Air Blank Won't Return to 0.000 After a Positive Test
Updated August 14, 2026 · 5 min read
What's happening
Every evidential breath test is bracketed by air blanks — the instrument samples room air before and after the subject test to confirm there's no residual alcohol left in the sampling system. A clean air blank reads 0.000. If the follow-up blank after a positive test comes back elevated instead, something between the mouthpiece and the fuel cell is still holding alcohol, or the fuel cell itself isn't clearing between samples.
Most of the time this is transient, and it clears with a longer wait and a fresh ambient sample. Occasionally it doesn't clear at all, and that points to a hardware fault rather than anything the operator did wrong.
Try these in order
- Give it more time between tests. A high blank immediately after a positive result can simply be residual alcohol still working its way out of the sampling path. Wait a few extra minutes past your normal interval, then run another air blank on its own, with no subject involved.
- Rule out ambient contamination. Alcohol-based hand sanitizer, cleaning products, or anything alcohol-based used near the instrument right before testing can throw off a blank. Move the unit away from any recent spray or spill and re-test.
- Check the mouthpiece. Confirm you're using a fresh, unused disposable mouthpiece for each subject test, seated fully on the sampling port. A reused or improperly seated mouthpiece can carry residue into the next sample.
- Run a check against your dry gas standard. If you have a current dry gas standard (105L ethanol standard) on hand, run an accuracy check per your normal procedure. If the instrument reads meaningfully off from the known value, note the reading — that's useful information for the next step.
- Run a few more air blanks with nothing else changed. If it's still elevated after all of the above, and it isn't a one-time fluke, stop testing on that unit and treat it as a hardware issue rather than continuing to run subjects through it.
When this means the unit needs service
If air blanks keep coming back elevated after you've ruled out ambient contamination, given it adequate time between samples, and confirmed the mouthpiece and dry gas standard aren't the issue, the most likely cause is the fuel cell or the internal sampling board holding residual alcohol rather than clearing it. This isn't something that can be adjusted or corrected in the field — it requires the unit to come in for evaluation and, if confirmed, a fuel cell or sampling board replacement.
Continuing to use an instrument that won't blank clean risks an inaccurate result on your next test, so pull it from service once you've confirmed the reading is real and repeatable.
When to call us
- Air blanks stay elevated across repeated attempts, with normal wait time and a clean sampling environment.
- An accuracy check against your dry gas standard comes back noticeably out of tolerance.
- You need the unit to come in for evaluation — we can arrange a shipping label and, where available, a loaner while yours is serviced.
Call (513) 891-0868 or email service@fosterinstruments.com and describe what you're seeing, including any readings you noted along the way.
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Still stuck?
If this didn't fix it, or the next step involves opening the case, recalibration, or anything that could affect accuracy — stop and call Foster. We'll get you sorted or schedule service.