AutoZone and some of the other chain after-market parts stores will read and clear the codes without charge when the check engine light comes on. You should go to see them.

The check engine light came on when the second of two glow plugs on my '98 e300td went bad. The light stayed after I installed new glow plugs. I went to the AutoZone store near me and they cleared the codes without charge. The light has not come on since then. The guy was gracious and didn't act like he was doing me a big favor.