ZKTeco Fingerprint Not Recognised? Fixes for 'Please Try Again' and Failed Verification

A fingerprint terminal that keeps answering "please try again" costs more than the annoyance. Queues form at the door, staff punch for each other out of frustration, and managers start adding manual corrections. The cause is usually not a faulty terminal. It is the sensor, the finger, the way it is pressed, or an enrolment that was weak to begin with.

This guide works through ZKTeco fingerprint verification faults in order, from the quickest check to the last resort. The advice comes from ZKTeco's own manuals, including the troubleshooting sections of the F18 and its 2.4-inch terminals manual, and its installation guide for the F7. Menu names vary by model, so check yours.

Symptoms: what you are seeing

Step 1: Clean and test the sensor

ZKTeco's manuals open their troubleshooting with the same check. When the sensor cannot read or verify a fingerprint effectively, check whether the fingertip is wet, or the fingerprint sensor is wet or dusty, then clean both and try again.

1. Wipe the sensor glass with a clean, dry, lint-free cloth. Dust, grease and sunscreen build up on a terminal that has been used all day.
2. For stuck-on dirt, ZKTeco's installation guide for the F7 says sticky tape can be used to lift it off the sensor.
3. If the sensor has a plastic protective film from the factory, check that it has been removed. ZKTeco's guidance for new installations is to remove it.
4. Look for scratches. A scratched sensor cannot be cleaned back. After years of use, scratching is a normal cause of a terminal that has gradually got worse, and it needs repair or replacement.

Then test it. On models with an Autotest menu, go to Menu, Autotest, Test Fingerprint Sensor and press a finger on it. ZKTeco's manual says the collected fingerprint image is shown on the screen, so you can check that it is clear.

• A clear image means the sensor works. The fault is in the finger, the placement or the enrolment.
• A blank, striped or partial image means the sensor, the cable behind it or the board needs repair. ZKTeco's F7 guide lists a loose sensor cable among the causes of an unresponsive sensor.

Step 2: Fix how the finger is pressed

Most "faulty terminal" calls end here. ZKTeco's manuals are specific about how to use the sensor:

• Use the index, middle or ring finger. Avoid the thumb and the little finger, which the manual says are difficult to press accurately onto the reader.
• Press the centre of the fingerprint on the centre of the sensor, flat and pressing firmly. The F18 manual illustrates three wrong ways: too close to the edge, too low and held vertically.
• Cover a large area of the sensor. A fingertip that only touches with its tip gives too little to match.
• Keep the same finger and the same angle that the person enrolled with. A person who enrolled with a flat press and then verifies with a quick tap will fail.
• If the finger is too dry, ZKTeco's manual says to blow air onto it and try again.
• If the finger is wet, dry it first. Wet or sweaty skin smears the image.

ZKTeco also states that it accepts no liability for recognition problems caused by incorrect use. That is worth knowing, because it means training is part of the fix: a short demonstration at induction, and a poster of the correct placement at the door, prevents most of these calls.

Step 3: Re-enrol the people who fail

If the sensor works and one person still fails, the stored template is probably weak. This is common when someone enrolled in a hurry, with a dirty finger, or with a light touch. ZKTeco's installation guide for the F7 gives the guidance in one line for users whose fingerprints sometimes cannot be verified: the fingerprint quality is poor, so choose a fine fingerprint when enrolling, cover a larger area of the sensor, test it after enrolment, and enrol more fingerprints.

1. Delete or overwrite the old fingerprint for that person and enrol again. On ZKTeco terminals this is under Menu, User Mgt., then the user, then Fingerprint.
2. Clean the sensor and the finger before you start.
3. Press the same finger three times when prompted. The F18 manual shows a green indicator when the fingerprint has been enrolled successfully.
4. Test straight away. Walk away from the sensor, come back and verify, the way the person will in real life. If it fails now, repeat the enrolment with another finger.
5. Enrol two or three fingers, on both hands where you can. Then a cut, a plaster or a dirty finger does not lock the person out. Check your model's capacity first, because each finger uses one template. The ZKTeco F18, for example, stores 3,000 fingerprint templates, so three fingers each reduces the number of people it holds.
6. Offer a fallback. ZKTeco's F7 guide notes the device supports 1:1 matching and password identification, so a person whose finger is hard to read can enter their user ID and then a finger, or use a password, instead of a 1:N search. Remember a password is easy to share, so use it for exceptions only.

If a person still cannot enrol a usable fingerprint, the cause may be the skin, not the terminal. Move on to Step 7.

Step 4: Check that the right method is switched on

Sometimes the fingerprint is fine and the terminal is not set to accept it, or is asking for a combination.

