I occasionally get readings of 10-15ppm during slow flight, and got steady 20ppm one time when I left the baggage door open. But I don’t get readings from the heater, so you should definitely get your system checked.
I trained in Katana’s, and they produce ~10-15ppm readings much of the time (DA20-A1’s) since the air intakes are rather close to the exhausts. Once I saw that I never took one above 5000’.
Carrying a sensitive CO detector can be enlightening. I had it go off in a taxi in Bellingham WA. I asked the taxi driver if he got headaches, and he said all the time. I told him he might want to keep the windows open until he had the car checked.
At San Carlos which is located next to Highway 101, when there is a gentle breeze blowing from the freeway direction at rush-hour I get readings of 10-12ppm on the ramp!