Rep Power: 0

My son's 2003 Civic EX CEL came on a couple of weeks ago. I went into Autozone and had them scan the system. P0138 showed up - downstream O2 sensor. I replaced that and reset it, but the CEL came on again after 10 or so miles. I noticed once that when I hit a heavy bump going over a railroad track that it went out for a while, but came back again after 10 or so miles. I read on one forum that it's possible that a water leak might be the cause of a short. I thought the small leak we have on the driver's side/front was a totally separate issue. Anyway, friends thought that maybe the sunroof drains were clogged. I tested those by pouring water down the holes and it drained fine. So... I started checking the wall behind the engine and found the attached:

In the attachment, you can see that the previous owner must have removed the grommet that was there and put another wire through the wall. Is it possible that water gets in through this spot?

I still have no idea as to what could be causing the CEL to display, but maybe one thing at a time. Thanks for any input.

 
Rep Power: 144

As far as the water leak goes, are you sure the water is draining to the ground properly? Sure it is draining out of the sunroof but to where? I had an older Maxima that had a leak from the sunroof and the water sure did drain, right onto the drivers feet! On that vehicle there is a hose that connects to a nipple that carrys the water down to the ground. This had disconnected and the water was just splashing around in the A-beam and getting everything soaked. Not totally sure, but you can probably take the plastic molding off from the inside to see that the water is draining all the way down the ground, rather than going into the car.

Since the CEL is coming on over bumps, there is likely an intermittent short somewhere along the line. Keep that water out or it's not too long until you will need a complete ECU replacement. Play and massage the connection from the O2 plug to as far as you can go and see if that trips the short.

Better yet, get a code reader and if you can track the live data, you will be able to see the short in progress and narrow down the search. You can buy a small device, the ELM327, a code reader that transmitts wirelessly to your iPhone, Android or BT-enabled laptop. It's like $15 on Amazon and you can find many free apps that will link to it.