Mercedes-Benz has warned owners of some of its cars there's a risk they could catch fire.
And the firm can't do anything about it because it can't get the parts needed for a fix.
The company says the problem concerns a possible leakage in the vehicles' coolant pumps.
It says there's no point doing a recall until the required components become available.
In the meantime it advises owners of affected vehicles to reduce usage to a bare minimum, and to drive in a "particularly prudent" manner.
Germany's Bild newspaper published a copy of a letter the Daimler unit sent to 800,000 owners.
The firm confirmed the letter was authentic, but wouldn't comment on the number of vehicles involved.
It says the fault affects a wide range of cars, including sedans and SUVs.
Like many automakers, Mercedes has been battling a global shortage of parts including for computer chips.