My 2000 E430 just had a check engine light on today, but the display on the instrument says no malfunction. I checked the gas tank cap and it is very tight, battery is fully charged and the car is running fine as normal. I do not have OBD II scanner to read the code(s).
What are possible problems ? Oxygen sensor ?
Which auto part stores scan the code for free: Autozone, Pepboys or Kragen ?

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'00 E430 Brilliant Silver/Ash