996 Error Code 1117

[LarryHughes]

I have a 996, 2001 911, 3.4, 128,000 miles, runs great.
Very rarely, I get an error code of 1117 which indicates a heater problem on the 'downstream' O2 sensor. The problem is, my car doesn't have a 'downstream' O2 sensor. Has anyone experienced this?
The Bentley manual indicates my car has a downstream sensor but that ROW doesn't. My car is USA but no downstream sensor.
Thoughts? Thanks.

[JFP in PA]

If it lacks the second sensor, it either is not a US car, or someone modified it. All legitimate US cars were delivered with four O2 sensors, as required by the EPA.

[LarryHughes]

I took another look and JFP is right. There are two O2 sensors on each bank. What threw me was that the "downstream" sensor isn't downstream but is about 1/4 of the way into the catalytic converter. Will replace both O2 sensors in the cats. Thanks.