Yes their wiring degrades just like the upper harness. Not being on the top of the engine, the cooking temp is lower..

I think DaveM has a writeup out there somewhere and some pics on his website. He's a keeper of W124 data.

You could have a bad sensor in the oil pan.. bad noise I didn't understand what you are describing. Fairly easy to inspect with the splash shield off. They are much cheaper than the upper harness.

Think the P-clamp around the trans dip-stick tube is a real bugger to get off in the replacement. But if you've been doing service on your 400E... you know how things go!

Michael