Start with the batteries

A monitor that will not turn on, shuts off in the middle of inflating, or reports a problem with cuff pressure often has nothing wrong with it except tired batteries. Most upper arm monitors run on four AA batteries. One manufacturer's manual says its low battery symbol begins flashing when the batteries are about three quarters used. When the symbol flashes the moment you press the power button, the batteries are spent, and that monitor will refuse to measure until you change them.

Battery trouble can also hide behind other codes. One manual's remedy for its "abnormal cuff pressure" code includes replacing the batteries, because a pump running on weak power may not inflate the cuff properly.

A few habits prevent most battery trouble.

1. 1 Replace all four batteries during the same period. One manual warns that mixing old and new batteries can shorten battery life or cause the monitor to malfunction.
2. 2 Match the plus and minus ends to the markings inside the compartment. The same manual states that the device will not work if the batteries are installed incorrectly, the usual reason a monitor with brand new batteries stays dark.
3. 3 Wait a moment after the monitor turns off before swapping batteries. One manufacturer asks for at least 2 seconds so the unit powers down fully.
4. 4 If the low battery symbol still shows after you install fresh batteries, take a measurement. On at least one monitor, the symbol clears only after the unit completes a reading.
5. 5 Use the battery type the manual specifies. If rechargeable batteries are allowed, the manual may limit you to one chemistry, such as nickel metal hydride.
6. 6 Remove the batteries if you will not use the monitor for several weeks. Both manuals ask for this because batteries left in place can leak and damage the unit.

One manufacturer rates its monitor at roughly 700 readings on a set of alkaline batteries but only about 200 on cheaper manganese batteries, tested at an inflation pressure of 180 mm Hg at room temperature, with shorter life in the cold. At the four readings a day that guidelines suggest during a monitoring week, 700 readings works out to close to half a year of daily use. If you are changing batteries far more often than that, a loose cuff may be making the pump inflate repeatedly.

An AC adapter sidesteps the issue. Most manufacturers sell one separately, and at least one states that its monitors run on the adapter with no batteries installed at all. Use only the adapter made for your model.

Check the air plug and hose

The cuff connects to the monitor through a flexible hose that ends in a small plug. That plug has to go into the socket on the monitor firmly, "as far as it will go" in one manufacturer's words. A plug seated only part way lets air escape, and the pump cannot build pressure.

Two error displays point directly at this connection. One monitor shows a code that its manual translates as "the pressure value did not increase during the inflation." Another shows an "abnormal cuff pressure" code, and the manual's first instruction is to check that the cuff is correctly connected and not too loose. If you see either, pull the plug out, look at it for cracks or a bent tip, and push it back in until it stops.

Then run your fingers along the hose and feel for kinks, splits, or a section that has gone stiff. One manual warns not to twist the hose during a measurement, because a blocked hose can leave the cuff inflated and squeezing your arm. A cuff that stays tight longer than usual, or deflates in jerks, often has a partly blocked hose.

Cuffs wear out well before monitors do. If the plug is firmly in, the hose looks fine, and the cuff still will not hold pressure, the air bladder inside the cuff may have developed a slow leak. Replacement cuffs are sold for most monitors and cost far less than a new monitor.

One more safety point from a manual: never inflate the cuff without wrapping it around the arm, so do not press start with the cuff lying on the table.

Read the error code by its cause

Codes vary by brand, but the problems behind them fall into a handful of groups.

1. 1 Signal too weak or no pulse detected. The monitor could not pick up your pulse through the cuff, usually because the cuff is loose, placed too high or too low, turned so the artery marker faces the wrong way, or wrapped over clothing. Reposition the cuff on bare skin, about 1 to 2 cm (roughly half an inch to three quarters of an inch) above the crease of the elbow, with the artery mark over the inside of the arm in line with your ring finger, and repeat.
2. 2 Movement or unstable signal. Both manuals include a code for a reading disturbed by movement. Talking, shifting in the chair, flexing your hand, or a deep sigh during inflation can trigger it. Sit back, rest the arm on the table with the palm up, breathe normally, and say nothing until the cuff has fully deflated. A 2017 systematic review of measurement error found that talking during a reading raised systolic readings by 4 to 19 mm Hg across the studies it examined, so this code is doing you a favor.
3. 3 Abnormal result. One monitor displays this when the numbers it calculated do not make sense. Another shows a generic error when the systolic and diastolic values come out within 10 mm Hg of each other, which almost always means the cuff was applied wrong. The manual's remedy is to apply the cuff correctly and measure again. Do not write down a reading that arrived with one of these codes.
4. 4 Out of range. One monitor shows HI when the pulse or the cuff pressure exceeds what it can measure, and LO when the pulse is too slow to count. Its advice for HI is to relax for 5 minutes and try again. A HI code by itself is a device message. For repeated very high readings, see the last section of this article.
5. 5 Too many errors in averaging mode. Monitors that take three readings in a row and average them will stop if several attempts fail. Read through the setup checklist, then start the sequence over.
6. 6 Internal error. The manual's fix is a reset: remove the batteries, press the power button once with the batteries out, then reinstall them. If the code returns after a reset, the manufacturer wants a call.
7. 7 Communication failed. On monitors that send readings to a phone by Bluetooth, a separate code appears when the transfer fails. The manual's steps are to press the power button or reinsert the batteries, then try again after checking that the phone's Bluetooth is on, the app is open, and the phone is within a few feet. A failed transfer does not affect the reading itself, which stays in the monitor's memory.