• Verification mode. A user or group set to "fingerprint and password" or "card and fingerprint" will fail if only one is provided. Check the user's verification type under User Mgt. and the group's setting.
• 1:1 or 1:N. In 1:N the person presses a finger and the terminal searches all stored fingerprints. In 1:1 the person first enters their user ID, then a finger. The F18 manual describes both. Staff who expect to press only a finger will fail when the terminal is waiting for an ID first.
• Duplicate punch period. ZKTeco terminals have a Duplicate Punch Period (m) that does not save repeated punches within a set number of minutes. A person who punches twice may think the terminal failed, when it has simply ignored the second punch.

Step 5: Adjust sensitivity and match threshold

If the sensor is clean and enrolments are good, but a whole site still struggles, look at the settings. These are on models that offer them, and they are in the fingerprint settings under Menu, System, Fingerprint.

FP Sensor Sensitivity. ZKTeco's manual for its 2.4-inch terminals says to use the default level, Medium. It adds that when the environment is dry, making fingerprint detection slow, you can set it to High to raise sensitivity. When the environment is humid, making fingerprints hard to identify, set it to Low. Match the setting to where the terminal is, such as an air-conditioned lobby or a damp loading bay.

Match threshold. The same manual explains the two thresholds. The 1:1 threshold applies when the person enters an ID first, and the 1:N threshold applies when the terminal searches all fingerprints. A match passes only when the similarity is higher than the threshold. ZKTeco gives these recommended values:

Lowering the threshold makes the terminal more forgiving and also less secure. Change one setting at a time, in small steps, and test with the people who were failing. Do not lower it on a door that protects cash, stock or a server room.

1:1 Retry Times. Under 1:1 verification, a person who presses the finger improperly can retry without re-entering their ID. The manual allows between 1 and 9 retries.

Fingerprint Image. You can set the terminal to show the fingerprint image on the screen when enrolling or matching. Turn it on temporarily at the door: staff can see what the sensor sees, and you can spot a poor press or a dirty sensor immediately.

Step 6: Rule out faults that look like fingerprint faults

Not every rejection is the fingerprint. ZKTeco's manuals list others:

• "Invalid time zone" after a successful verification. The person is not allowed through at that time. The manual says to ask the administrator to check whether the user has the privilege within that time schedule. See our guide to door not unlocking.
• Verified, but the door does not open. Check the user's privilege and the lock wiring. This is an access fault, not a fingerprint one.
• A tamper alarm. The device and its back plate are not fixed together properly, so the tamper switch on the back is triggered.
• A wrong time on the punch. If the person verifies but the record is stamped incorrectly, see ZKTeco wrong time and punch time errors.

Step 7: Know when fingerprint is the wrong method

Some workforces cannot use fingerprints reliably, however well the terminal is set up. This is a property of the people and the job, not a fault.

• Construction, maintenance and cleaning staff. Hard manual work wears fingertips and fills the ridges with dust, cement and grease.
• Kitchens, laundries and healthcare. Hands are often wet, gloved or washed many times a day.
• Hot and humid sites. Sweat and moisture smear the image.
• Older staff and anyone with dry skin conditions. Fine ridges flatten with age.

For these sites, change the credential, not the settings:

• Face recognition needs no contact. The ZKTeco SpeedFace-V5L is an example. See our face recognition terminals.
• Cards or key fobs are cheap and simple, and work with gloves. Their weakness is that they can be lent to a colleague, so they are weaker for attendance than a biometric.
• Fingerprint plus card, so that a person whose finger fails can still get through on the card, with the fingerprint recorded for audit.
• A terminal with a different sensor. Many ZKTeco terminals use an optical sensor, which is sensitive to wet and dirty fingers. Ask us which models are better suited to your conditions.

For a site-by-site comparison, see our guide to ZKTeco, Suprema and Hikvision for enterprise access control.

A checklist for your site

1. Clean the sensor with a dry, lint-free cloth, and book a regular clean.
2. Check the sensor image with the Autotest, and check for scratches.
3. Train staff: index, middle or ring finger, centre of the finger on the centre of the sensor, firm and flat.
4. Re-enrol anyone who fails, with a clear finger, and enrol two or three fingers each.
5. Set sensitivity to match the environment: Medium, or High if dry and Low if humid.
6. Change the match threshold only if needed, in small steps and never on a high-security door.
7. If a whole team still fails, switch the credential: face, card or both.

Related guides: ZKTeco BioTime device offline and ADMS sync errors, Hikvision iVMS-4200 employee enrollment, and Suprema BioStar device connection and user sync.