ZOLL M Series Operator's Manual page 22

REPLACE CO2 MODULE
ZERO CO2 MODULE
ZEROING CO2 MODULE
Module defective, incompatible, under
temperature, or temperature unstable for
more than 5 minutes.
Module cable connected to M Series unit
for the first time. Probe drift error detected,
or negative CO detected.
2
Module zeroing in progress.
EtCO - 20
2
Replace module.
Perform module zero as described in
"Zeroing the LoFlo CO2 Module" on page 8.
Wait for module zeroing to finish.