Write down or photograph the exact code when it appears, because support staff will ask for it.

Fix the cuff before you blame the monitor

Some monitors show a cuff fit symbol. One manufacturer displays one icon when the cuff is placed well and a different one when the fit is "suboptimal, but it is still ok to measure." Another flags a cuff fit error when the cuff is loose and warns that the reading may be wrong. If the fit symbol appears reading after reading, the wrap needs work.

One manual states that it is normal for the cuff to feel very tight. The American Heart Association's measurement guidance asks for a cuff that is snug with comparable tightness at the top and bottom, sitting on bare skin, with the center of the bladder over the artery. A sleeve pushed up above the cuff should be loose; one manual warns that a tightly rolled sleeve can prevent an accurate reading.

Check the printed range and the index line whenever you unpack a monitor or someone else has used it. Cuffs carry their arm circumference range on the fabric, and an index mark should land inside a printed zone once the cuff is wrapped. If the index falls outside the zone, the cuff is the wrong size for that arm, and one manual's instruction is to contact the seller for a replacement cuff. In the Cuff(SZ) randomized trial published in JAMA Internal Medicine in 2023, a standard adult cuff used on people who needed an extra large cuff read systolic pressure 19.5 mm Hg too high, and used on people who needed a small cuff it read 3.6 mm Hg too low. A monitor can be perfectly calibrated and still give a misleading number if the cuff is wrong. A separate article in this series covers how to measure your arm and find your size.

What the irregular heartbeat symbol means

One manufacturer defines an irregular heartbeat as a beat that varies from the average of all beats detected during the measurement, and its monitors show a symbol when that happens. The same manual warns that a very weak or irregular heartbeat can make it difficult for the monitor to determine your blood pressure, because automated monitors work by sensing the pulse through the cuff as it deflates.

The symbol is not a diagnosis, and movement during the reading can set it off as easily as a true rhythm change. If it appears once, sit quietly and repeat. If it appears on most readings over several days, the manual's advice is to mention it to your health care professional, who can decide whether it deserves a closer look. Bring the monitor along; the pattern of symbols stored in its memory is useful information.

Readings that look wrong without any error

Not every troubling result comes with a code. A number that seems far too high or too low, or two readings a minute apart that disagree by 15 mm Hg, is usually a technique or timing problem. One manufacturer's frequently asked questions page notes that a person's pressure can vary by as much as 30 to 50 mm Hg from day to day, which is why guidelines ask for an average of many readings rather than trusting any single one.

The 2017 systematic review mentioned earlier, which pooled 328 studies, catalogued how much everyday conditions push readings around. A full bladder raised systolic readings by 4.2 to 33 mm Hg. Cold exposure raised them 5 to 32. Recent caffeine raised them 3 to 14. Holding the arm below heart level raised systolic by 3.7 to 23. Legs crossed at the knee added 2.5 to 14.9. Skipping the 5 minute rest added 4.2 to 11.6. Any of these can produce a number that looks like an equipment fault.

When a reading surprises you, use this short sequence before suspecting the monitor.

1. 1 Empty your bladder, sit with your back supported and feet flat, and rest quietly for 5 minutes.
2. 2 Confirm the cuff is on bare skin, snug, with the index in range, the middle of the cuff at heart level, and the arm resting on a table.
3. 3 Take two more readings 1 minute apart without talking.
4. 4 Compare the second and third readings with each other rather than with the first. If they agree within a few mm Hg, the monitor is probably fine and the first number reflected something else.

Monitors are built to a laboratory tolerance of plus or minus 3 mm Hg, which both manufacturers state in their specifications. When researchers tested 85 home monitors brought in by their owners, 69 percent were off by 5 mm Hg or more on systolic or diastolic pressure compared with careful manual measurement, and 29 percent were off by 10 or more. Those findings argue for having your monitor compared with the office equipment, which the American Heart Association recommends at your first visit after buying it and then once a year or as the manufacturer directs. A separate article in this series explains what to bring to that appointment.

When to stop troubleshooting

Run the full reset once: fresh batteries installed correctly, plug seated, hose inspected, cuff rewrapped on bare skin with the index in range, 5 minutes of rest, two quiet readings. If the same error returns after all of that, the next step belongs to the manufacturer.

Three pieces of information in your manual tell you what to expect. First, the stated service life. One manufacturer gives 5 years or 10,000 readings for the monitor and 2 years or 5,000 readings for the cuff; another gives 5 years for the monitor and 2 years for the cuff when used six times a day. Second, the accuracy check interval. One manual recommends testing for accuracy every 2 years or after the unit has been dropped. Third, the warranty. For the monitors reviewed here, coverage ranged from 2 to 5 years on the unit and 1 to 2 years on the cuff, and a cuff that stops holding pressure inside its warranty period is a repair the manufacturer should handle.

Do not open the case. One manual states that attempting repair voids the warranty. Call the support line listed in the manual instead, with the model number (on the back label or under the battery cover), the exact code, and a list of what you already tried.

A separate situation calls for a different response. If a reading shows 180/120 mm Hg or higher and you feel fine, the American Heart Association's advice is to wait at least 1 minute and take another reading. If the second reading is still that high, contact your health care professional as soon as possible, because that level of pressure needs attention even without symptoms. If a reading of 180/120 mm Hg or higher comes with chest pain, shortness of breath, back pain, numbness or weakness, a change in vision, or trouble speaking, do not spend time checking batteries or rewrapping the cuff. Call 911.