Warning light When pressing the 'on' switch, the vacuum doesn't start. The warning light just comes on. I am sure that the socket I am using is OK- could this be the motor at fault?

It sounds like there may be a problem with the vacuum's sensor that detects when the bag or filter is full. Here are some troubleshooting steps you can try to fix the issue:

1. Check the bag and filter: Make sure that the bag and filter are properly installed and not damaged. If the bag or filter is damaged or not properly installed, it can cause the sensor to malfunction.
2. Reset the sensor: Some Nilfisk vacuums have a reset button or switch that can be used to reset the sensor. Check the owner's manual or the vacuum itself for a reset button or switch and try resetting the sensor.
3. Clean the sensor: Dust and debris can sometimes accumulate on the sensor and cause it to malfunction. To clean the sensor, turn off the vacuum and unplug it from the electrical outlet. Locate the sensor and use a small brush or compressed air to carefully clean it.

If these troubleshooting steps do not resolve the issue, it is possible that the sensor itself is damaged or malfunctioning. In this case, it may be necessary to have the sensor repaired or replaced by a professional.

Before you g ponying up dough, check for clogs, debris in the cabin, and filter issues first: https://youtu.be/a8Qt8xEfqLw. This solved my issue for the price of a bent up coat hanger, and tons of gentle fidgeting and patience. No more red triangle !

the vac filters are blocked it will shut off the power untill you clean the filters or replace

First thing I would check are the motor brushes.

I have a Silver Moon S558, when I turn it on the power light comes on and a few secords later the red warning light comes on. What is causing the vacuum not to work? One day I was vacuuming, turned it off and a few minutes later went to turn it on and nothing, but these lights turning on. Is there a fix?

check for clogs, and also check the brush rollers' belt.
Really, i think you should take it to a vacuum shop and have it inspected for problems

It means to check the bag. Your bag is either missing or misaligned and has slipped past the little clear sensor or bag lockout plunger in the corner of the bag chamber at the door.
Remove the bag turn on the vac and push this switch, the vac should start when the switch is depressed. If not the internal switch inside the vac is misaligned or damaged. Please post back to confirm how you make out

I dismantled the top part of the machine from the bottom and there stuck right down in the bowels of the tubes was a piece of card, a dress label! somehow it was only making a noise when the machine was in an upright position. It took a bit of stretching and maneurering but once it was out the problem was solved.

WARNING WARNING Danger danger Will Robinson! The power head "wand" and the hose from the body of the vacuum have shorts in them. While my wife was using this machine.. she started smelling plastic.. seeing whitish smoke and then sparks... The pin connector arc'd through the plastic... Some research on Google found that this has been a large problem for Electrolux, when i tried to call the factory.. robots.... no solution. Then I contacted a distributor and they informed me that it was not really that big of a problem. Only later to say, that they had replaced ALOT of them. My unit was past its warranty period so I just hardwired it. (now the unit is only two pieces, body and hose w/wand.)

There is an electronic circuit board on the motor that may have gone bad and will have to replaced, part # 06836740 and sells for around $75.00, for model S516. But if this a S316, then it could be the speed control mounted on the motor, Part # 04416342 and sells for around $83.00. Both of these models are Cat Naad Dog so you need the model number berfore you order a part.