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Intermittent LAN Failure Clues
Posted to Technical Discussion Forum on 2/27/2017 8 Replies

We see a lot of intermittent failures in the dealership. Most are either LAN or infotainment related. In either case, although the problem may not be current in the shop, there are usually clues to follow. How you follow these clues will usually mean success or failure in your repair attempt.

A 2016 Colorado came in today with an intermittent failure. It has 28,000 miles and the customer states that the ABS and SIR lights come on once a week or so for a few seconds and then go off. The truck ran normally and no lights were illuminated when I drove it around the block.

Doing a full module scan with GDS2, I saw codes set only in three modules, the EBCM, PSCM and SDM. This is clue #1 - all 3 modules are the only modules on the Chassis Expansion Bus. Let's see who is active on that bus.

[2016 Chevrolet Colorado WT, ECM/Inputs/Outputs Scan Data]

OK, all 3 modules are present and currently communicating. Now, what codes are set?

[2016 Chevrolet Colorado WT, ECM/Inputs/Outputs Scan Data]

There's obviously a communication issue on the Expansion Bus. All 3 modules set U0077. This is clue #2 - When a modules is unable to communicate over the bus for 3 or more attempts, it will set U0077. At this point, it will suspend all message transmissions and will use default values for all parameters received over the bus. This will cease when communication is restored. So, all 3 modules lost communication at the same time. This points to a total LAN failure, not an individual module. How's the LAN voltages look?

[2016 Chevrolet Colorado WT, ECM/Inputs/Outputs Scan Data]

Well, we know that the problem is intermittent. The Chassis Expansion Bus looks normal right now. What can cause the problem? An open is unlikely as all three modules are equally affected. This leaves a short to power or ground. An intermittent short to power is very unlikely and I would expect a grounded LAN harness. Since we're dealing with only 3 modules in a LAN, it shouldn't be too hard to locate. With the LAN voltages on the screen, I started at the EBCM, then the PSCM and gently wiggled and pulled slightly on the harness with no change. The harness then goes up and under the UBEC before leading inside the truck to the SDM. When I shook the upper part of the harness, I saw this...

[2016 Chevrolet Colorado WT, ECM/Inputs/Outputs Scan Data]

There's our ground out. Both CAN+ and CAN- are being pulled down. Following the harness past the UBEC, I saw it lying on an A/C line with the conduit worn in a concave fashion against the aluminum tubing. I pushed it against the tubing and saw this...

[2016 Chevrolet Colorado WT, ECM/Inputs/Outputs Scan Data]

Well, that's pretty obvious, isn't it? Repairing the rubbed through wiring, installing new conduit and routing the harness away from the tubing solved the problem. With an intermittent, just follow the clues and proceed in an orderly, systematic fashion and you will be more successful with these type of failures.

James from Florida

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car Vehicle Data

2016 Chevrolet Colorado LT 2.5L

Engine2.5 L
Trans6-speed Automatic (Electronic)