Canon i-SENSYS MF729Cx Important Safety Instructions And Installation Manual page 577

Envelope Switch
Adjust this setting if envelope printouts are light, or envelopes do not feed smoothly.
Envelope 1
Envelope 2
<Menu>
Switch> Select <Envelope 1> or <Envelope 2>
<Envelope 1>
Prints on envelopes with normal fixing power.
<Envelope 2>
Prints on envelopes with fixing power stronger than <Envelope 1>.
Special Mode P
If the printed paper curls, select <On>.
Off
On
<Menu>
Clean Fixing Assembly
Clean the fixing unit after a toner cartridge is replaced or if black streaks appear on printouts.
<Adjustment/Maintenance>
<Adjustment/Maintenance>
<Special Processing> <Special Paper Processing>
<Special Processing> <Special Mode P>
<Envelope
<On>
Fixing Unit