On 20 May 2018, the DVSA changed what a failed MOT looked like. Not which cars failed — or at least, not straightforwardly — but how the failure was recorded, described, and handed back to the owner. The change was visible and substantial. But some of its effects got misread at the time, and still do.
What existed before
From 1967 until May 2018, the MOT had two formal outputs: pass and failure. Testers could note advisories — items worth watching — and hand these to the driver verbally or on paper. But the certificate said pass or fail, and that was that.
In practice, this meant a car with a badly corroded subframe might receive a handwritten note while still leaving the garage with a pass certificate in hand. What counted as failure, and how tester discretion was applied, varied between stations in ways the DVSA acknowledged but could not easily measure. The advisory system existed largely outside any formal audit trail.
The 2018 categories
The new system introduced four classifications for defects, derived from EU Directive 2014/45/EU — agreed before Brexit, and implemented on schedule:
Dangerous — an immediate risk to road safety or the environment. A car that receives a Dangerous classification cannot be driven away from the test station under its own power. The registered keeper is responsible for arranging recovery or repair on-site.
Major — a fault that significantly affects road safety or the environment, but stops short of an immediate danger classification. Major is an automatic fail. The car can be driven — the owner is not legally prevented from driving it home — but the MOT has been failed and the vehicle is not compliant.
Minor — a defect with no significant effect on road safety at the time of test, but which could develop. Recorded on the pass certificate. Does not cause a failure. The car leaves with a pass.
Advisory — an item worth monitoring, but not yet defective. Also recorded on the certificate. No effect on pass/fail status.
The old advisory category did not disappear; it became one of four formal classifications, each now logged to the central DVSA database in real time. For the first time, the full defect picture was attached to the vehicle’s permanent test record rather than existing as a paper note in the driver’s glovebox.
For more on the history that led here, see the full history of the UK MOT test.
Diesel particulate filter testing
The 2018 reform also tightened the rules around diesel particulate filters. From May 2018, a DPF that has been removed or visibly tampered with — detectable by a tester looking at the filter housing and comparing it with the original specification — is an automatic Major failure.
This was a specific response to a rising practice: DPF removal as a maintenance shortcut. A blocked DPF is an expensive repair. Some garages and independent operators were removing them instead, returning the car to free-flowing exhaust at the cost of legal and environmental compliance.
The new rule meant that a tester who found evidence of DPF deletion — a missing canister, a straight-through pipe where the filter should sit, or soot deposits inconsistent with a functioning filter — could fail the vehicle without needing to rely solely on the smoke opacity test. Prior to 2018, a deleted DPF on a car that had been recently serviced to pass the smoke test was difficult to fail on emissions grounds alone.
Post-2018, the DVSA inspection manual updated guidance on what constitutes visible evidence of removal versus routine DPF damage. Testers are not expected to conduct a forensic examination, but the rule gave them a clear legal basis to fail vehicles where deletion was apparent.
Brake fluid and brake-pad thresholds
The 2018 revision updated several threshold values for specific components. Brake fluid and brake-pad wear moved from advisory-level items — flagged for the owner’s attention but not formally recorded — to Minor or Major categories depending on severity.
Brake fluid with water contamination above a certain percentage had previously fallen in a grey zone. From 2018, high moisture content moved to an advisory, recorded on the certificate, with significantly degraded fluid meeting the threshold for a Minor or Major classification depending on the level measured.
Brake-pad wear thresholds were similarly reclassified. Pads worn to a level that a tester would previously have noted verbally now appear on the pass certificate as a Minor, creating a paper trail that follows the vehicle. A buyer running a DVSA history check on a used car will now see every Minor and Advisory from previous tests — not just whether it passed.
Steering components received parallel treatment. Play in steering joints, column wear, and rack condition were reclassified within the new four-tier framework, with DVSA updating the inspection manual sections on Class 4 steering assessment to give testers clearer pass/fail/Minor/Major decision criteria.
Why the failure rate appears to have jumped
After May 2018, some commentators noted that first-attempt MOT failure rates increased. The reading requires care.
The DVSA’s test data — released annually under the Open Government Licence — records outcomes by test result. Post-2018, a vehicle leaving with a Minor is a pass. A vehicle with a Major is a fail. Before 2018, some of the items now triggering a Major failure were dealt with as advisories and the car left with a pass. The same car, tested on 19 May 2018, might have received an advisory and a certificate. Tested on 21 May 2018, it might have been failed for a Major.
So the first-attempt failure rate did increase for some defect categories. But this does not mean cars got worse. It means the threshold shifted. Items that were previously pass-with-advisory became fail-with-Major. The car’s actual condition was identical; the recorded outcome changed.
Comparing pre- and post-2018 failure rates as if they measure the same thing is a methodological error that several media reports made. The DVSA was clear about this in its 2018 reform documentation, though the nuance did not always survive into the coverage.
The useful comparison is pass certificate quality. Post-2018, a clean pass with no Minors is genuinely meaningful. Pre-2018, a pass might have concealed several items now classified as Minors or low-tier Majors. In that sense, the post-2018 pass certificate carries more information — and arguably, more honest information — than its predecessor.
What the data shows since 2018
DVSA publishes annual test results covering all MOT classes. The 2023 test year data — covering approximately 38 million Class 4 tests — records defects at the individual item level, broken down by make, model, and defect category.
Lighting remains the single most common failure item by volume. Brake condition and tyre condition follow. The DPF-related defect codes introduced in 2018 show up as a measurable category, though their share of total failures is relatively small — most modern diesel owners are not removing filters.
The Minor category, which had no equivalent before 2018, now accounts for a significant share of the total defect record. These are vehicles that passed but left with recorded faults. Pre-2018, a substantial fraction of these would have appeared on no formal record at all.
What changed for owners
The practical effect depends on what kind of driver you were before 2018.
If your car had genuinely marginal items — borderline pads, a slightly weeping steering rack, a DPF showing early stress — your outcome changed. Previously those might have been advisories. Post-2018, they appear on the certificate, follow the vehicle’s history, and are visible to future buyers.
If your car was in sound condition, the reform made little day-to-day difference. The certificate became more detailed. The information handed back on a failure became more granular. The tester’s job became more structured, with less room to exercise personal discretion about what warranted a note versus a recorded defect.
For used car buyers, the change was unambiguously positive. The DVSA’s free history check now surfaces every defect category from every test since digitisation — not just pass/fail, but the full Minor and Advisory record. A vehicle with a pattern of recurring Minor defects, passed each time but never fully resolved, will show that pattern clearly. The pre-2018 equivalent was invisible to anyone who hadn’t been present at the tests.
See also: MOT exemptions for classics, EVs, and other corner cases.