Siemens SIMATIC S7-400 Reference Manual page 226

Analog modules
5.17 Diagnostics functions of analog modules
Diagnostic message
Reference channel error
Underflow
Overflow
Run time calibration error
226
Possible cause of error
The reference junction connected at chan‐
nel 0 is faulty due to a wire break, for exam‐
ple.
The reference temperature value transfer‐
red is not within the value range.
Input value below undershoot range; possi‐
ble cause of error: wrong measuring range
selected
With the measuring ranges 4 to 20 mA and
1 to 5 V, if necessary by polarity reversal of
sensor connection
Input value exceeds overshoot range
A wiring fault has occurred on a channel
during the calibration cycle
Remedy
Check the connections
Reassign the parameters for the reference tem‐
perature
Set another measuring range
Check the connections
Set another measuring range
Eliminate the wiring fault (fault remains until the
next calibration; in other words, a maximum 6 mi‐
nutes or until there is a STOP-RUN transition of
the CPU).
S7-400 Automation System Module Data
Reference Manual, Ausgabe 11/2016, A5E00850736-08