The Chevy 1500 5v reference problem... from hell

Chevy trucks are not normally very hard to diagnose. Some of the more complicated newer systems like electronic parking brake can have huge repair costs if you do not know what you are doing but they are fundamentally simple systems. In this case of a 2014 GMC Sierra 1500 it was anything but simple. It might seem that way typed up cleanly and clearly here but in reality it was a day long endeavor to try to solve a 5V reference problem and did not conform to Occam's razor philosophy...

Customer dropped it off because it kept saying on the dash to shut the vehicle down as there as no oil pressure. Furthermore it had a pile of codes that were reporting as "not active" stored in the computer. P0016, P0191, P0192, P0340, P0513, P0521, P0532, P0641, P2227, P2230, C0035, U0073, U0077, C0242, B1325, U0164. Where do we start? Well all of the codes relating to the sensors are on a single 5V reference circuit coming from the computer... lets start there.

• Common rub points on the harness: no issue.

• 5V reference circuit for all these sensors tested fine at each of their connectors as well

• Load tested wiring for entire 5V circuit between each sensor

• Checked resistance of each sensor compared to a stack of new sensors ordered in for just such a purpose

After a stroke of luck finding oil coating Camshaft Position sensor and wiring into its X154 connector we thought we had it figured out. Oil was actually leaking through the electrical connector on the sensor itself. Replacing the part and cleaning up the oil, however, did not correct the issue. Could there actually be more then one issue with the same system? What next?

What we ended up doing is hooking up a DMM (Digital Multi Meter) to the 5V reference circuit and one-by-one unplugged one sensor at a time with the key on. After a period of time the 5V reference signal would drop down to around 3V often times briefly requiring the continual monitoring of the DMM during this whole process. Eventually we ended up at the Fuel Tank Pressure sensor that, when unplugged, was the first sensor to show us no voltage drop on the reference signal over the course of the half hour we monitored. We installed a new fuel tank pressure sensor, put the tank back in position and let it run outside for the rest of the day. Finally, the problem with the vehicle was solved but we still had to sell the cost of the repair to the customer.

One of the hardest things to do is explain a bill to a customer who is not a part of our field. Many people think a code is all you need to be able to know what part needs to be fixed. Every day I am trying to explain to people that just because a code is there does not mean that the specific part named in the code is the actual problem. That Evap small leak codes don't actually refer to a specific part but rather the interactions of parts connected to the fuel tank through hoses, canisters, solenoids, electrical connectors, fuel vapors themselves, gas caps, filler necks, so on and so foreth.