Why does my 2018 GMC Acadia show Service Steering Assist warning?

Quick Summary

• Issue: Service Steering Assist message
• Vehicle: 2018 GMC Acadia SLT V6
• Problem: Thermostat opening too early
• Solution: Thermostat replacement
• Location: Omaha service

Summary

“Service Steering Assist” warnings in a 2018 GMC Acadia are often not caused by steering issues, but rather by cooling system malfunctions that trigger safety system shutdowns. Gateway Auto’s diagnostic testing in Omaha confirmed that this warning resulted from diagnostic trouble code P0128, which occurs when the thermostat opens too early causing the control module to affect a multitude of sub systems because of it's changed behavior, which affects the electric steering assist.

Our ASE-certified technicians found the thermostat opening at 160°F instead of the specified 194°F, causing the engine computer to disable electric steering assist after testing a multitude of possible factors. Replacing the thermostat housing restored correct cooling system operation and cleared both the check engine light and Service Steering Assist message.

5 Signs Your Acadia Has Thermostat-Related Steering Warnings

1. “Service Steering Assist” or “Reduced Steering Assist” message on the dashboard
2. Check engine light with code P0128 (coolant thermostat range/performance)
3. Engine takes longer than normal to reach operating temperature
4. Poor heater performance or lukewarm cabin heat
5. Slightly lower than normal temperature gauge readings

What We Found: 4 Connected Cooling System Issues

1. Thermostat Opening Too Early

Thermostat began opening at 160°F instead of the specified 194–225°F range, preventing proper engine warm-up.

2. Code P0128 Triggering Steering Assist Warning

The engine computer disabled steering assist in response to the cooling fault as a draw/protective safety measure.

3. Low Coolant Level

Slightly low coolant discovered during inspection, suggesting a minor system leak.

4. Leaking Radiator Tank

Pressure testing revealed a cracked radiator tank on the passenger side causing gradual coolant loss.

Our 3-Step Diagnostic and Repair Process

1. Computer Diagnostics and Warning Analysis
   → Retrieved code P0128 and confirmed the Service Steering Assist warning was caused by a cooling system fault, not a steering failure.
   Learn more about our Check Engine Light Diagnostic Services.
2. Thermostat Performance Testing
   → Ran engine for 15 minutes with diagnostic tools monitoring coolant temperature. Verified thermostat opening 34°F too early.
3. Thermostat Housing + Radiator Replacement
   → Installed new thermostat housing assembly with correct 194°F specification and refilled cooling system with fresh coolant.

3 Ways to Prevent Thermostat-Related Warnings

• Address check engine lights early
• Schedule routine inspections including cooling system pressure tests to catch leaks.
• Monitor temperature gauges for abnormally low readings that indicate thermostat issues.

Customer Reviews

⭐⭐⭐⭐⭐ “Appreciated the thorough testing to confirm the thermostat was actually opening too early. They showed me the temperature readings that proved the diagnosis.”

Quick FAQ

Q: Why does a cooling problem cause a steering warning?
A: Modern vehicles use electric power steering controlled by the engine computer. When faults like P0128 appear, the computer reduces steering assist to protect system integrity or due to other possible factors that can trigger the code. The steering system itself is fine—the warning reflects a cooling system safeguard, not a steering malfunction.

Q: How do I know if it’s really the thermostat or the steering system?
A: Professional diagnostics can differentiate between the two. Cooling-related warnings will appear with code P0128, while genuine steering faults produce codes specific to steering sensors or motors.

Q: Will the steering warning come back after thermostat replacement?
A: If the thermostat caused the code, the warning should not return. However, if leaks persist—like the radiator crack found on this Acadia—the code and warning may reappear until all cooling issues are resolved.

Don’t Ignore Steering Assist Warnings

“Service Steering Assist” messages often point to cooling system problems, not steering failures. If you’re seeing this message or a check engine light, schedule your GMC Acadia Service Steering Assist warning fix today at Gateway Auto in Omaha today. Our ASE-certified technicians will pinpoint the true cause—whether it’s a thermostat issue or a deeper cooling system fault—and get you safely back on the road.