The check engine light illuminates because the car's onboard diagnostic system (OBD-II) has detected a problem. This problem could be anything from a loose gas cap to a serious engine malfunction. The fact that it happens at 55 mph or with the heater on is just when the symptom becomes noticeable. The actual problem could be manifesting itself under those conditions, or it might be unrelated.
Here are some possibilities:
* Speed-related issues: A sensor malfunctioning at higher speeds (like a speed sensor, mass airflow sensor, or throttle position sensor) could trigger a code. However, this is not directly linked to 55 mph specifically – it's the stress on the system at higher speeds.
* Heater-related issues: A problem with the heater system itself (like a faulty blower motor resistor or a short circuit) might also set off a check engine light if it draws excessive current or is related to a control module that also interacts with the engine control system. This is less common.
* Underlying issue made apparent under load: The extra load on the engine when accelerating to and maintaining 55 mph or running the heater might highlight an already existing problem (a failing sensor, a small vacuum leak, etc.) that only becomes apparent under increased demand for power.
To find the real reason:
You absolutely need to get the diagnostic trouble codes (DTCs) read from your car's OBD-II system. This can be done using an OBD-II scanner (available at most auto parts stores) or by a mechanic. The codes will give a much more precise indication of what's wrong. Simply knowing the speed or heater usage doesn't help pinpoint the problem; it's just a clue that needs further investigation.
