1994 Oldsmobile Cutlass Ciera S 3.1 V-6 Engine Code "M"
My Digital EGR
I call this subject My Digital EGR because of so much confusion about EGR valves.
I'm writing about the digital EGR that fits my 1994 Olds 3.1 V-6, code "M". 

I went to a new "Automotive How To" site to see how accurate their information is. A good test is to see how they present the EGR, which is a simple little gadget that that is bolted on to your engine and causes a lot of confusion among do-it-yourselfers.

I inserted the specs of my engine and asked for information about servicing the EGR. A page appeared which explained my EGR and how to test it, remove it, and reinstall it. The problem is -- it is not my Digitally Operated EGR. It was a detailed description of a Vacuum Operated EGR. The page also showed the estimated cost of their pictured Vacuum EGR:

Vacuum Operated EGR.
GM Tri-solenoid Digitally Operated EGR.
Costs for 1994 Oldsmobile Cutlass Ciera 6 Cyl 3.1L
DIY Costs   Est. $408.54 parts only         Shop Costs  Est. $663.90 in parts and labor
The actual cost of the Vacuum EGR (pictured) is about $50.00 to $60.00. A used GM Digital EGR can be bought from about $40.00 and up (used) or from about $250.00 to $500.00 new, depending on where you buy it. My used Digital EGR pictured above cost me $43.00 and is guaranteed. Mine came from a salvage company which had a wrecked 1994 GM vehicle that had the recently replaced Digital EGR that I needed. It doesn't have enough miles on it to be dirty and it is a genuine GM EGR.
Another item to look out for is the pin lettering. The two diagrams below are of the same Digital EGR but from different commercial Web sites. Regardless of how they are lettered on a diagram the positive input pin is the first lone pin. On picture number 1 it is labeled D -- and on picture number 2 it is labeled A.  The only pin that is lettered the same on both diagrams is pin "B". But that first lone pin is the positive voltage input, regardless of what they name it. The other 3 pins are the negative side of each solenoid.
Picture 1.
Picture 2.
Instead of wasting your time with my words and pictures, I'll make a suggestion: If you don't know the OEM part number for your vacuum or digitally operated EGR, and the vehicle is between the early 1970's and 1999 go to Tomco, Inc (a link is below) and see if your vehicle is listed. If so, you will find which original equipment EGR fits your vehicle and what other vehicles use your EGR  
There are several subjects listed on that page. Scroll down to this one: EGR VALVES. There are other EGR titles, so make sure you choose EGR VALVES. A PDF will open that shows the vehicles. A small sample of the many hundreds of vehicles on 82 pages, is below.
I began this study after my Check Engine Light came on. I used my code scanner to find the problem which pointed me to the EGR valve. I went to Google and a search for EGR brought up 4,830,000 listings. Digital EGR brought up 387,000 listings and GM Digital EGR found 835,000 listings. Information is almost as varied as the number of listings.
But another picture a few pages later shows the same solenoid coils receiving voltage from pin D. But this time, with a resistor in parallel with each coil. If the coil is open (bad) the resistor would still show a reading on the ohm meter and unless you know what the exact value should be, you wouldn't know whether the coil is good or bad.

This is the pin lettering I use. 

With a resistor or without a resistor?  I'm not going to open a good EGR to see whether it has a resistor.

Below is one of my EGR's. See the single pin D at the top of the socket? D is the pin that connects to (+) 12 volts.  The other 3 pins are the (-) negative side of each solenoid.

I like the answer one Internet poster gave to an EGR question. He said, "If the engine is running good and you have no complaints, place a piece of tape over the Check Engine Light and drive on."

Of course that is not acceptable if you need to have an emissions test. Also you would not know if the light was warning you of another problem. So, better fix the EGR.

One of the best articles I've found about EGR operation was written by Henry Guzman, ASE Master Tech, L1 Certification with over 20 years experience at the time of writing. His article, written in 1997, covers both vacuum operated and digitally operated EGR's. The link below will take you to his article.

More Digital EGR is on the next page.

The following two pictures are on different pages from one Web site, showing the same digital EGR. 

The top picture shows pin D supplying voltage to solenoids A, B and C. The solenoids are shown as coils. An ohm meter would show an open (bad) coil as zero ohms.