Weak or poor signal indicator on a pulse oximeter
The oximeter is telling you it cannot see a clean pulse at the finger, so the numbers on the screen may be wrong. The usual causes the manuals list are movement, a cold hand or poor circulation, the finger not pushed fully in, nail polish, and bright light.
Check your oximeter’s manual — makers differ.
What it is for
Do not rely on the readings while it shows. Keep the hand still and at heart level, warm the finger, push it fully in, or try another finger. Nonin says that if the Onyx Vantage 9590’s pulse quality light “blinks yellow or red, try another finger”. Masimo lists, for dim numbers on the MightySat, “Measurement site may be poorly perfused” among the causes and repositioning the finger among the fixes. Beurer says of the PO 45 that when its symbol appears “the measurement signal is unstable, and the readings shown are invalid”.
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
- A small light that changes colour, numbers that turn dim, or a separate symbol beside the readings. Each maker shows it differently.
- Names makers use
- Pulse quality indicator, green, yellow or red (Nonin Onyx Vantage 9590); Dimmed numbers and the SIQ line (Masimo MightySat Rx); Inaccurate measurement symbol (Beurer PO 45); Unstable readings (Microlife OXY 300 and OXY 200 troubleshooting)
Where makes differ
Nonin uses a tricolour light that blinks with the pulse: “Green indicates a good pulse signal”, yellow a marginal one and red an inadequate one. Masimo dims the numbers “when confidence in the value is low” and draws a shorter SIQ line or pulse bar. Beurer’s PO 45 has its own inaccurate-measurement symbol; its PO 30 and PO 60 have none. Microlife’s OXY 300 and OXY 200 have no separate indicator, only the bar and, on the OXY 200, dashes. Nonin also warns that “in some circumstances, the device may interpret motion as good pulse quality”, so a good signal indication is not a guarantee.
What it does not mean
It is not a warning about your oxygen or your heart: it is about the measurement. It does not mean the oximeter is broken; those faults have their own error codes.
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 4 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) — Activating the 9590: the tricolor pulse quality indicator blinks at the pulse rate — green good, yellow marginal, red inadequate pulse signal; “If the pulse quality indicator blinks yellow or red, try another finger.” Checked 2026-09-29.
- Nonin: Nonin Onyx Vantage 9590 — instructions for use leaflet (P/N 114847-000-02, 11/2022, PDF) — Cautions: “In some circumstances, the device may interpret motion as good pulse quality. Minimize patient motion as much as possible.” Checked 2026-09-29.
- Masimo: Masimo MightySat Rx fingertip pulse oximeter — operator’s manual (LAB-9121C, PDF) — Features: “Numbers will dim when confidence in the value is low”; the SIQ line and pulse bar heights assess confidence. Checked 2026-09-29.
- Masimo: Masimo MightySat Rx fingertip pulse oximeter — home care manual (PDF) — Troubleshooting, numbers appear dim: brightness set low, bright light or sunlight, incorrect finger placement, “Measurement site may be poorly perfused”; reposition the finger. Checked 2026-09-29.
- Beurer: Beurer PO 45 pulse oximeter — instructions for use (2020-06-12, German/English, PDF linked from beurer.com) — Unit description: 2. Inaccurate measurement; Operation note: the symbol means “the measurement signal is unstable, and the readings shown are invalid”. 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 shown unstable — finger not inserted deep enough or excessive patient movement; retry, sit calmly. Checked 2026-09-29.
Written in our own words; quotations are the makers’ wording.