T3400 crashed - amber flashing start button, no diagnostics

The computer crashed while I was away.
The front panel start button flashes amber - repeating 2 (or 3) rapid flashes followed by a short pause and a single longer flash. (The repeated flashes are too rapid to distinguish whether there are 2 or 3 flashes.) The front panel does not show ANY diagnostic lights. (I have looked with a magnifying glass with the cover plate removed.) Also, there are no beeps. Nothing is displayed on the monitor. If I continue to hold down the start button for about 5 seconds, everything stops completely and the button light turns off.

The power supply fan is very slow. (When the system had been running normally, the power supply fan was moderately slow. Since the fan wasn't running on high I assume there wasn't an overheating problem.)

After disconnecting all external peripherals and pressing the rear power supply diagnostics button, the adjacent diagnostics light turns solid green and the CPU fan is very fast. According to a Dell diagnostic video (https://www.youtube.com/watch?v=udGOe7LQJEI), this should indicate a problem with one of the external peripherals. However, no external peripherals were connected.  This video further states (minute 1:00), "If the power supply light remains off with all external devices disconnected then we'll need to identify the internal component causing the power failure." However, my power supply light remains on. (Conversely, here (https://www.dell.com/support/article/us/en/04/sln153751/no-post-no-power-no-video-on-a-dell-desktop-computer?lang=en#No_Power) Dell states, "Power off computer, leaving the computer plugged in. Press and hold the power supply test button on the rear of the power supply unit. If the LED next to the switch illuminates, the problem may be with your system board.")

With all other internal connectors still connected, I disconnected and the reseated the power supply connector to the motherboard. This had no effect.

I considered replacing the coin battery on the motherboard but the User's Guide says that the computer should run without a battery (p. 249) and there was no indication that the battery might be bad (incorrect dates and times, p. 249).

The DC power supply connector P15 (black plastic female (two rows of 3 pins) coming from the bottom of the case) is not connected. The T3400 user manual (p. 154) states that this connector is for the 525 watt power supply only. However, I can't see where this might connect to any socket or any cable within reach of the 6" P15 wires. (The Power Supply DC Connector Pin Assignments (p. 149) doesn't show the P15 connector as being part of any of the power supply cables.)

The computer is always plugged into an APC Back-UPS 1300 so a power surge seems an unlikely culprit. No external peripherals or internal components have recently installed or removed.

Specs --
Purchased 2008
Service Tag:
Express Service Code:
Product Key - XHJTY-76KKH-BKKK6-RC8DV-PK2MM
Stock except for 12 GB memory (originally 8 GB; in 2014 two 2 GB modules were replaced with two 4 GB Crucial modules)
Power supply: Dell 525 watts (model H525E-00, P/N HP-D5251A001 LF)
Windows 7 Pro

This is my main computer so I can't work without it.

Thanks for any suggestions.
Don C.