Air Blank Won't Return to 0.000

Stop here first if you are mid-test

If you are running a DOT breath alcohol test right now and the air blank is reading above 0.000, the procedure is fixed and it is short. Do this before any troubleshooting.

1. Run one more air blank. That is the allowance, one repeat.
2. If the second reading is still above 0.000, stop using that instrument. Do not keep retrying it and do not continue the test on it. Move to another instrument if one is available.
3. Take the instrument out of service. An EBT pulled for failing to perform an air blank accurately cannot be used for testing again until an external calibration check is run against a known standard and the instrument is found within its tolerance limits.

The rest of this guide is about the unit once it is out of service. It is not a way to get a running test finished.

Why this matters for the test itself

An air blank samples the instrument's chamber to confirm no alcohol is left from a previous sample or from the surrounding environment. It is what establishes that the reading you are about to take belongs to the subject and nothing else.

Under DOT rules, an air blank before the confirmation test that does not read 0.000 is a fatal flaw. The test has to be cancelled. A cancelled test is neither a positive nor a negative, it does not count toward the employer's random testing requirement, and it does not give the employer grounds for a non-DOT test in its place. Follow your employer's procedure and notify your DER as you normally would for a cancelled test. This is worth knowing before you are standing in front of a subject, because the instinct to retry until it clears is exactly what turns a recoverable situation into a documentation problem.

Working out what caused it

Once the unit is out of service, these are worth checking in order. The first three are environmental and cost nothing to rule out.

1. Give it more time and blank again. Residual alcohol from a positive sample can still be working its way out of the sampling path. Wait past your normal interval and run an air blank on its own, with no subject involved.
2. Rule out ambient contamination. Alcohol-based hand sanitizer, cleaning products, or a recent spill near the instrument can register. An air blank checks the environment around the device as well as the chamber, so move the unit away from anything recently sprayed or wiped and blank again.
3. Check the mouthpiece. Confirm a fresh, unused mouthpiece for each subject test, seated fully on the sampling port.
4. Run the external calibration check. This is required before the instrument goes back into service anyway, so run it against your dry gas standard per your QAP. Record the reading. If it comes back outside your tolerance limits, that is your answer, and it also has consequences for tests already run, see below.
   On the I-900 you do not need to work out the corrected value of your gas standard by hand. The instrument has an internal barometric pressure sensor, calculates the target value itself, and displays the acceptable range it will pass against.

What "within tolerance" actually means

Your own instrument's QAP governs, so check yours. For the Intoxilyzer 900, CMI's QAP sets it out concretely, and these are the numbers worth knowing before you run the check:

• Verify against a standard between 0.040 and 0.110 BrAC, manufactured to a tolerance of 0.002 BrAC.
• An acceptable verification is the chosen concentration within 0.005 BrAC as displayed by the instrument.
• Calibration verifications should not be spaced more than 30 days or 50 tests apart, whichever comes first. More often than that is fine. Keep the log, it is your proof the interval was met.
• The instrument comes out of service if it fails a calibration verification or fails its self-diagnostics.

If the display shows an E followed by a number, that is an error code. Look it up in your operator handbook, which will tell you whether the fault needs factory maintenance rather than anything you can resolve on site.

If the calibration check is out of tolerance

This one carries backward. When a calibration check on an EBT falls outside the tolerance in your QAP, every alcohol test of 0.020 or greater between that failed check and the last valid check is cancelled. Do not work through this alone. Get your DER involved and call us, because the affected window needs to be identified accurately from the instrument's own records rather than estimated.

When the unit needs to come in

If air blanks stay elevated after adequate wait time, a clean environment, and a fresh mouthpiece, the likely cause is internal. On the I-240 and I-240D specifically, we have traced this to the fuel cell or the sampling board holding residual alcohol instead of clearing it. That is not a field adjustment and not a calibration drift you can correct on site. The unit comes in, we evaluate it, and if confirmed it needs a fuel cell or sampling board replacement.

We won't predict which one it is, or how long, before the unit is actually on the bench. The symptom narrows it down, the evaluation confirms it.

When to call us

• Air blanks stay elevated across repeated attempts with normal wait time and a clean sampling environment.
• Your external calibration check comes back outside your tolerance limits, especially if tests have been run since the last valid check.
• You need the unit to come in. We can arrange a shipping label and a loaner while yours is serviced.
• You are unsure whether a test needs to be cancelled. Ask before the paperwork is filed, not after.

Call (513) 891-0868 or email service@fosterinstruments.com. Have your instrument model and any readings you noted, including the calibration check result if you ran one.