C1700 & C2100 Trouble Codes, Can Find NOTHING

[theoneandonlywhytry]

1999 GMC K1500 Suburban, 5.7L, 4L60E with 230K miles.

For the last 6 months I have been getting codes C2100 (does not give a definition on this one) and C1700 ("Left Rear Sensor Circuit Failure") intermittently and never at the same time, and I can almost time when I will get it.

After about 20 miles and a few starts the C2100 comes on and it trips the CEL, I know it is coming because when I watch the fuel system status it will go from "using O2 sensors for mix" to "using O2 sensors for mix but there is a fault". If I am on the highway for over 30 to 45 mins, the C1700 kicks on.

There is not bad idle or performance issues with this I notice and I find nothing online besides that it is a chassis code and somewhat generic. I have read everything online that comes up on these...

Since it says there is a fault in the fuel system status I assumed O2 maybe, but all my O2 sensors put out voltage within spec and work when checked according to my scanner when I look at each bank and I have no other codes.

I have replaced the following in the last 6 months in the fuel system, not to try to find the issue but just for preventative maintenance.

New fuel pump
New fuel filter
New spark plugs
Clean throttle body and intake
New PCV valve + grommet
Cleaned EGR
Some fuel system cleaners here and there
and more

I thought maybe a bad ground? I really do not know where to go from here since everything looks in order and checks out, I have never had any other codes with this truck in the last 6 months of ownership and 3,500 miles drive time besides a P0161 that went away and never came back over 3K miles ago after some fuel system cleaner.

I would drive this truck anywhere, but these two codes just keep coming back and I can find nothing about them.

Can anyone help?

 

[east302]

Have you tried a different scanner to see if those are the correct numbers for those C codes? Usually, they’re in the C0## format and associated with the ABS system. Any oxygen sensor or fuel (misfire, rich, etc.) should be an emissions P code.

 

[theoneandonlywhytry]

do you know what that might be on in the rear? i cannot get any ABS codes, lights, and it works if i really hit the brakes and get on it. the left rear circuit failure could be on the actual rear or maybe a left rear circuit in the module.

 

[east302]

There are three circuits - the two front wheel speed sensors and the vehicle speed sensor (on the transfer case, I believe) which covers both rear wheels. There aren’t sensors for each rear wheel. That’s why I was wondering if your code reader was flubbing the number somehow or if it was a false code.

 

[theoneandonlywhytry]

gotcha, yeah i get this on 3 different scanners and the one i usually use i can real time read a ton of circuits so only one left is a snap-on, which i can use so maybe i will try that.