Centurion o2 ppm issues

So we have 3 new ITW/EAE Centurion ovens that all have issues when running nitrogen to keep the o2 ppm below 1000. It is always much higher, sometimes gets lower when no boards are loading in. They've had us replace valves and do an upgrade kit that feeds n2 into the valve box that switches the atmosphere and still doesn't really make a difference.

Was curious if anyone else has experienced this issue and have found a proper way to deal with it.

Any help would be greatly appreciated

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