Q: 1988 mercedes 420sel how do you self diagnostic check check engine light is on continually in the car

If the vehicle has an 8 (or 16) pin data connector, you can use a reader and refer to a MB Fault Code Manual for Model Years 1988-1997. The blinking codes are also documented in the Factory Service Manual (copies are still available and you will need if you are working on the car). Four blinks refers to the air flow position sensor which you can test but, again, it must be diagnosed with the manual in hand. If you desire that a certified mechanic read the codes and resolve any found issue, the recommended service is a Check Engine Light diagnostic. If you have further questions or concerns, do not hesitate to re-contact YourMechanic because we want you to make the most of your repair dollars and help you to get the best possible results.