I got my first Check Engine Light a few days ago. 145,000 miles.
It comes and goes, but mostly stays on.
It seems to run OK, but that is a big yellow light. Makes it hard to see the TRAC light.
Causes:
A code P0137 may mean that one or more of the following has happened:
- Faulty O2 sensor Exhaust leak near the rear sensor
- Plugged catalyst
- Short to voltage on O2 signal circuit
- High resistance or open on O2 signal circuit
Possible Solutions:
- Replace faulty sensor
- Repair exhaust leak near the rear sensor
- Check for restriction in catalyst and replace as necessary
- Repair short, open, or high resistance on O2 signal circuit

My question is whether any other GXP owners have had this code, and did a new sensor fix it, or is there regular "GXP" cause for P0137?
I have a hard time believing that I would be one of the first.

I found a couple references from SAMzGXP, but he was dealing with a bunch of issues at the time.

Other news... seat heater out. CD player inop. Using some oil - need to look for a leak.
You know... the usual.

I have the SSEi, so different setup. I had same error. Replaced O2 sensors, same problem. Ended up being a bad cat converter. In my case, may have been caused from driving with a misfiring cylinder or, although unlikely- Wife not willing to pay for premium fuel for over six years.

That'* what I'm afraid of. I did replace an O2 sensor on my 3800 Impala last year, and that took care of it.
Many years ago, my F150 had the the same code, but that turned out to be a broken wire.

I bought a OEM direct replacement for the CAT, but didn't have the time to put it on. Went to a local muffle shop, he stated his generic CAT would install better and would be cheaper. The generic cat for the 3.8L installed was around $100.00, much cheaper than I thought.

Delphi sensor ES20094 will run about $57 at Amazon delivered.
It'* too cold to work on it now, and beside, the light has been off for 2 days.
I can only hope? ...Never happen. I'm not that lucky.

The CEL kept coming back intermitently,
I ordered the ES20094 sensor. It looked the same and plugged right in.
The rear sensor is located on the exhaust pipe after the converter. It'* not very hard to get to. You'll need to remove an aluminum stamped cover that'* held on with 4 pop rivets.

The good news is I still had plenty of rivets from my snowmobiling days.
The bad news... the threads stripped on the old sensor taking it out.

I had to get an M18 -1.5 Tap to chase the threads. Same threads as many spark plugs.
I'll drive it for a few days and see if that takes care of it.

How many senosrs does the N* take? I have a 05 and fuel mileage is horrible. Its at 68k and I think Ill throw some new plugs and sensors in her just for good measure. Interested in location, quanity and OE manufacture and numbers. Thank

What do you call horrible mileage? I've never seen under 17.4 mpg average, best mileage was 25.7 mpg. Double check your normal maintenance items (air filter, oil and filter, etc). I doubt it'* the plugs because they are generally a good plug up to and over 100,000 miles. Do you notice any misfires at start up? If so, could be one or more of the ignition coils. You are getting really close to the end of their expected life span, so just something to look into. Normally, sensors will throw a code if there is an issue so probably won't need to mess with those either.

Last fill up was 14.9 mpg. Thats with hwy/city . The plugs and sensors or on the way out as they do not perform as optimal on the last half of their life. I change plugs and sensors 65-75k miles on cars with 100k mile plugs. If you wait till a light comes on then you already been running the car poorly anyways for awhile. Sensors are know to start to read slow and heaters to go out so I just consider it perventative maintance. Im going to have to get a service manual. Are the cheap ones people selling on Ebay on CD any good?

depends on which one you get. Some of the sellers will copy pages out of a generic manual (including 3.8 info) and sell you a PDF of unrelated material.
Others are selling the shop manual on CD.