Do I need to use magnetic ink or toner when printing checks?

To process checks, banks’ automated check sorting equipment relies on numeric information that appears at the bottom of checks and is printed in magnetic ink. This information is known as the check’s magnetic ink character recognition line, or MICR line, and contains information such as the routing number of the bank on which the check is drawn, the account number on which the check is drawn, and the check serial number. Generally applicable industry standards for original checks long have required the MICR line to be printed in magnetic ink; the need for magnetic ink on original checks is not the result of the Check 21 Act. Only the MICR line of a check must be printed in magnetic ink. The rest of the information on the check, such as the date, the payee name, and the amount, can be printed in regular, non-magnetic ink.

If you make payments by printing checks at home and the checks you use have pre-printed MICR lines, then the rest of the information that you print on the checks need not be in magnetic ink. By contrast, if you must print a check’s MICR line because it is not preprinted on the check, you should print the MICR line in magnetic ink.

https://www.federalreserve.gov/paymentsystems/regcc-faq-check21.htm