* Shared Computer Systems: The engine control module (ECM) and the transmission control module (TCM) might be interconnected or even a single unit in your car. A fault detected by the ECM, like a faulty sensor (oxygen sensor, mass airflow sensor, etc.), might lead to incorrect data being sent to the TCM, causing it to shift improperly.
* Electrical Issues: A problem causing the check engine light could be a wiring harness fault, a ground issue, or a faulty sensor that impacts both engine and transmission operation. These electrical problems disrupt the proper functioning of both systems.
* Low Transmission Fluid (Indirect): While not directly related to the check engine light, certain engine problems (like a leak or faulty sensor related to fuel) might cause the engine to misfire or run inefficiently. This could lead to increased heat and stress on the transmission, eventually leading to low fluid levels or damage, resulting in poor shifting. This might trigger a check engine light *later* as a consequence of transmission damage.
* Torque Converter Issues: Engine problems can sometimes strain the torque converter, affecting its ability to smoothly engage and disengage, resulting in poor shifting and potentially triggering a check engine light (indirectly).
In short, the check engine light is a symptom of an underlying problem. The poor transmission shifting is either a *symptom of the *same* underlying problem* or a *consequence of the underlying problem leading to other damage*.
You need to get the check engine light diagnosed. Using an OBD-II scanner (available at auto parts stores) will give you a trouble code that will help pinpoint the exact problem. This is crucial before attempting any repairs to avoid further damage to your transmission. Ignoring the issue could lead to significant and costly transmission repairs.
