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The thermostat housings are a known area on earlier E39s to develop cracks and leaks over time due to their exposure to thousands of heat cycles, which cause the plastic in these units to weaken. This relatively inexpensive item should be on everyone’s list for preventative maintenance, or whenever replacing your thermostat.
If you're replacing the housing, it's a good idea to change the thermostat at the same time (found at the bottom of this page). Make sure you have enough coolant on hand before starting the job too.
Please note: Part numbers are provided for reference only. For correct compatibility to your vehicle, please follow our fitment advice