P0125 on Mercedes-Benz GLS
Factory diagnostic protocol and troubleshooting wire for fault code P0125 specifically for the Mercedes-Benz GLS (2020–2026, 3.0L Turbo I6 (GLS 450) / 4.0L Bi-Turbo V8 (GLS 580 / Maybach)).
1. Symptoms of P0125 on Mercedes-Benz GLS
When the Engine Control Module (ECM/PCM) on your Mercedes-Benz GLS (3.0L Turbo I6 (GLS 450) / 4.0L Bi-Turbo V8 (GLS 580 / Maybach)) logs fault code P0125, drivers commonly observe:
2. Most Likely Causes on Mercedes-Benz GLS
Historical warranty claims and technical repair bulletins for Mercedes-Benz GLS point to the following failure distribution:
3. Step-by-Step DIY Repair Guide for Mercedes-Benz GLS
Check that engine coolant is at full mark and no external radiator or hose leaks exist. on the Mercedes-Benz GLS equipped with 3.0L Turbo I6 (GLS 450) / 4.0L Bi-Turbo V8 (GLS 580 / Maybach).
Start cold engine and watch ECT climb. If it plateaus below 170°F (77°C) while driving, thermostat is stuck open. on the Mercedes-Benz GLS equipped with 3.0L Turbo I6 (GLS 450) / 4.0L Bi-Turbo V8 (GLS 580 / Maybach).
Drain 1 gallon of coolant, remove thermostat housing bolts, install new OEM temperature-rated thermostat (typically 180°F–195°F) with jiggle valve at 12 o'clock. on the Mercedes-Benz GLS equipped with 3.0L Turbo I6 (GLS 450) / 4.0L Bi-Turbo V8 (GLS 580 / Maybach).
Fill cooling system with 50/50 prediluted coolant, run engine with spill-free funnel to purge air bubbles. on the Mercedes-Benz GLS equipped with 3.0L Turbo I6 (GLS 450) / 4.0L Bi-Turbo V8 (GLS 580 / Maybach).
Verify scan tool Fuel System Status changes from 'Open Loop' to 'Closed Loop' within 5 minutes. on the Mercedes-Benz GLS equipped with 3.0L Turbo I6 (GLS 450) / 4.0L Bi-Turbo V8 (GLS 580 / Maybach).
Equipment for Diagnosing Mercedes-Benz GLS (P0125)
OEM Thermostat & Gasket Assembly
Direct-replacement thermostat calibrated to factory operating temperature for Mercedes-Benz GLS.
AstroAI Auto-Ranging Digital Multimeter
Essential for testing 0.1V–5.0V sensor reference signals, ground continuity (<0.20Ω), and harness resistance.
BlueDriver Bluetooth Pro OBDII Scan Tool
Reads and clears enhanced codes (ABS, Airbag, Transmission, Engine) & live freeze-frame telemetry on phone.
Other Common Fault Codes on Mercedes-Benz GLS
Frequently diagnosed powertrain, emissions, and sensor trouble codes logged on the Mercedes-Benz GLS:
4. Mercedes-Benz GLS P0125 FAQs
What causes code P0125 to trigger on a Mercedes-Benz GLS?
On the Mercedes-Benz GLS (3.0L Turbo I6 (GLS 450) / 4.0L Bi-Turbo V8 (GLS 580 / Maybach)), P0125 is most frequently caused by Thermostat stuck open or opening prematurely or Low engine coolant level / air pocket around ECT sensor.
Where is the OBD-II port located on my Mercedes-Benz GLS?
On the Mercedes-Benz GLS, the 16-pin DLC diagnostic connector is located Driver lower knee bolster trim panel. Plug in any standard OBD-II scanner with the ignition in the ON position to read freeze-frame parameters.
Can I safely drive my Mercedes-Benz GLS with code P0125?
Yes, for short distances. Fuel economy will drop 15–25% as the engine remains trapped in open-loop enrichment. Given that P0125 carries a Medium severity rating, we advise avoiding heavy towing or aggressive throttle inputs until sensor readings are verified.