Lenovo T420 - Manual (Page 68)

Before servicing ThinkPad T420 and T420i
Some models of the ThinkPad T420 and T420i you are servicing might have the SIM card that the customer
has installed.
If the computer you are servicing has the SIM card, remove it before you start the servicing.
To remove the SIM card, you need to remove the battery pack first. (See “1010 Battery pack” on page 63.)
After you finish the servicing, make sure that you insert the card back into the slot firmly.
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62 ThinkPad T420 and T420i Hardware Maintenance Manual