This could happen right after replacing the batteries, after using the tachometer for a long time, or when using it for the first time.
Insert the new batteries after performing the following reset operation. This procedure completely discharges the condenser in the HT series Handheld Digital Tachometer.
Reset operating procedure
1. Remove the batteries.
2. Press and hold the power (measure) switch for several seconds.
3. Put in the new batteries.
Replace all the batteries at the same time. If you
mix used batteries and new ones, the lifetime of the batteries will
be shorter due to their potential difference.