Problem booting Rpi5 - green LED flash pattern

I just took delivery of an RPi5, case and power supply from Pimoroni today. Fitted the Pi in the case and powered it on, but it doesn’t seem to want to boot. Nothing appears on the HDMI output, and the green LED flashes repeatedly in a 4 slow - 5 quick cycle. Fan runs normally.

Full LED sequence when power is applied:

STEADY RED - for 1 sec
STEADY GREEN - for 5 secs
FLASHING GREEN - 4 slow, 5 fast flashes, pause for 2 secs, then repeat

The SD Card is a brand new SanDisk Extreme Pro, which I flashed using the Raspberry Pi Imager v1.8.4. I initially chose the Raspberry Pi OS (64 bit, recommended). When that gave the above result, Id downloaded the tarballs for Bookworm (arm64 and armhf), checked the SHA256 hashes (all good), then extracted and flashed each in turn, using the “Use custom” option on the imager. In all cases, the image was verified successfully, so I assume the SD card is fine. Th result on powering up was exactly the same.

Can anyone suggest what the problem might be? I’ve not seen any info on the net about the flashing LED pattern and what it might mean. Thanks in advance!

Remove the SD Card and boot up, you should get a diagnostic / info text screen.
I would also try your monitor on the Pi’s other video output jack. Making sure its actually plugging in all the way.

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Might be worth trying to boot without an SD card, to see if the behaviour changes.

It’s also worth trying a different SD card; just because it verifies on your machine doesn’t (inexplicably) guarantee that the Pi will play nicely with it, from rather frustrating experience…

That said, I’m sure I saw on the official forums someone with exactly the same pattern, and the green name there pretty much said “dead board” so it might be an RMA job. Of course, I can’t find that thread any more :-(

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Good suggestions @ahnlak and @alphanumeric, I’ll try with a new SD card and with another HDMI cable.

Tried with a new SD card, a different HDMI cable, and both HDMI outputs. No luck. have logged a support ticket asking for returns procedure, but no response after a week! Can any one tell me if this lack of response is to be expected?