Why would your car start check engine light is slower on?

The "Check Engine" light on your car's dashboard is designed to alert you to a potential problem with the engine or other critical systems in the vehicle. When it comes on, it's usually accompanied by a slower idle or rough running engine. Here are some reasons why your car might start with the "Check Engine" light on and idle slowly:

1. Oxygen Sensor Issues: A bad or faulty oxygen sensor can disrupt the air-fuel mixture in the engine, leading to the "Check Engine" light coming on and causing a slow idle.

2. Spark Plug Problems: Worn-out or damaged spark plugs can hinder proper ignition, resulting in a rough idle and the activation of the "Check Engine" light.

3. EGR Valve Malfunction: A malfunctioning Exhaust Gas Recirculation (EGR) valve can cause a slow idle and illuminate the "Check Engine" light. The valve helps control exhaust gas recirculation and reduce emissions, and a fault can disrupt engine performance.

4. Vacuum Leaks: Leaks in the vacuum system can cause the engine to run lean (too much air, not enough fuel), leading to a slow idle and illuminating the "Check Engine" light.

5. Idle Air Control (IAC) Valve Issues: The IAC valve controls the amount of air that bypasses the throttle valve when the engine is idling. If the IAC valve is faulty, it can cause a slow or erratic idle and trigger the "Check Engine" light.

6. Throttle Body Problems: A dirty or clogged throttle body can disrupt airflow and cause the "Check Engine" light to come on. This can also result in a slower idle or stalling issues.

7. Fuel Injector Issues: If the fuel injectors are faulty or clogged, they may not deliver the right amount of fuel to the cylinders, causing the engine to run lean or rich and resulting in a slow idle and the activation of the "Check Engine" light.

8. Catalytic Converter Problems: A faulty catalytic converter can cause a slow idle and trigger the "Check Engine" light. The converter helps clean up exhaust emissions, and a problem with its efficiency or functionality can affect engine performance.

9. Mass Airflow Sensor (MAF) Malfunction: The MAF sensor measures the air entering the engine. A faulty sensor can provide incorrect data, affecting the air-fuel mixture and leading to a slow idle and the "Check Engine" light coming on.

Note: It's important to have the "Check Engine" light diagnosed by a qualified mechanic or through appropriate diagnostic tools to determine the exact cause of the issue. Ignoring the light or delaying repairs may lead to more severe problems down the line.