Your best getting the codes read on the ECU first. If you have a laptop you can do this yourself, buy an interface cable on eBay and get some free OBDII reader software. There's plenty of free ones out there that can read basic fault codes and reset them.
It could well be the lower lambda sensor, it does sit at a level where water could get at the connection. While they are sealed with a rubber grommet on the connection they do perish over time.
Be a good idea to get some electrical contact cleaner and some dielectric grease to clean the connection and help keep moisture out. Might be an idea to do any other connections you can get at while your down there.
The scuttle drains are a good shout too even if they aren't related to the issue, I did mine a few weeks ago and the drivers side one was badly blocked. Seems to be quite a bit smaller than the passenger side so wouldn't take much to bung it up. Can grease the wiper system too while everything off and you have access.
