This happened to the Lexus. It suddenly wouldn't warm up the cabin when I let the engine warm every morning on the driveway. Very irritating when it's well below zero - my fingers were frozen holding the steering wheel. But I noticed that if you rev the engine around 2000, the heater started and it got warm, but back at tickover it was freezing again. I asked a garage bloke. "Top up the damned radiator with water, idiot" he said. And I did, about three cm. And lo and behold, that's what it was. It's back to toasty again. Something to do with it having another radiator in there somewhere and needs to get pressure up - that happens if you rev the engine but needs more water to get the pressure correct. I didn't understand the gubbins either but it worked.
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