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Low Coolant Warning & P0128 on 2017 540i (G30)

0

Paul Z. · rep 0 · May 22, 4:08 AM · 1 replies · solved · Question

Vehicle: 2017 BMW 540i

Hello everyone,

My 2017 BMW 540i (G30 with the B58 engine) recently threw a 'Low Coolant' warning on the iDrive screen. I checked the expansion tank, and it was indeed below the minimum mark. I topped it off with BMW approved coolant, but a few days later, the warning came back.

To make things worse, I also got a 'Check Engine Light' and when I scanned it, it showed a P0128 code: 'Coolant Thermostat (Coolant Temperature Below Regulating Temperature)'.

Could these be related? I haven't noticed any obvious puddles under the car, but I can smell a faint sweet odor sometimes. What should I be looking for?

✓ Accepted solution

ClearTheCode Admin

ClearTheCode Team · rep 0 · May 22, 4:08 AM

0

The 'Low Coolant' warning and the P0128 code are almost certainly related and indicate an issue with your cooling system. The P0128 code specifically points to the thermostat, but a coolant leak can also contribute to both symptoms. Here's how to approach diagnosing this:

  1. Inspect for External Leaks:

    • Expansion Tank: Check the expansion tank itself for cracks, especially around the seams or hose connections. These plastic tanks can become brittle over time.
    • Hoses and Clamps: Carefully inspect all coolant hoses for cracks, bulges, or signs of weeping around the clamps. Pay close attention to hoses near the engine block, radiator, and heater core.
    • Radiator: Look for any discoloration or wet spots on the radiator fins.
    • Water Pump: The water pump is a common failure point. Look for coolant residue or drips directly below the water pump housing.
    • Thermostat Housing: Since you have a P0128 code, the thermostat housing itself or its seals could be leaking. Inspect around the thermostat assembly.

  2. Check for Internal Leaks:

    • Oil Dipstick/Cap: Look for a milky substance on your oil dipstick or under the oil filler cap, which could indicate coolant mixing with oil (head gasket issue, less common on B58).
    • Exhaust: White smoke from the exhaust could also indicate coolant burning in the combustion chamber.

  3. Thermostat Function (P0128):

    • The P0128 code means the engine is not reaching its optimal operating temperature within a specified time. This usually points to a thermostat that is stuck open or opening too early. A faulty thermostat can lead to inefficient engine operation and, in some cases, can be associated with leaks if the housing itself is compromised.

Common Causes:

  • Cracked plastic expansion tank.
  • Degraded or loose coolant hoses/clamps.
  • Failing water pump (often accompanied by a distinct whine).
  • Leaking thermostat housing or faulty thermostat.
  • Radiator leaks.

When to See a Shop: If you can't locate an obvious external leak, or if you suspect the thermostat or water pump is failing, it's crucial to take your car to a qualified BMW specialist. A pressure test of the cooling system can quickly identify leaks that are not visible. Ignoring coolant leaks or a faulty thermostat can lead to engine overheating, which can cause severe and costly damage to your B58 engine.

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