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Like you, I had a day when the heater worked fine—not particularly hot, but adequate. The following day, there was a noticeable lack of heat. I determined that the heater matrix was clogged. To clear it, I disconnected the pipe from the thermostat housing to the heater. After blowing out the pipe, I filled it with distilled vinegar and let it sit for a few hours. I should have left it overnight for optimal results. Then, I blew it out again, pumped through some clean water, and filled it with coolant. The heater started working much better, but it was still nowhere near hot enough. I should have left the vinegar in for longer!