Model Reports 8 min read Updated 11 Sep 2026 DVSA open data · 2,631 tests analysed

Mercedes-Benz Slk 2004: reliability & common MOT faults

906 first-attempt MOT failures were recorded on the 2004 Mercedes-Benz Slk in 2025. Figures are among first-attempt failed MOT tests, not among all tests.

Coil spring (front) was the most common reason a 2004 Slk failed its MOT — 11.7% of its failed first attempts.

First-attempt pass

65.6%

of 2,631 first tests (2025)

First tests

2,631

test year 2025

Failed first attempts

906

failed first-attempt tests

MOT pass rates

First-attempt and official-style rates among class 4 first tests of the year.

First-attempt pass

65.6%

of 2,631 first tests (2025)

Official-style pass

70.5%

incl. same-day PRS fixes

First tests

2,631

class 4 · first MOT of the year

Typical age 21.0 yrs · median mileage 81,758 mi

vs 2024: +1.0 pp first-attempt

Note:

  • Class 4 cars · first MOT of the year per vehicle · P/PRS/F outcomes (2025 data).
  • PRS (fixed at the station the same day) counts as a fail in the first-attempt rate.
  • Raw outcomes for this cohort — not adjusted for age, mileage, or peer mix.
  • MOT doesn't test engine or gearbox internals.

Common faults

Failure patterns that show up more often on this registration year than on similar cars.

These MOT failure patterns show up more often on this registration year than on similar cars of the same class, age, and mileage.

Shares below are measured across the 903 of 906 failed first attempts (99.7%) that recorded an odometer reading in 2025 — the subset the age-and-mileage comparison can be computed on.

Standout: Sub-frame (rear) — 5.5% of this year's failed first attempts

Recorded on 50 of those tests. It tops the table on elevation — about 25× more often than on similar cars of the same age and mileage. That is what makes it unusual for its class, which is a different question from what is most likely to fail; the buying checklist ranks by that. The full ranked list is in the table below.

No patterns met the strongest callout thresholds on this page; showing the highest-lift rows that still cleared the display floors.

Fault pattern Location Failures vs similar cars Share of fails Confidence
Sub-frame (rear)
Suspension > Sub-frames > Sub-frame
 Rear 50 ×25 5.5% Likely
Electronic stability control
Brakes > ABS / EBS / ESC > Electronic stability control
 Any 33 ×6.9 3.7% Likely
Steering rack (front)
Steering > Steering gear > Steering rack
 Front 58 ×6.2 6.4% Likely
Coil spring (front)
Suspension > Springs > Coil springs > Coil spring
 Front 106 ×3.5 11.7% Likely
Coil spring (rear)
Suspension > Springs > Coil springs > Coil spring
 Rear 62 ×2.8 6.9% Possible
Ball joint (front)
Suspension > Suspension arms > Ball joint
 Front 97 ×2.3 10.7% Possible

Only patterns that clear minimum sample and elevation thresholds are shown (at least 20 failures and about 2.0× more often than similar cars).

Note: Rates and comparisons are among first-attempt failed tests, not all MOTs. Patterns come from MOT defect codes — not manufacturer service bulletins, recalls, or a diagnosis of any individual car. Failures and advisories are listed separately.

Wear patterns

Usage and mileage effects that often track road use rather than model design.

These patterns look like wear or usage effects rather than model-specific design faults. Tyres, brake friction material, and alignment-related defects often track mileage and road use.

Pattern Location Failures vs similar cars Share of fails Confidence
Tread depth (rear)
Tyres > Tread depth
 Rear 43 ×2.5 4.8% Wear
Note: These patterns are not treated as a model design fault in our common-faults ranking.

Advisories

Notes recorded at the test that appear more often than on similar cars.

Advisory notes recorded on failed first-attempt tests that appear more often than on similar cars.

Advisory pattern Location Notes vs similar cars Share Confidence
Wheel bearings (front)
Suspension > Wheel bearings
 Front 74 ×5.2 8.2% Elevated
Sub-frame mounting prescribed areas (rear)
Suspension > Sub-frames > Sub-frame mounting prescribed areas
 Rear 29 ×3.5 3.2% Elevated
Flexible brake hoses (rear)
Brakes > Flexible brake hoses
 Rear 73 ×2.6 8.1% Elevated
Note: Advisories are not a fail rate — they flag issues noted at the test, often before they become failures.

