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This laptop had no signs of life and when the charger was plugged in the led was not coming on.
First step was to test the charger to see if it actually works or not, this is usually neglected and it's easy to overlook it but rarely it happens to be the problem and it will save you time if you realise this after you have taken your device apart.
After confirming the charger was fine I hooked the laptop to the Bench Power Supply and noticed that the current was jumping from 0 to 20mA continuously which is not a good sign and immediately I thought it's a faulty startup chip and it will take a long time to get a replacement.
Normally all laptops take between 10-20mA at all times while switched off.
I start to disassemble the laptop and there is a total of 16 screws circled in the picture below.
One is covered by a red plastic and four are covered by the top rubber feet and under the bottom feet there aren't any screws.
HP 15-p246sa Not powering ON
 Pull the rubber feet out and under you will find a screw that holds the black plastic in place which hides another screw underneath.
HP 15-p246sa Not powering ON
HP 15-p246sa Not powering ON
The last screw you see once you pull the DVD Drive out and some other configurations might be three screws there.
HP 15-p246sa Not powering ON
There are no screws under the keyboard because it is a single unit with the palmrest.
Now you can start to pry around the edges to separate the palmrest from the bottom cover.
Once loose lift carefully while disconnecting the trackpad ribbon cable, keyboard ribbon cable and lastly the power button cable. 
The motherboard has four screws and a fifth screw belongs to the wifi card that needs to be unscrewed and wifi card taken out.
Now you can start to disconnect all cables and gently lift the motherboard, the charging port is plugged underneath and is a good idea to leave it plugged because it's easier to hook it up to your power supply for voltage testing.
The charging port has a screw holding down a metal bracket keeping the port secured.
HP 15-p246sa Not powering ON
I started to probe around for voltages and there was 19.5V at the charger connector and voltage present at the first Mosfet.
I went straight away to the startup chip to check for 3.3V and the voltage was jumping from 0 to 0.5v which is unusual.
Most power supplies around the board were showing 0 or 0.5v jumps and at this point I had to start from the charging port connector.
I pulled out a schematic and started to trace the 19.5v from the charging port up to a mosfet marked as PQ20 and there the voltage was stopping, there was 19.5v on the Drain but no output on the Gate.
You can see our PQ20 pictured below.
HP 15-p246sa Not powering ON
By injecting voltage to the gate with a Linear Power Supply I noticed how something on the board started to take around 1.5 amps which was the current limit set on the power supply and at this point something should start to get hot.
Shortly after noticed that PD10 circled below, was getting extremely hot and after checking with the multimeter in diode mode it was indeed shorted. 
I looked it up using the part number from the schematic, always use that and not what it says on the component because often Manufacturers write their own numbers and you cannot identify the component.
The part number of the faulty diode is P4SMAJ20A and I found the exact same diode on a donor board from an HP G6 which has BV written on it but it's exactly the same part.
Below you can see the replacement diode soldered and the faulty one desoldered next to it.
The laptop works as it should now. 
HP 15-p246sa Not powering ON
HP 15-p246sa Not powering ON
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