Mercedes GLS X166 (2015 – 2019)

Reliability score : 6.2/10

The Mercedes-Benz GLS (Generation X166) is not an entirely new model, but a deep facelift of the GL (X166) launched in 2012. This nomenclature change at the end of 2015 aimed to align it with the S-Class, officially positioning it as the "S-Class of SUVs". It is a luxurious behemoth measuring over 5.10 meters and offering 7 real seats. While the ride comfort (thanks to the standard AIRMATIC air suspension) and soundproofing are imperial, this flagship requires a maintenance budget commensurate with its badge. In terms of reliability, the record is mixed: the V6 petrol engines are robust, but the V8s (M278/M157) suffer from costly design flaws, and the onboard electronics as well as the air suspension sometimes age poorly.

✅ Strengths

Exceptional roominess (7 real seats usable by adults)
Imperial ride comfort (AIRMATIC suspension)
Soundproofing worthy of an S-Class
Towing capacity (up to 3,500 kg)
Interior finish quality and materials

⚠️ Weaknesses

Disastrous reliability of the petrol V8s (M278/M157)
Prohibitive maintenance and repair costs (suspension, brakes, XL tires)
Size unsuited to European city centers and underground parking lots
Colossal weight (over 2.4 tons) rapidly wearing out the running gear
COMAND infotainment system aging compared to current standards

🎯 Verdict

The Mercedes GLS X166 is a formidable travel machine, but it represents a significant financial risk on the used market if chosen poorly. If you are looking for a petrol model, the GLS 400 / 450 (V6 M276) is by far the most rational and reliable choice. The V8 versions (GLS 500/550 and AMG 63) are to be avoided unless you have a substantial repair budget and have had the cylinders inspected by a professional before purchase. The diesel (GLS 350 d) remains relevant for high-mileage drivers, provided you check for the absence of leaks at the oil cooler. In all cases, plan for a large annual budget for routine maintenance and the inevitable quirks of the air suspension.

Common issues

Défaillance de la suspension pneumatique (AIRMATIC) High

🔧 80 000 – 150 000 km

💶 1 000 – 3 000 € (par composant : compresseur, ressort pneumatique)

Vérifier l'affaissement du véhicule et le bon fonctionnement du compresseur.

Fuites d'huile moteur (joints de cache-culbuteurs, boîtier de filtre à huile) Medium

🔧 100 000 – 180 000 km

💶 400 – 1 200 €

Inspecter régulièrement le dessous du moteur pour toute trace d'huile.

Problèmes électroniques du système COMAND/Infotainment Medium

🔧 Variable

💶 500 – 1 500 € (réparation ou remplacement de module)

S'assurer du bon fonctionnement de toutes les fonctions électroniques et de l'écran.

Défaillance des actionneurs de volets de chauffage/climatisation Low

🔧 90 000 – 160 000 km

💶 300 – 800 €

Tester toutes les fonctions de climatisation et chauffage avant l'achat.

Estimated maintenance costs

Oil change	£100 – 180
Timing chain	£2 200 – 4 000 (si remplacement nécessaire)
Complete brakes	£650 – 1 200
Estimated annual budget	£900 – 1 800

Indicative estimates (independent garage, UK market). Prices vary by region and year.