Intermittent oil pressure gauge operation on 2003 Trailblazer 4.2

Two times now, like every other start oil pressure gauge is 0. The engine runs fine, oil level is fine, thinking of replacing pressure switch, ie sending unit after checking connections of course

. Was curious this is a simulated pressure gauge as I read on some other posts?

Supposedly the gauge unlike the stepper motor I need to replace for the speedometer is actually voltage controlled by a circuit board in the cluster.

Any knowledge and advice is appreciated in advance thanks!

Are you certain the oil pressure is ok? If you have any doubt it would probably be a good idea as a first course of action to verify that measurement using a shop gauge right at the sensor port.

My best understanding is if there is no oil pressure your engine will sound it’s discontent, a check engine light will come on and die a painful death, being as how every other start it seems the oil pressure is ok and the lack of cel or engine noise I am assuming the pressure is good.

You are probably correct.

The PCM monitors the oil pressure switch and transmits a “value” to the cluster to be displayed.

If you have a scan tool that shows data you can view the oil pressure switch input when the problem occurs, this may indicate if it is the switch or gauge that is failing.

You might be able to measure the resistance right at the oil pressure sensor terminals, at least see if it is changing in a believable way as the engine goes from stopped to running. You have to disconnect the connector first. That assumes your oil pressure sensor works as I imagine, variable resistance strain gauges bonded to the non-oily side of the diaphragm in the sensor. The posts above refer to it as a switch, that would imply it is for a dummy light, but I’m assuming b/c you have a gauge, and it’s the gauge reading that is causing you doubts, the gadget in question, it’s actually a sensor.

This u-tube vdo might be of interest.

If its a strain gauge, it won’t work disconnected. It needs a bias voltage to function.

Pretty sure the TB is a sender not switch. The gauge fluctuates indicating an analog function not binary.

Check out the replacement shortcut described on trailvoy. Much simpler than manual describes…

Does that video show 265k on dash? Pretty good for a high tech 4.2 tb motor.

From what I can see the 4.2L engine uses a 2-pin (tan/black & purple wires) oil pressure switch, and the 5.3L uses a 3-pin (grey, black, and tan/white wires) oil pressure sensor.

So at this point I’m not sure how the pcm figures out for the 4.2 L engine what the analog oil pressure is to display for the gauge. OP, just curious, how many pins are on your oil pressure switch/sender?

Switches can also be one pin. And senders that are resistive can be two. Number of pins is not indicative of function in this case. Parts websites are often confusing or wrong on what parts fit what applications

@GeorgeSanJose I have not looked at it but all I have read it is the 2 wire for 4.2 6 (mine) and 3 wire for 5.3 v8.
Stolen From Trailvoy

“Like you said, ours is just a 12 PSI switch (which is what the link points to) - it’s the PCM that keeps the gauge around 40 PSI and makes the needle arbitrarily move with RPMs. It’s really just a dummy gauge.”

Which makes me think it might be the gauge, I am going to start tomorrow with the easiest and not too expense first parts thrown at it, the switch.

No reading on the gauge for the trip home, just had to go out and it is working again. Maybe it just does not like the cold. 165k and regular oil changes for the life of the car. I also checked at the dealer and the oil gauge and components are not subject to any recall.

@Barkydog Your cluster uses a stepper motor for ever single gauge in the cluster

GM is well known for having gauge stepper motor problems. When acting up, they either get stuck at one end or the other, and sometimes they shake

There are companies out there that will take your cluster and replace all the motors with updated parts. The good news is that no reprogramming will be required. The bad news is you’ll be without a cluster while waiting for the repairs to be completed

I believe there are even youtube videos showing how to replace the motor yourself, if you’re brave enough

For example, this company repairs your cluster

Yes @db4960 Stepper motors are certainly involved in the speedometer and tach, but have found various opinions about the oil pressure gauge. I am confident enough to replace stepper motors, got the gps for speed, and tach whatever if it works or not, we will see what happens after pressure switch.

"Like you said, ours is just a 12 PSI switch (which is what the link points to) - it's the PCM that keeps the gauge around 40 PSI and makes the needle arbitrarily move with RPMs. It's really just a dummy gauge."

Wow, a dummy gauge with numbers and hash marks…seems somewhat deceitful to me. Thinking back, my daughter’s Cavalier’s temp gauge acted “normally” at idle, rising to 2/3 or so, then the fan came on and then back down to 1/2. My more recent Hondas’ temp gauges all just went to 1/2 and stayed there no matter what. I’m thinking they were dummy gauges, too. At least they didn’t have numbers on them.

There’s a component behind the instrument cluster called LOGIC.

Or in other words, a computer.


@tester would the fail of an oil pressure switch not cause a check gauges engine light? Trying to piece it all together, Thanks.

The oil pressure switch isn’t an emissions control device.

So it won’t cause the Check Engine light to come on.


@tester I would guess an aftermarket real oil pressure gauge would be the only way to monitor oil pressure? Though I might refer to video posted previously where oil pressure drops and check guages light comes on.

Posted by @GeorgeSanJose


I would also replace the pressure switch. It’s cheap and easy to do.

But if that doesn’t fix it, then you may be looking at a new instrument cluster.

Because one of the functions of the instrument cluster is, to turn on the Check Gauges light if any of the gauges go out of range.

That’s where the logic comes in.


@tester I am glad we are on the same page, though logic does not mean logical, I think a real oil pressure gauge would be logical…