2020 Cadillac CT6 warning light illuminates intermittently indicating a drop in engine power.

Posted by Tony Brown on

Car: Cadillac

Model: CT6

System: Engine

Year model: 2020

 Issue: The customer reported that recently the vehicle intermittently prompted a warning light indicating a reduction in engine power.

 Fault diagnosis: Start the vehicle road test, after driving for a period of time, the engine fault light was on and the instrument prompted the fault information of engine power dropping. It was then verified that the problem described by the customer was true.

 Check whether there are modified parts, etc. In this case, the vehicle did not have any modifications and program changes.

 Connect the diagnostic device (GDS) to the vehicle and read the fault code as U0625. The engine control module lost communication with fuel rail pressure sensor 1, U0670 lost communication with fuel temperature sensor 1, U067 lost communication with fuel temperature sensor 2, U101B lost communication with fuel rail pressure sensor 2, and U18A2 lost communication with fuel pump driver control module.

 Checked the repair newsletter and TAC technical briefing and found no repair information and solutions for similar failures. Next, carry out the system test according to the diagnostic process of "U0625" in the diagnostic information program of the manual. According to the diagnostic result, it could be judged that the circuit was normal. When interchanged with the normal fuel rail pressure sensor, the fault did not reproduce.

 Since the fault was an intermittent fault, and it could not be determined that the sensor was 100% faulty, the relevant lines were checked. When the probe was used to measure the fuel rail pressure sensor wiring harness plug, it was found that the No. 3 female terminal was relatively loose, as shown in Figure 1.


Figure 1

 We have re-organized the fault and measured the circuit. By looking up the diagnostic fault information in the maintenance manual, we basically determined the direction of the fault diagnosis. It was necessary to check the circuit between the engine control module and the fuel pressure sensor again.

Secondly, we compared the fuel rail pressure sensor control mechanism of the new 2.0T engine and the old LNF model engine: LTG, LSY engine fuel rail pressure sensor transmits fuel pressure and temperature information to control module K20 through serial data; The engine control module (ECM) of the model engine provides the 5V reference voltage to the 5V reference voltage circuit and provides the ground to the reference ground circuit. The engine control module receives the varying voltage signal on the signal circuit. The signal voltage becomes high when the fuel pressure becomes high. When the fuel pressure becomes lower, the signal voltage becomes lower correspondingly and no temperature signal appears.

Use a probe to check the female connector of the fuel rail pressure sensor wiring harness for resistance. After checking, it was determined that the No. 3 female terminal was loose, and the fault was caused by poor contact. Next, measure the resistance and voltage of the circuit, finding which was within the normal range.

After processing the faulty pins, the fault did not reproduce after repeated test runs. It could be then concluded that the fault point was caused by the loose and poor contact of the No. 3 pin of the wiring harness connector of the fuel rail pressure sensor.

Fault summary: When repairing faults, you cannot blindly replace parts, and you must find the root cause of the fault. When measuring the pin of the line plug, the test pin should feel the damping of the inserted pin to determine whether the pin is too loose. At the same time, the test pin should be inserted to feel the damping, but it does not need to be inserted too much. For example, in this case, the loosing pin leads to poor contact and failure.

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