gpu has fallen off the bus, how can I find the real reason. the gpu was sent to repair, but no problem was found. the error message is:

[405535.804298] pcieport 0000:00:1c.4: AER: Correctable error message received from 0000:00:1c.4
[405535.804305] pcieport 0000:00:1c.4: PCIe Bus Error: severity=Correctable, type=Physical Layer, (Receiver ID)
[405535.804306] pcieport 0000:00:1c.4: device [8086:7a3c] error status/mask=00000001/00000000
[405535.804308] pcieport 0000:00:1c.4: [ 0] RxErr (First)
[405535.845994] pcieport 0000:00:1c.4: AER: Correctable error message received from 0000:00:1c.4
[405535.846000] pcieport 0000:00:1c.4: PCIe Bus Error: severity=Correctable, type=Transaction Layer, (Receiver ID)
[405535.846000] pcieport 0000:00:1c.4: device [8086:7a3c] error status/mask=00002000/00000000
[405535.846001] pcieport 0000:00:1c.4: [13] NonFatalErr
[405535.857037] NVRM: GPU at PCI:0000:04:00: GPU-939f3e08-5287-0355-e07d-0f1ebc95deef
[405535.857052] NVRM: Xid (PCI:0000:04:00): 79, pid=”, name=, GPU has fallen off the bus.
[405535.857054] NVRM: GPU 0000:04:00.0: GPU has fallen off the bus.