2021 Mercedes-Benz AMG GT CEL P012800 Coolant Temp Below Spec

Owners of the 2021 Mercedes-Benz AMG GT may encounter a Check Engine Light (CEL) illuminated on their dashboard, accompanied by the diagnostic trouble code P012800. This code specifically points to an issue where "The Coolant Temperature is Below the Specified Temperature." Mercedes-Benz has released Technical Service Bulletin (TSB) LI20.10-P-072886, version 19, dated February 11, 2026, to address this concern for vehicles equipped with M176, M177, and M178 engines.

Symptoms

The primary symptom of this issue is the illumination of the Check Engine Light (CEL) on the instrument cluster. When scanned, the vehicle's engine control unit (ME Control Unit) will log the fault code P012800, indicating that the engine's coolant is not reaching its optimal operating temperature. The TSB notes that the symptom appears under "Power generation > Engine management > Indicator lamp > Engine diagnosis > lit."

Fault Codes / DTCs

The specific diagnostic trouble code associated with this condition is:

Code	Description	Module
P012800	The Coolant Temperature is Below the Specified Temperature	ME Control Unit

Important Warnings

Mercedes-Benz explicitly states in TSB LI20.10-P-072886 that this particular issue is "not an issue that affects drivability or performance and is not a safety concern." The manufacturer assures that "The vehicle remains safe to operate with this code, but the measures listed in the 'Remedy' section are recommended." Owners can continue to drive their 2021 Mercedes-Benz AMG GT safely, but addressing the underlying cause is advised to ensure optimal engine efficiency and performance.

Diagnosis / Trigger Conditions

At the time of the TSB's publication, the exact cause of the Mercedes-Benz AMG GT P012800 code was still "Under investigation." However, the presence of the code suggests that the engine's cooling system, particularly the thermostat, may not be functioning correctly, leading to prolonged engine warm-up times or consistently lower-than-optimal operating temperatures.

Repair Procedure Summary

Addressing the Mercedes-Benz AMG GT P012800 code involves a multi-step process for technicians:

1. Warranty Check: Technicians must first use the Vehicle Master Inquiry (VMI) system to determine if the vehicle is covered under the Emission Extended Warranty for the Engine Thermostat, which includes a potential software update. This warranty applies to various models and platforms from model years 2019-2023.
2. NCU Repairs: If the 2021 Mercedes-Benz AMG GT is covered by the Emission Extended Warranty, the technician will follow specific repair steps outlined in the Mercedes-Benz Network Communication Unit (NCU) system.
3. Software Updates & Remedy Instructions: If the vehicle is not covered by the extended warranty, technicians are instructed to check for available software updates for the ME control unit. Following any necessary software updates, they are to proceed with the detailed "Remedy Instructions" provided in an attached document (LI20.10-P-072886 Remedy Instructions v2.docx). This attachment contains the comprehensive steps for resolving the issue, which may include thermostat replacement.

Another related WIS-Reference, AR20.00-P-1142-04A, titled "Pour in coolant and bleed coolant system," suggests that proper coolant filling and bleeding procedures are also part of the overall repair process.

Required Tools or Parts

Resolving the Mercedes-Benz AMG GT P012800 issue may require the replacement of the engine thermostat. The TSB lists the following part numbers, depending on the specific engine variant:

• A1772007901: Thermostat for M177LS2 and M177LSE engines.
• A1772008801: Thermostat for M176, M177LS1, and M178 engines.

Technicians will need access to the specific "Remedy Instructions" document (LI20.10-P-072886 Remedy Instructions v2.docx) for detailed repair steps, which was not included in the primary TSB document due to formatting issues.

Owner Action

If your 2021 Mercedes-Benz AMG GT displays a Check Engine Light with the P012800 code, it is recommended to schedule an appointment with an authorized Mercedes-Benz service center. While the issue is not a safety concern, addressing it ensures your vehicle operates as intended. Inform the service advisor about the specific fault code and mention TSB LI20.10-P-072886. The dealership will be able to check for applicable extended warranties, perform necessary software updates, and carry out any required repairs, such as thermostat replacement, according to the manufacturer's guidelines.

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Source: NHTSA ID: 11028810