Oil Pressure gauge stuck at maximum, coolant gauge stuck at maximum, speedometer erratic (either not working or working after startup)

Replace the instrument cluster . GM had stepper motor issues 2003 to 2006 . Pin out won't help you , The info for the instrument cluster gauges come's over class 2 serial data buss (network) from the PCM - engine computer . Do you know what GM class 2 data network is ? All gauge info comes over one wire from the PCM .
How to repair GM Instrument Cluster Speedometer Gauge Stepper
Buick Class 2 Operation Diagnosis 1998

Hooking up a Tech 2 scan tool an checking for code's an data to the instrument cluster would be the correct diagnostic procedure . But you wouldn't know that , not being a Chevy technician . Not having factory service repair info. manual .

Speedometer
The IPC displays the vehicle speed based on the information from the PCM. The PCM calculates the vehicle speed from the vehicle speed sensor (VSS) circuit. The IPC receives a class 2 message from the PCM indicating the vehicle speed. The PCM also calculates the vehicle speed based on the wheel reference vehicle speed class 2 message from the EBCM. The PCM uses the vehicle speed information from the EBCM as a backup. The IPC displays the vehicle speed information from the EBCM when the PCM detects a malfunction in the vehicle speed sensor circuit.
The speedometer defaults to 0 km/h (0 mph) if the IPC detects a loss of class 2 communications with both the PCM and the EBCM.

Engine Coolant Temperature (ECT) Gage
The IPC displays the engine coolant temperature on the analog ECT gage as determined by the PCM. The PCM monitors the signal circuit of the engine coolant temperature sensor. The IPC receives a class 2 message from the PCM indicating the engine coolant temperature. The IPC illuminates the check gages indicator when a high engine coolant temperature condition exists. The engine coolant temperature gage defaults to 75°C (160°F) or below when one of the following conditions occur:
• The PCM detects a malfunction in the engine coolant temperature sensor signal circuit.
• The IPC detects a loss of class 2 communication with the PCM.
For additional informatin on the engine cooling system, refer to Cooling System Description and Operation in Engine Cooling.
Engine Oil Pressure (EOP) Gage
The IPC displays the engine oil pressure on the analog EOP gage as determined by the PCM. The PCM monitors the signal circuit of the engine oil pressure sensor. When the engine oil pressure sensor resistance is high, the engine oil pressure is high. When the engine oil pressure sensor resistance is low, the engine oil pressure is low. The IPC receives a class 2 message from the PCM indicating the engine oil pressure. The IPC illuminates the check gages indicator when a low oil pressure condition exists. The engine oil pressure gage defaults to 0 kPa (0 psi) when the following condition occurs:
• The PCM detects a malfunction in the engine oil pressure sensor signal circuit.
• The IPC detects a loss of class 2 communication with the PCM.
Fuel Gage
The IPC displays the fuel level on the analog fuel gage as determined by the PCM. The PCM monitors the signal circuit of the fuel level sensor. When the fuel level sensor resistance is high, the fuel gage indicates full. When the fuel level sensor resistance is low, the fuel gage indicates empty. The IPC receives a class 2 message from the PCM indicating the filtered fuel level percent. The PCM uses 2 filters for the fuel level percent. The slow filter is used during normal operation. The fast filter is used when the engine is just started or when the vehicle is in Park or Neutral. The fast filter allows the fuel gage to reach the current fuel level quickly. The fuel gage defaults to empty when the following conditions occur:
• The PCM detects a malfunction in the fuel level sensor circuit. Refer to Diagnostic System Check - Instrument Cluster .
• The IPC detects a loss of class 2 communications with the PCM.
Reviewing the fuel system description will further explain how the system functions. Refer to:
• Fuel System Description in Engine Controls - 4.3L
• Fuel System Description in Engine Controls - 4.8L, 5.3L, and 6.0L.

GM used a bunch of "stepper motors" in the dash cluster in this and many other vehicles in this time that were prone to failure. Speedo, Oil pressure, Temperature, Gas gauge - you name it - they were all the same stepper motor.

You can replace them yourself for short money - but you need soldering tools and skills. It's a great time to replace dash lamps, too. Check out eBay for parts.
Escalade Speedometer eBay

And this video to learn how to do the work:

Chevrolet GM Speedometer Gauge Repair Removal Instrument Cluster

Your oil pressure gauge works with the oil pressure sensor, which is screwed into the block, to transmit a reading. A faulty oil pressure sensor can cause the reading on the gauge to be erratic. You may have a bad oil pressure sensor. You should also inspect the wires going to the sender to make sure they are not loose, corroded, or shorted. If a new sending unit doesn't work, you can check the oil pressure with a manual pressure gauge. Testing for oil pressure involves fitting the oil pressure gauge with an adapter. Then you will need to determine if you have a gauge problem or an actual oil pressure problem.

You'll want to remove the electrical oil pressure gauge and check the oil pressure with a manual gauge to verify low oil pressure. If the oil pressure tests good with a manual gauge, then you either have a faulty oil pressure sender, or gauge in the dash (I rarely see faulty gauges). If the pressure is still low, suspect worn crankshaft bearings. There is also a possiblity that the screen to the oil pump is clogged. This can be visually checked by removing the oil pan and cleaned.

Most likely your Instrument Cluster contains Electronic "Stepper Motors" on all Needle Indicators. If this is the case, you should consult the Factory Website to detrermine if this is the case. I have replaced many in my 2004 Chevrolet Silverado which has 6 Stepper Motors. These typically exhibit erratic indications prior to total failure. The first to go was the Speedometer followed by Oil Pressure and eventually Temperature, Voltmeter and Tachometer. Obviously your vehicle is no longer covered under warranty. I strongly suggest that you find a Speedometer repair facility and have it checked professionally.

FastestRobe

It might not be anything wrong with your fuel sending unit. Keep an eye on your other gauges. I own a 2004 Pontiac Bonneville GXP. All of my gauges have started acting erratically. It started with my Temp. gauge, moved on to my speedometer, and now my oil pressure , fuel level and batty volt gauges are affected too. Its a very common problem with GM vehicles.

HI, the oil pressure sending unit may be malfunctioning at this point. this will cause erratic readings. if your oil level is proper and there are no apparent oil leaks, you will need to replace the oil pressure sending unit. this will fix the erratic oil pressure readings.

Concerning the gas gauge. this device also has a sending unit that sends the correct fuel level reading to the gas gauge via wire transfer. this sending unit is located and attached to the fuel pump assembly inside the gas tank. replacing this device will fix this issue as well.

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get the original company service and repair manual from the given link it will help you to fix all the facing problem along with diagram

Are you referring to the oil pressure gauge on the dash? If so, that is most likely the oil pressure sending unit having gone bad. The sensor runs about $40 - 50 (at least the last time I had to replace one on that engine). They are not terribly difficult to replace and I believe its a three wire sensor located back side of the engine, on the passenger side area. Its in a kind of tight spot and takes a bit to get it completely screwed out, but it can be done.

When they go bad, they typically will cause the gauge on the dash to read maximum oil pressure. You should be okay to run like that until you can get a new sensor as long as it is reading maximum oil pressure on the gauge and not low or no oil pressure on the gauge.

for the oil pressure guage issue you need to change the oil pressure sending unit.