Have I got a faulty CPU?

Computer will not start. No sound, no beep, no noise, no fans, no lights.
If I unplug the power plug for the CPU on motherboard I get lights fans and hard drive spinning.
Does this mean the CPU is faulty?

 

Look closely to see if the pins on the cpu got bent. You might as well take it out anyway.

 

Not a new build. Has been working ok.
Do we agree that the CPU is the problem as it will light up, fans come on, and hard drive will spin when I take out the CPU power plug on the motherboard

 

Could you post a make, and model?

There are a multitude of reasons why a PC will not boot, and the CPU being rot is only one of them.

 

No. I don't agree - not yet - not without first ensuring your power supply is working properly. Note fans spinning only indicates the presence of +12VDC. Your PSU must also provide +5VDC and +3.3VDC.

So if me, especially before dinking with fragile devices, I would swap in a known good PSU before thinking about spending money on a replacement.

 

Make and model as requested Medion Pc Mt14. I will try and get spare psu as advised.

Why will the fans, lights,harddrive come on when the CPU power is unplugged on the motherboard yet none of them when the CPU power is plugged in?

 

As soon as you press the button, one of the first things the BIOS does is look for the presence of the CPU. If no CPU, the boot process halts immediately. So typically when there is a fault, you might see the fans spin a half turn or so, and that's it. In your case, when the CPU power connected, there may be a serious fault with the CPU or related circuits and perhaps shorting any power straight to ground before hitting the fans - or signaling (by drawing too much current) the PSU to shut down. It is impossible to tell without putting the motherboard and CPU in a test rig - and probably only the motherboard maker has a test rig for that board.

Testing PSUs with a multimeter is not advisable. To test any power supply properly (computer PSU, battery, car engine, etc.), it must be tested while under a variety of realistic loads! This requires measuring the voltages on all the pins while the PSU is connected to the motherboard and the computer powered on. That requires poking (with some considerable force) two hard and sharp, highly conductive meter probes into the main power connector, deep in the heart of the computer. One tiny slip can destroy the motherboard, and everything plugged into it. And of course, digging around inside exposes the many ESD sensitive devices to highly destructive ESD potentials too.

To properly and conclusively test a power supply unit, it must be tested under various realistic "loads" then analyzed for excessive ripple and other anomalies. This is done by a qualified technician using an oscilloscope or power analyzer - sophisticated (and expensive) electronic test equipment requiring special training to operate, and a basic knowledge of electronics theory to understand the results. And besides the proper loading issue, most multimeters are not designed or capable of measuring ripple. Therefore, conclusively testing a power supply is done in properly equipped electronic repair facilities.

So I reiterate what I said above, "I would swap in a known good PSU".

 

The exisiting PSU is a 350W. I installed a 300W PSU and the results were the same.
While I had the existing 350W PSU out I installed this in an older system and it worked ok.

This should rule out the PSU!

What next

 

First of all thank you for your replies. It turns out that this computer( a friend's) is still under guarantee. Will post again if I discover what caused the failed boot up.