Safety Warning
DIY auto repair can cause serious injury, fire, or vehicle damage. These guides are for informational purposes only. Always follow OEM torque specs, wear PPE, and consult a certified mechanic if you are unsure. You are solely responsible for your safety.
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HOMESERVICE MANUALSGMC1999FORWARD CONTROL 5.7 R, AUTOMATICREPAIR AND DIAGNOSISEXTERNAL PAGESDIFFERENT CARSECTION 139 (ENGINE CONTROL SYSTEM - SELF-DIAGNOSTICS)DIAGNOSTIC TESTSDTC P0128: ECT BELOW THERMOSTAT REGULATING TEMPERATURETEST DESCRIPTION
1999 GMC Forward Control 5.7 R, Automatic
Test Description
1999 GMC Forward Control 5.7 R, AutomaticSECTION Test Description
WARNING: This page is about a different car, the 2003 Pontiac Vibe. However, it is still accessible from the selected car via links, so may be relevant.
The numbers below refer to the step numbers in the diagnostic procedures.
- 1 - The Diagnostic System Check - Engine Controls prompts the technician to complete some basic checks and store freeze frame data on the scan tool if applicable. This creates an electronic copy of data taken when fault occurred. Information is then stored in scan tool for later reference.
- 2 - This step compares the ECT temperature to IAT temperature. This inspection is to be performed when vehicle has reached ambient room temperature. At that time both surrounding air temperature and engine coolant temperature are nearly equal. If ECT sensor and circuit are OK, both sensors should indicate the same temperature.
- 3 - This step checks temperature of engine coolant increases at least 36°F (20°C) within 10 minutes. If temperature does not increase at least 38°F (20°C) there is a fault with ECT sensor or engine cooling system.
- 6 - Fault not present indicates the condition that caused DTC P0128 to set is intermittent and not currently present. If no other DTCs are stored, refer to DIAGNOSTIC AIDS for additional information on diagnosing an intermittent DTC P0128.
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When to See a Mechanic
Stop DIY work and contact a certified mechanic immediately if any of the following apply:
- • You smell fuel, burning insulation, or see smoke.
- • Brakes feel soft, pull hard to one side, or make grinding noises.
- • The engine overheats, stalls repeatedly, or misfires under load.
- • You are missing required tools, torque specs, or safe lifting equipment.
- • You are not confident in the next step or safety outcome.