09/26/2025
Case Study of the Week
2019 Hyundai Santa Fe
I apologize in advance for this lengthy explanation. It will be worth taking time to read. This Case Study will be separated into two Customer’s Complaints (same customer and same problem), Initial Checks, Tests, and Fixes.
Customer’s Complaint:
Vehicle will display several warning lights on cluster and vehicle will still run, but has shifting problems, RPM, fuel gauge inop, airbag, side object, hill decent.
Initial Check:
Visited this vehicle several times, but the faults would not be active upon first couple of diagnostic attempts. Codes would reveal about eight modules not communicating and flagging U-codes for it. Finally got vehicle and brought it to my house and attempted to get the vehicle to go into its fault condition. After checking it in late evening time, finally able to get the faults to be duplicated.
Test:
After looking at data and many circuits tested, it was determined that all of the P-CAN bus modules were going offline. Checked again the next morning and vehicle operated fine again. I discovered that the fault would occur after the lights were turned on, or daytime running lights cut off in the late evening time due to daylight sensor.
Fix:
After reviewing wiring diagrams and other testing of circuits, I determined that the IBU module was the problem and replaced it with a new module. Repair fixed the problem and vehicle was delivered back to customer (TO BE CONTINUED).
*Three months later the dreaded phone call. The original problem was back.*
Customer’s Complaint:
Same faults, same conditions to make the faults occur.
Test:
More testing and several phone calls with a Hyundai tech on a diagnostic tech support, determined the P-CAN bus was still shorting to voltage when lights were coming on or daytime running lights went off because of the daylight sensor. The culprit was finally located under the center console and under gear shifter on the floor. In a nice little indention, sat a junction connector with P-CAN data wires and illumination wires joined (shorted) together with Coke, Pepsi, or maybe Dr. Pepper syrup, not sure, which apparently is a particularly good conductor.
So now after finding this, the problem makes sense. With lights on or daytime running lights cutting off, all of the interior illumination lights would come on and short the P-CAN data circuit to voltage.
Fix:
Repaired connector and the problem was fixed for real this time.
Lost a lot of sleep over this one.
