose clamps are tight. Clean off any seals and gaskets.
Clean the ion source and multiplier if there is a significant loss of sensitivity. Sensitivity will slowly decrease over time but a sudden drop in sensitivity means that something is very wrong. Pull the source out and make sure that the filament is properly aligned with the electron hole. Make sure that the electron multiplier is clean and is getting enough power. An old electron multiplier can cause the MS to fail. For dirty ion sources, clean the source or bake it out at high temperatures.
Determine the cause of no signal within the MS. No signal can be due to no ion beam caused by a faulty power supply, the isolation valve being too closed or open and the wrong computer parameters set within the system.
Ensure that the sector slits are properly aligned if the peak shape in the spectra is bad or resolution is low. This can be caused by high pressure within the system, misaligned slits or an ion source that has started to fail.