Dashes instead of numbers on a pulse oximeter (no reading)
The oximeter is switched on and has a finger in it but cannot get a usable pulse signal, so it shows no numbers rather than a guess. On some models the last numbers stay frozen for a few seconds before the dashes appear.
Check your oximeter’s manual — makers differ.
What it is for
Count it as no reading. Take the finger out, warm it if it is cold, put it back fully in with the nail up, keep still, and wait. If dashes keep coming back, try another finger or check the batteries. Nonin says that on the Onyx Vantage 9590, if the signal is inadequate, “the last measured SpO2 and pulse rate values freeze for 10 seconds and are then replaced with dashes”, and that the display “will go blank after 30 seconds of no readings or poor readings”. Microlife says the OXY 200 shows «--» and «---» when the signal detected is insufficient or weak.
These pages report what each maker’s manual says and quote it wherever health is involved. They do not diagnose anything, they are not medical advice, and they do not tell you whether an oxygen or pulse reading is right for you or what to do about it; only your own oximeter’s manual and your doctor or nurse can do that for the person you are looking after.
- What it looks like
- Short horizontal dashes where the SpO2 and pulse numbers should be.
- Names makers use
- Dashes after 10 seconds of inadequate signal (Nonin Onyx Vantage 9590); «--» and «---» (Microlife OXY 200); Blank display (Nonin, after 30 seconds without readings)
Where makes differ
Not every model shows dashes. Microlife’s OXY 300 lists “SpO2 or pulse rate do not display normally” in its troubleshooting table instead, with causes that include the finger not being inserted correctly and an SpO2 “too low to be measured”, and for the last case says to measure again and, “if you determine the product is working correctly, consult your doctor”. Nonin’s 9590 switches itself off after a 2-minute period of inadequate pulse signals.
What it does not mean
Dashes are not a zero reading and do not mean there is no pulse or no oxygen. They are not an error code either: the oximeter is working but has nothing reliable to show.
Other symbols with the same shapes
Sources
No published standard we could check defines what a fingertip pulse oximeter’s display shows, so this meaning is taken from 2 makers’ own manuals and support pages, each fetched and read on the date shown:
- Nonin: Nonin Onyx Vantage 9590 finger pulse oximeter — instructions for use (P/N 10906-001-04, 2020, PDF) — Equipment response time: with an inadequate signal “the last measured SpO2 and pulse rate values freeze for 10 seconds and are then replaced with dashes”; auto shut-off after a 2-minute period of inadequate pulse signals. Checked 2026-09-29.
- Nonin: Nonin Onyx Vantage 9590 — instructions for use leaflet (P/N 114847-000-02, 11/2022, PDF) — Cautions: “This device’s display will go blank after 30 seconds of no readings or poor readings.” Checked 2026-09-29.
- Microlife: Microlife OXY 200 fingertip pulse oximeter — instruction manual (IB OXY 200 S-V10, 2026-09-14, PDF) — When the signal detected by the pulse oximeter is insufficient or weak, values are displayed as «--» and «---». Checked 2026-09-29.
- Microlife: Microlife OXY 300 fingertip pulse oximeter — instruction manual (IB OXY 300 N-V9, revision 2023-11-30, PDF) — Troubleshooting: SpO2 or pulse rate do not display normally — finger not inserted correctly or SpO2 too low to be measured; retry, and consult your doctor if the product is working correctly. Checked 2026-09-29.
Written in our own words; quotations are the makers’ wording.