I have an intermittent code 26. Every time I clear it, a few weeks later the check engine light is on again. The cold engine shift delay works every morning. The vacuum lines look visually fine and are tight at the firewall. The light comes on when restarting the engine warm, and it is lit immediately at startup. What else should I check for?

This time I also have a Code 2, which I believe is an Oxygen sensor fault. I wonder if the two are somehow related. Please help!

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2004 E500 4Matic 138,000 miles