Alco-Sensor FST possible results

Alco-Sensor FST possible results

Details are everything when it comes to impaired driving investigations. An impaired driving allegation, whether administrative or criminal offence, must be supported either with observed indicia of impairment, a physical coordination test or evidence from an approved screening device, also known as a breathalyzer.

The Alco-Sensor FST is the approved instrument for many police forces across Canada. They use the device to gather evidence that a person operated a motor vehicle while impaired by alcohol. It is not the only test police use at the roadside. They also conduct physical coordination tests such as the standard field sobriety test (SFST). This blog, however, will focus on the Alco-Sensor FST and the readings it can give.

Lawyers who know these readings can be alert to errors and challenge police evidence.

What are the possible Alco-Sensor FST Valid Test Results?

First things first, the Alco-Sensor FST only has three possible valid test results:

  • Digital Numerical Reading 0 to 59 mg%
  • WARN
  • FAiL

The Alco-Sensor FST programmed for use in British Columbia does not display a digital reading for any specific blood-alcohol content (BAC) greater than 59 mg%.  Police officers are not able to submit evidence that a driver was impaired because they gave a specific reading on an Alco-Sensor FST.  A FAiL reading does allow the officer to form the grounds to believe the driver is impaired by alcohol; it does not allow for an allegation of impaired driving by itself.

The background colour will change to amber and the display will read “WARN” if the reading obtained is between 60 and 99 mg%. If the reading is 100 mg% or more, the background will turn red and the display will read “FAiL”.


It is important to know how to correctly operate an Alco-Sensor FST. The acceptable operating temperature of the device is -12C to 55C.

The manufacturer requires officers do not conduct tests until 15 minutes after the time they believe a subject last consumed alcohol or five minutes after the time they believe a subject has smoked or had anything in their mouth.

The reason for this is to prevent mouth alcohol. Any residual alcohol in the mouth can give a falsely high BAC reading.  Any other objects, such as coins, gum or candy, must be removed from the mouth at least five minutes before starting a test so as not to interfere with the test.

Other messages

Like any tool, the Alco-Sensor FST is only as good the person wielding it. Police officers must be trained before they can use an ASD and they must be able to show they understand how to use the device. They must be able to describe the correct sequence of messages that appear on the device if called upon to do so in court.

According to the operator’s manual, “failure to identify and articulate these messages can raise concerns about the proper operation of the device”. Lawyers too should be aware of these things.

With that in mind, here are some of the Alco-Sensor’s messages that are important to know about.

Blow Void

Indicates that the subject has not met the minimum breath flow requirements after 3 opportunities to provide a sample.


No breath sample had been provided within the 3-minute time interval to provide a sample.  The device powers off automatically.


The device automatically runs an air blank test to make sure there is no residual alcohol before starting a test. An officer must wait for the device to show a “0” reading followed by “WAIT” and then “Blow” and a double beep sound before instructing a subject to give a breath sample. Officer’s may be required to show they know this sequence and they followed it later in court.


If the blank test is not “0”, the device will show the message “NULL/FAiL” and abort the test sequence. Officers must then ensure there is no residual alcohol contamination and perform another blank test that returns “0” before the test can resume.

Alco-Sensor FST flow messages

In many cases, subjects may provide an inadequate breath sample. The Alco-Sensor FST can display several messages in this case and it is important to know the difference between them. This is especially vital when someone has been charges with failure or refusal to comply with an ASD demand.

  • Flow LOW: the subject did not provide a constant breath flow above the minimum rate required. The subject is given three attempts to provide an adequate sample before the test is aborted.
  • Flow INS: not to be confused with “insufficient flow”, this means the subject’s breath flow rate is inconsistent.
  • Flow CUT: the subject’s breath flow was stopped too abruptly or there was an attempt to suck back the breath as the sample is being taken.
  • Flow HIGH: the subject’s breath flow was above the maximum allowable flow rate.

In this judicial review of an Immediate Roadside Prohibition, a police officer submitted evidence to the Superintendent of Motor Vehicles describing a driver’s attempts to provide a breath sample as “insufficient flow”. The Superintendent accepted the officer’s testimony, but a judge later found that the general term, “insufficient flow” was not one that would be found on an ASD.

The judge overturned the adjudicator’s decision, stating: “In making the most favourable possible assumption about what [the police officer] meant by ‘insufficient flow’, I find that the adjudicator effectively gave the police evidence a presumption of reliability.”

Expert assistance

I have been a Qualified Technician since 1992 and I have instructed and coached many other officers in the correct operation and calibration of a number of approved screening devices, including the Alco-Sensor FST. As such, I know that any procedural or operating error in the ASD evidence can be enough to create reasonable doubt in court.

If you need help with an impaired driving case or you are interested in using an expert witness to help your case, Gottgetreu Consulting can help. Get in touch here.