This isn’t as common as one might expect. It’s taken me some time to research the cause. 
The ‘Exclamation’ is an indication to the detection of an onboard fault. This fault can usually be resolved by hard resetting the unit. Unplugging for a period of several minutes and then switching it on. 

If it still persists, you’d be require to contact Dyson directly. I suspect this is as they have access to more in-depth diagnostic, than available via their website. 

This resource may help guide you. HP04 Troubleshooting.

David.

Thanks David.  I tried all the remedies.  It may need service.

Hi @rschilke 

@DavidTaylor is right. The ‘Exclamation’ is an indication to the detection of a fault. 

'Power Cycling/Restarting' your air treatment machine usually resolves this, as it resets the onboard PCB (Power Control Board). 

Failing the above, Reverting your air treatment machine back to it's factory settings may also resolve the issue. If you haven’t already tried this, I’d recommend giving it a go!

Our How to: Book a repair with Dyson article can help should you need to contact Dyson. 

Thanks, 

Stuart

Can we just take a moment to celebrate how hard @Stuart works! Forever sharing his knowledge and bringing attention to bespoke workaround! 

Kudos to you! Keep up the hard work !! 

I am having the exclamation point problem. I’ve turned my machine on and off several times, tried all the advice here, and still the exclamation point.

I called customer service and they told me to take the machine in to a service centre in Calgary - I live in Vancouver! The call centre did not give me any other options and I am very disappointed. I saved up to buy my Dyson and I expected a longer life than three years and much better customer service.