The light appears on the way to work and is gone by the afternoon. Next cold morning, back again. Warm days, nothing at all — which makes it easy to talk yourself into ignoring it. A fault that tracks the temperature is telling you something specific. On a BMW, MINI or Porsche, an oxygen sensor heater circuit that has gone high-resistance will pass on a mild afternoon and fail at 35 degrees, and Pontiac gets a lot of 35-degree mornings between now and Thanksgiving. Quick Summary: Key Takeaways Cold-start-only codes are real: A heater circuit fault sets when the sensor warms too slowly, which happens first in cold weather. [...]