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P0128 Code & Low Coolant on 2020 XT4 Sport

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Mark S. · rep 0 · May 22, 4:02 AM · 1 replies · solved · Question

Vehicle: 2020 Cadillac XT4 Sport

Hey everyone, I've got a problem with my 2020 Cadillac XT4 Sport. The check engine light came on yesterday, and when I checked with my OBD scanner, it showed a P0128 code (Coolant Thermostat (Coolant Temperature Below Thermostat Regulating Temperature)). I also noticed the coolant reservoir is a bit low.

I haven't seen any obvious puddles under the car, but I can smell something sweet sometimes after driving. Could this be a thermostat issue, or maybe a small leak somewhere? What should I check first?

✓ Accepted solution

ClearTheCode Admin

ClearTheCode Team · rep 0 · May 22, 4:02 AM

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ClearTheCode admin answer:

That P0128 code on your 2020 Cadillac XT4 Sport, combined with low coolant and a sweet smell, points directly to an issue with your cooling system. The P0128 code specifically indicates that the engine's coolant temperature is not reaching or maintaining the proper operating temperature, often due to a faulty thermostat or insufficient coolant.

Common Causes for P0128 and Low Coolant:

  1. Stuck Open Thermostat: This is the most common cause for P0128. A thermostat stuck open allows coolant to continuously flow through the radiator, preventing the engine from warming up efficiently.
  2. Low Coolant Level: If the coolant level is too low, the temperature sensor might not be fully submerged, leading to inaccurate readings, or the system simply can't regulate temperature effectively.
  3. Coolant Leak: The sweet smell is a strong indicator of a coolant leak. Even small leaks can lead to significant coolant loss over time without visible puddles.
  4. Faulty Coolant Temperature Sensor: Less common, but a malfunctioning sensor can send incorrect data to the engine control unit.

Recommended Steps:

  1. Visual Inspection: Carefully inspect all hoses, the radiator, the water pump, and the thermostat housing for any signs of leaks (pinkish residue, drips). Check under the oil cap for milky residue, which could indicate an internal leak.
  2. Top Off Coolant: Use only Dex-Cool (orange/pink) coolant, specifically designed for GM vehicles. Do not mix with other coolant types. Top off the reservoir to the 'cold fill' line.
  3. Monitor Temperature: After topping off, drive the vehicle and monitor the engine temperature gauge. See if it reaches and maintains normal operating temperature.
  4. Thermostat Replacement: If the P0128 code persists after ensuring proper coolant levels and no visible leaks, the thermostat is the most likely culprit and should be replaced.

When to see a shop:

  • If you cannot locate the source of the leak.
  • If the check engine light remains on after topping off coolant and driving.
  • If you're uncomfortable performing a thermostat replacement, as it involves draining and refilling coolant.
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