The key contact has gotten contaminated. They are a black conductive rubber pill pressed against circuit traces on a board bellow the keys. Remove that key and maybe others to get at the contacts in silicone strips. Clean ONLY with 99% isoprophyl alcohol and Q tips. Clean both the pills (two per note) and the circuit traces they touch. You have to release the spring at the back of each key to remove the key.
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The key contact has gotten contaminated. They are a black conductive rubber pill pressed against circuit traces on a board bellow the keys. Remove that key and maybe others to get at the contacts in silicone strips. Clean ONLY with 99% isoprophyl alcohol and Q tips. Clean both the pills (two per note) and the circuit traces they touch. You have to release the spring at the back of each key to remove the key.