If you are buying one

What this year's MOT record means for the decision in front of you.

Nothing separates this year from the Slks either side of it, so buy on the individual car. The checks below are the items that actually failed 2004 Slks, commonest first.

What to check on a 2004 Slk

The items that failed 2004 Slks most often, as a share of the year's failed first attempts. This is what goes wrong, not what is unusual — the table further up the page covers that.

  • Coil spring (front) — recorded on 11.7% of failed first attempts
  • Ball joint (front) — recorded on 10.7% of failed first attempts
  • Headlamp aim — recorded on 8.3% of failed first attempts
  • Malfunction indicator lamp — recorded on 7.6% of failed first attempts
  • Steering rack (front) — recorded on 6.4% of failed first attempts

Every figure here is a first-attempt MOT rate from DVSA open data for this model line. MOTs do not test engine or gearbox internals, so this is a pre-purchase checklist, not a full reliability score.

FAQs

Short answers about how to read this model-year guide.

Coil spring (front) was the most common reason a 2004 Slk failed its MOT — 11.7% of its failed first attempts. In 2025, 65.6% of 2,631 first MOTs on the 2004 Mercedes-Benz Slk passed first time (PRS — items fixed at the station the same day — counted as a fail). The official-style rate including PRS is 70.5%. Among 906 failed first-attempt MOT tests (test year 2025), Sub-frame (rear) appears more often than on simil…
Ranked by how often they were actually recorded, the items that failed 2004 Mercedes-Benz Slks most often at the first attempt were Coil spring (front) (11.7%), Ball joint (front) (10.7%) and Headlamp aim (8.3%) — each as a share of the year's failed first attempts. That is what goes wrong on the car. It is a different question from which patterns are unusual for the class, which the common-faults table answers.
Among failed first-attempt tests we highlight patterns that appear more often than on similar cars. Top example: Sub-frame (rear) (5.5% of those tests; 50 observed failures; about 25× more often than similar cars). These are statistical signals, not a diagnosis of any individual car.
Sub-frame (rear) shows up more often than on similar cars (5.5% of those tests; 50 observed failures; about 25× more often than similar cars). That does not prove a design fault — age, mileage, and how the car was used still matter. Treat it as a pre-purchase check point, not a manufacturer service bulletin.
Common MOT problem areas for the 2004 Mercedes-Benz Slk include Sub-frame (rear), Electronic stability control, Steering rack (front). These patterns show up more often than on similar cars after we filter out sparse noise — not a full list of every possible fault on an individual car.
Advisories flag issues noted at the test and are not a fail rate. We show advisory patterns that appear more often than on similar cars among failed first-attempt tests, separate from common failure rows. Use them as early-warning checks, not as a pass/fail score.
This page highlights MOT failure patterns for the 2004 Mercedes-Benz Slk (registration year) using UK DVSA open data for the selected test year. Patterns are compared with cars of a similar age and mileage. It is a buyer checklist from MOT defect statistics — not a full service history or manufacturer service-bulletin list.
No. MOT tests do not cover engine internals, gearboxes, or many electronic modules. Patterns here come from MOT defect statistics only and should not be read as engine or gearbox reliability scores.

About this data

How this page is built and what it can (and can't) say.

These figures come from UK MOT open data for this model year. We surface common failure patterns against similar cars — not a full reliability score, and not a pass/fail verdict on any individual vehicle.

2024 2025

What we include

UK Class 4 cars only. Normal MOT tests (not retests); first test per vehicle per calendar year; results pass, PRS, or fail.

Same-day repairs

PRS means the car failed items that were fixed at the test station and then passed the same day. We count PRS as a first-attempt fail in headline rates so same-day repairs do not hide problems.

How we compare

We compare this model year with other Class 4 cars of similar age and mileage in the same test year. This model family is left out of the peer group so the car is not measured against itself.

What this does not cover

  • MOTs do not cover engine internals, gearboxes, or many electronics — this is not a full reliability score.
  • Common faults come from MOT defect stats, not manufacturer TSBs or recalls.
  • Age and mileage matching reduces — but does not remove — differences in how cars were used and maintained.
  • We never invent pass rates, star scores, or ranks when those data marts are missing.

Contains public sector information licensed under the Open Government Licence v3.0.

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