Lenovo T420 - Manual (Page 51)

Error messages
Table 3. Error messages
Symptom or error FRU or action, in sequence
Fan error. 1. Fan.
2. Thermal grease.
3. System board.
Thermal sensing error. System board.
This system does not support batteries that are not
genuine Lenovo-made or authorized. This system will
continue to boot, but may not charge unauthorized
batteries.
Attention: Lenovo has no responsibility for the
performance or safety of unauthorized batteries, and
provides no warranties for failures or damage arising out
of their use.
Replace the battery.
Chapter 4 . Related service information 45