A fluctuating temperature gauge in your 2020 Traverse paired with a P0128 code points to a cooling system that isn't maintaining proper operating temperature. The engine is running too cool—which sounds harmless but actually affects performance, emissions, and fuel economy.
Decoding P0128
The P0128 code means "Coolant Temperature Below Thermostat Regulating Temperature." Translated: your engine isn't warming up fast enough or staying warm enough. The ECM expects to see a certain temperature within a specific time frame after cold start, and it's not happening.
Why Temperature Fluctuates
In a properly functioning system, the temperature gauge should rise to the middle range after warmup and stay there steadily. Fluctuation happens when:
- Thermostat stuck open: Coolant circulates constantly instead of being blocked during warmup
- Thermostat opening early: A weak thermostat spring causes premature opening
- Coolant level low: Air pockets cause erratic temperature readings
- Coolant temperature sensor issue: The sensor itself sends incorrect readings
- Head gasket leak: Combustion gases entering cooling system (less common)
Symptoms You May Notice
- Temperature gauge moves up and down while driving
- Heater blows lukewarm or inconsistent air
- Check engine light with P0128 stored
- Slightly reduced fuel economy
- Engine takes longer than usual to warm up
Diagnostic Process
- Verify coolant level: Check when cold—low coolant causes erratic gauge behavior
- Monitor actual temperature: Use a scan tool to watch coolant temp during warmup
- Thermostat test: A properly functioning thermostat should keep the engine at 195-220°F
- Inspect for leaks: Coolant loss requires finding and fixing the source
- Check the sensor: Compare scan tool reading to an infrared thermometer reading on the thermostat housing
Repair Costs
- Thermostat replacement: $200-$400 (part and labor)
- Coolant temperature sensor: $100-$200
- Cooling system flush and fill: $100-$200
- Thermostat housing replacement: $250-$450
The thermostat is the most common cause and a relatively straightforward repair on the 3.6L V6.