Warranty Work Advice

Hey guys just wanted some advice (Longish Post).

Had a CEL come on a couple of months ago. Went to the dealer to get it fixed/serviced they notified me that the CEL was due to either oxygen sensors being broken or my ecm being broken. They also noted that since there was some mud under the truck they will not be warrantying it. I was a little surprised but just got the part numbers for the oxygen sensor and left

I did want to note that ever since having my drive shaft boot torn open (thanks guys for referring me to Advanced Driveline) I've taken it easy on the car but I do occasionally go to Tosohatchee or the bird wildlife preserve so the truck sees dirt and mud.

Anyway I put the Oxygen sensor myself (had a non Toyota mechanic check it too) one of the CEL still on, so I decide to try another dealer since I moved to a different part of town. They inform me that they will not warranty it since one of the regional mechanics was also in for the day and he called the previous dealership and they noted that there was mud under the truck that's they they didn't initially warranty the truck.

Now I didn't submerge/watercross this truck but this truck has seen some dirt. It's pretty clean now and there shouldn't be any mud under the truck. The CEL came on randomly about 1.5-2 months after I went to Tosohatchee/Wildlife preserve. I was seeing if you guys think I should pursue this more (talk to GM or call Regional) or just pay to get it fixed

Thanks!