Same as any check valve.....one way air flow.
You want exhaust going back up INTO the AIR System when it is not activated or the pressure from the exhaust is more than the pump???????????????????????????
Check valve eliminates that possible.....BUT, you could have a crapped up /stuck check valve that is not functioning ...and that will stop the air flow from the pump from ever reaching where it is supposed to go...
I think you get it. It is all about controlled air-flow at pre- warm-up.
Emmissions Device ..
Be aware that most AIR codes are caused by hose vac leaks or no pump engagement due to fuse, etc....but if those are found to be OK and one has a high milage 02 sensor, that is always suspect as a code 4 can actually be a technical tip-off of an aging/weak 02 sensor ,where the AIR system has in reality nothing wrong with it at all. That is where understanding the realtionships of codes and systems interactions are pragmatic....
In other words, a weak/old sensor that has become less sensitive and lost some range capacity is no longer capable of reading the very lean condition present with AIR system activation and will in effect report an inaccurate finding back to the DM with a perfectly operational AIR system..it is the monitoring 02 sensors sensitivity signal that is faulty, not the system. Kinda like a bad temp gauge telling you the car is running hotter than it actually is...so you go change the thermostat before checking the temp gauge and wind up with the same problem...so then you change the radiator ... nope, that didn't work, etc ....Diagnose Twice, fix it once....
