There are several potential causes for the check engine light to come on in a 2017 Toyota RAV4. Understanding these can help you diagnose and possibly fix the issue. Here are the possible causes:

 
Loose or Faulty Gas Cap
 

• A common reason for the check engine light to illuminate is a loose or cracked gas cap.
• This can cause a reduction in fuel pressure and might allow fuel vapors to escape, triggering the light.

 
Faulty Oxygen Sensor
 

• The oxygen sensor monitors the level of unburned oxygen in the exhaust system.
• If it's not working correctly, your vehicle’s emission system isn't managed properly, leading to poor fuel economy.

 
Malfunctioning Catalytic Converter
 

• The catalytic converter helps reduce exhaust gases by converting carbon monoxide into carbon dioxide.
• A failing converter impacts engine performance, reduces fuel economy, and can lead to higher emissions.

 
Faulty Mass Airflow Sensor
 

• This sensor measures the air entering the engine to determine how much fuel to inject.
• A malfunction here can lead to issues like rough idling or stalling.

 
Ignition System Issues
 

• Problems with spark plugs or spark plug wires can hinder your engine’s performance.
• Misfires or delayed ignition can trigger the check engine light.

 
Faulty EGR Valve
 

• The Exhaust Gas Recirculation (EGR) valve helps regulate emissions.
• If it’s clogged or faulty, it can affect engine performance and lead to increased emissions.

 
Vacuum Leak
 

• A vacuum leak occurs when there is air leakage into the engine, affecting the air-fuel mixture.
• This can result in increased engine RPM, rough idle, or stalling.

 
Failed Sensors and Electrical Issues
 

• Various sensors in the vehicle, such as the coolant temperature sensor, might fail or provide erroneous data.
• Electrical issues related to the vehicle’s wiring or connectors can also illuminate the check engine light.

In summary, while some causes are minor and easy to fix, others might require professional attention. It's always advisable to have your vehicle scanned with an OBD-II scanner to identify the exact trouble codes and address the specific issues triggering the check engine light.