Challenging the Breathalyzer: How Drink-Driving Evidence Is Set Aside

Testing positive on the breathalyzer leaves many drivers feeling the case is already lost. It is not. The device's result is one piece of evidence, subject to strict technical and procedural requirements, not an incontestable truth. When those requirements are not met, the evidence can be challenged and set aside, and with it the basis for any conviction. As a firm focused on challenging breathalyzer evidence, in this practical guide we explain what that technical defence involves, when it applies, and which weak points are examined.

What Challenging the Breathalyzer Means

Challenging the breathalyzer is not about denying that someone was driving or inventing an alternative version of events. It is about technically questioning the validity of the test: whether the device was reliable, whether it was properly type-approved and calibrated, whether the reading was taken in line with the rules, and whether the driver's rights were respected. The aim is to determine whether that result can support a conviction for the offence under Article 379.2 of the Criminal Code (CP) or whether, instead, it must be excluded from the evidence.

The criminal framework is worth setting out. Driving with a punishable alcohol level is punished under Article 379.2 CP by 3 to 6 months in prison, or a fine of 6 to 12 months, or community service of 31 to 90 days, and, in every case, a driving ban for motor vehicles and mopeds of more than 1 and up to 4 years. The technical defence seeks precisely to avoid that outcome by attacking the evidence that supports it.

Type-Approval, Calibration and Verification of the Device

A breathalyzer is only valid evidence if it is a reliable instrument. That requires checking three basic facts that must be on record:

• The type-approval of the model under the metrology rules in force.
• A current periodic verification (verifications expire annually; a device whose verification has lapsed loses reliability).
• The calibration of the specific device used, identified by its serial number.

The defence can request the documentation for the specific device used at the checkpoint. If the breathalyzer's verification had expired, was not type-approved for evidential use, or its calibration cannot be established, the reliability of the result collapses. These material defects are one of the strongest routes for challenging the evidence.

It is worth distinguishing between the evidential breathalyzer, the one that produces a result with probative value, and the screening or pre-screening device sometimes used at first to decide whether a formal reading is warranted. Only the former, properly type-approved and verified, can support a conviction. If the police report blurs the two, or if the result relied on actually comes from the screening device, the defence can dispute that genuine evidential proof exists at all.

The Double Reading and the Interval

The breath test cannot be reduced to a single reading. The driver is entitled to a double reading with a minimum interval between the two, around ten minutes. That interval is not arbitrary: it allows the alcohol absorption and elimination curve to be assessed and anomalous or transient readings to be ruled out, such as those caused by very recent drinking or residual mouth alcohol.

When only one reading is recorded, when the interval was not respected, or when the two readings differ widely without explanation, the reliability of the result is compromised. The defence examines the police report closely to verify the exact time of each test, the interval elapsed, and the consistency between the two levels.

The time sequence also carries deeper weight. Alcohol is not distributed through the body instantly: there is a rising phase, in which the level is still climbing, and a falling phase, in which it declines. Whether the second reading is lower or higher than the first may be consistent with that curve or, conversely, point to an anomaly that calls for explanation. An expert report reconstructing that behaviour helps to put the figures in context and to argue what the actual level was at the wheel, rather than simply what the device displayed.

The Margin of Error in the Driver's Favour

This is one of the most decisive technical points. Every measuring device has a metrological margin of error recognised in its type-approval order. That margin must be deducted in the driver's favour, because any reasonable doubt about the true value of the level must be resolved in the way most beneficial to them.

The margin is especially relevant for readings sitting right at the edge of the criminal threshold. If, after subtracting the applicable error, the level falls below the limit that separates the administrative infringement from the offence, the result may cease to be criminal and the criminal proceedings lose their basis. Calculating and correctly applying that margin is one of the defence's first checks.

The Right to a Blood Counter-Analysis

After the breath test, the driver has the right to request a counter-analysis by blood test, under the General Traffic Regulations. It is a right of enormous significance and, at the same time, little known.

A blood test is more reliable than the breath sample and can return a level lower than the roadside breathalyzer reading. If that counter-analysis level falls below the threshold, the result ceases to be an offence. That is why it is wise to request the counter-analysis expressly and to ask for the request to be recorded in the police report. And that is why, when the officers fail to inform you of that right or prevent or hinder the test, that omission is one of the most frequent grounds for having the evidence declared null.

Procedure, Information and the Chain of Safeguards

Beyond the device, the challenge examines how the whole measure was carried out. The defence reviews whether the driver was clearly informed of the nature of the check and the consequences of the test, whether the minimum waiting time before blowing was respected so as to avoid the impact of residual mouth alcohol, and whether everything was correctly reflected in the police report.

Any relevant irregularity can have serious procedural consequences. Evidence obtained in breach of fundamental rights is null under Article 11.1 of the Judiciary Act, and that nullity can drag down the rest of the evidence that depends on it. Spotting those defects in the police report is the starting point of the whole strategy.

The reach of each defect should be kept in perspective. Not every error carries the same force: some compromise the validity of the evidence (and may lead to nullity), while others affect its reliability and translate into less weight for the judge to assess. A rigorous defence does not overstate: it grades each irregularity by its seriousness, argues nullity where it genuinely applies and, in the remaining cases, builds reasonable doubt out of the accumulation of technical shortcomings. That honest reading of the police report is what sets a solid challenge apart from a merely generic objection.

Why Refusing Is Not the Answer

A common mistake is to think that refusing to blow avoids the problem. It is exactly the opposite. Refusing the tests an officer lawfully requires is a standalone offence under Article 383 CP, punishable by 6 months to 1 year in prison and a driving ban of more than 1 and up to 4 years. That penalty is harsher than the one for the positive reading itself under Article 379.2 CP.

The right route is to take the test, request the blood counter-analysis, and keep the technical challenge for later, with legal assistance. Refusing does not remove the evidence: it adds a further offence, without the defence options that the technical route offers.

Consequences for the Driving Licence

The driving ban is no minor detail. It is worth knowing that, under Article 47 CP, where the driving ban exceeds two years it entails the loss of validity of the licence: it is not enough to wait for the sentence to end; the driver has to sit the test again to recover it.

That is why challenging the evidence is not only about avoiding the prison term or the fine: an effective defence can be the difference between keeping the licence and having to start the whole process of obtaining it from scratch. Every month of disqualification and every fraction of the level counts, and all of them depend on the strength of the evidence that supports the prosecution.

Technical Breathalyzer Defence in Madrid and Across Spain

The criminal defence firm Alonso Sala, based at Calle Velázquez 27 in Madrid and covering the whole of Spain, takes on the challenge of alcohol-test evidence in road-safety proceedings. We review the breathalyzer's type-approval and verification, the double reading, the applicable margin of error, the right to a counter-analysis, and whether the safeguards were respected in the police report, in order to build the strongest possible defence against the offence under Article 379.2 CP.