How do you test high pressure oil system on 2004 6.0 liter

No damage will occur. Have you checked the oil pressure with a manual test gage? If not do that and please understand GM oil pressure gages are prone to reading high pressure when the sender is defective.

High oil pressure usually isn't a problem.

Flush your oil system, and refill with new oil and filter

you might need new glow plugs

Oil pressure always changes with rpm.
At idle you should have around 10 psi, and it should max out at high rpm around 50 psi or so.
You not only can't have as much pressure at idle, but you don't need it either.

160....O2 Sensor Circuit No Activity Detected (Bank 2 Sensor 2)

P0161....O2 Sensor Heater Circuit (Bank 2 Sensor 2)

P0162....O2 Sensor Circuit Malfunction (Bank 2 Sensor 3)

P0163....O2 Sensor Circuit Low Voltage (Bank 2 Sensor 3)

P0164....O2 Sensor Circuit High Voltage (Bank 2 Sensor 3)

P0165....O2 Sensor Circuit Slow Response (Bank 2 Sensor 3)

P0166....O2 Sensor Circuit No Activity Detected (Bank 2 Sensor 3)

P0167....O2 Sensor Heater Circuit (Bank 2 Sensor 3)

P0168....Fuel Temperature Too High

P0169....Incorrect Fuel Composition

P0170....Fuel Trim (Bank 1)

P0171....System too Lean (Bank 1)

P0172....System too Rich (Bank 1)

P0173....Fuel Trim Malfunction (Bank 2)

P0174....System too Lean (Bank 2)

P0175....System too Rich (Bank 2)

P0176....Fuel Composition Sensor Circuit

P0177....Fuel Composition Sensor Circuit Range/Performance

P0178....Fuel Composition Sensor Circuit Low Input

P0179....Fuel Composition Sensor Circuit High Input

P0180....Fuel Temperature Sensor A Circuit

P0181....Fuel Temperature Sensor A Circuit Range/Performance

P0182....Fuel Temperature Sensor A Circuit Low Input

P0183....Fuel Temperature Sensor A Circuit High Input

P0184....Fuel Temperature Sensor A Circuit Intermittent

P0185....Fuel Temperature Sensor B Circuit

P0186....Fuel Temperature Sensor B Circuit Range/Performance

P0187....Fuel Temperature Sensor B Circuit Low Input

P0188....Fuel Temperature Sensor B Circuit High Input

P0189....Fuel Temperature Sensor B Circuit Intermittent

P0190....Fuel Rail Pressure Sensor Circuit

P0191....Fuel Rail Pressure Sensor Circuit Range/Performance

P0192....Fuel Rail Pressure Sensor Circuit Low In put

P0193....Fuel Rail Pressure Sensor Circuit High Input

P0194....Fuel Rail Pressure Sensor Circuit Intermittent

P0195....Engine Oil Temperature Sensor

P0196....Engine Oil Temperature Sensor Range/Performance

the oil pressure switch on a 6 liter diesel is right behind the oil filter housing
it has a 1 wire switch the oter switch close to it is the oil temp switch

The pressure to be most concerned with is the reading in gear at idle with engine warmed up...Low end really should not be below 20. If pressure wanders, change the sending unit and check actual pressure with a mechanical gauge if necessary. The age and quality of your oil, as well as temperature all play a part in pressure, as well as internal wear from time & miles of use. If pressure changes with rpm, that is normal. 40psi is good while driving.
need more info just ask.

this is an oil pressure problem, either a sensor is not sensing enough oil pressure, or the high pressure pump is not building enough pressure(possibly leaking or head is leaking). this engine requires 450 psi minimum oil pressure at the injectors to actuate the injectors.

yes they are you stated that you have mech oil pressure did you install a gauge?. this is a fuel line pressure sensor on the right bank at the frt of the head this sensor is rail pressure that high pressure oil pump creates check you pressure there that will elimnate that head the other side is a little more of a pain to test. one other thing to keep in mind is that the cranking speed is very important, and have you checked the oil level on a 7.3 deisel if you are more than 3 qaurts low on oil you will trap air in the high pressure pump. this will cause a hard start our even a no start

Could be one of many things:

-Clogged fuel filter
-Leaking fuel line
-Bad fuel pump
-High or low oil level
-Overdue oil change (Will explain both these possible causes later)
-Bad high pressure oil pump, ICP sensor (injection control pressure sensor), ICR (injection control pressure regulator), or bad HEUI injectors, or oil lines
-Leak in the high pressure oil system (unlikely because you would have noticed this... This does not go undetected as the underside of your vehicle would be literally covered in oil and dripping in a very short period of time after the leak develops)
(Powerstroke engines use a HEUI fuel injection system, "Hydraulically actuated/electronically controlled unit injection", which relies on a high pressure hydraulic pump that uses engine crankcase oil and pumps it at high pressure (500 psi or so). This oil is sent to your fuel injectors to provide means for actuating them and pressurizing the fuel into the cylinders. High or low engine oil level, or old, diluted, additive depleted oil will cause this system to aerate the oil and result in poor fuel injection capabilities)

There might be other causes but this is the Powerstroke`s weak point... The HEUI system was first designed by Caterpillar, which makes a lot of excellent/bulletproof HEUI based engines, both in passenger vehicle and heavy trucks and equipment. The Powerstroke is an engine designed and made by International Truck and Engine, which cannot seem to make a HEUI engine, in any market, whether passenger, heavy truck, or equipment that does not suffer from fuel injection issues.