I spent 3 years working for a suspension company that specialized in air suspension replacement systems. The last year of that was editing the website, and doing product research for the R&D department. The GM based systems are relatively simple. The only ones with an actual computer control are the later model
Cadillac systems, and the Buick Lucerne. This cuts down on diagnosis complexity.
The air systems on GM vehicles are similar to the air systems sold by companies like Air Lift and Firestone. The difference is, GM uses air shocks, similar to units sold by Gabriel (the "Highjacker") and Monroe ("Load Leveling") and coil springs that are a 'softer' rate than they would be without the air shocks, instead of air bladders that either fit inside of the coil springs or mount on top of the leaf springs.
Both the GM and the Air lift/Firestone systems use a compressor with a vent solenoid (to let air out when the system is sitting higher than it should), and a height sensor to turn the air compressor (or vent solenoid) on and off.
Diagnosis is fairly simple.
First; How many miles does the vehicle have on it? And have the rear shocks ever been serviced since it was new? Service life for most shocks is officially about 50,000 miles, as per manufacturer, either air or "standard." The air bladders on air shocks are generally made of rubber (or synthetic rubber). Exposure to; the atmosphere (air), heat, road salt, dirt, etc., causes the rubber to deteriorate. Add to that being inflated and deflated, stretching from the shocks moving up and down, causing them to stretch (think of squeezing on an inflated balloon) causes wear and tear on the rubber. If the shocks are over 10 years old, and have never been replaced, and you're having air system problems, they probably (95 times out of 100) have pinhole leaks. Most of the time, those leaks are next to impossible to see without actually removing the Shock, and fully extending it. The leaks tend to be in the "folds" of the air bladder, unless the shocks have been damaged.
When these leaks start, the air compressor and the height sensor, are fooled into thinking that either the air system has been deflated, or a heavy load has been placed in the rear of the vehicle. The compressor is turned on, and tries to bring the vehicle back to the proper height. The compressor will stay on until either it brings the vehicle back to the proper height, or the circuit breaker overheats, and cuts the power to the compressor, in an effort to save it. Most people never notice this with a GM vehicle, until they hear the compressor running with the car turned off, because unlike say, a Ford/
Lincoln/Mercury, most of the weight is still held up by the coil springs in the rear. Most 'older people' who are the first owners of these vehicles rarely drive them in situations where they would notice the poor handling from the loss of spring rate, so by the time the damage is noticed, the entire system is ruined.
Technically, if you changed the shocks before the problem started, or as soon as it happened, then you probably would have problems with the compressor. This rarely, if ever, happens. Almost everyone who has problems with the air system, doesn't know until either the compressor is running "every five minutes" or has died completely, and the rear of the car sags or "bottoms out" every time you put someone in the back seat, or a load in the trunk.
Once in a blue moon, the height sensors or an air line become damaged from debris flying up and hitting them, or they wear out, but this is almost never the case.
So, by the time you KNOW you have a problem, it's usually a full blown problem. That's why I (and every company that sells replacement parts, not just the one I worked for) recommend replacing the system as a whole.
If you think that is more expensive than doing things 'piecemeal' then consider that if you just throw a compressor on the vehicle, and don't replace the shocks, then that compressor will burn out in a short period of time. If it's a new or remanufactured compressor, then if you don't replace the air shocks when you put it on, you've just voided the warranty (if it had one), so you have to BUY ANOTHER COMPRESSOR when you buy those air shocks. That means more money.
If you replace the shocks without replacing the compressor, then if the compressor is weak and damaged and has to work against those new air bladders, then it will fail shortly after you replace them. This means you have to buy the compressor anyways, and if you have a shop doing the work, that means TWO LABOR BILLS.
So, if you actually want to 'save money' and not have problems, you have two choices, replace the components in the system with new parts, or 'convert' the system. Each of them have their own benefits and limitations.
Replacing the air shocks and the compressor maintains the load leveling capacity of the rear suspension. If you're someone who wants to 'keep their vehicle stock' then this is the way to go.
I recommend KYB replacement air shocks (all of the air suspension companies that have 'cheap kits' use Chinese shocks they get for about $5 each), and a new or rebuilt compressor (most of the 'new' compressors on the market are Chinese, no matter who you buy them from).
If you aren't worried about "staying stock," then consider a 'conversion kit.' Not all 'kits' are the same. The first 'kits' that came out were just shocks with coil springs on them, instead of air bladders. These shocks will 'hold the car up,' but they don't have the proper spring rate, and tend to 'sit high' and make alignment of the rear suspension next to impossible.
The company I worked for actually did "R&D work" and developed a proper kit. The kit they developed actually replaced the air system, and made it work like a 'normal' suspension, by replacing the 'soft' coil springs, with what would have been the 'proper rate' springs if it never had air in the first place. They replace the air shocks with 'conventional' shocks. That way, the suspension sits at the proper height, and has the same load capacity as stock.
Most people think the 'ride quality' is affected by the air system, but that really isn't the case. Although 'air springs' (the air bladders) do have less 'rebound' (bounce) than coil springs, they only affect about 30% of the spring rate on this system. 95 out of 100 people in a 'blind' (meaning they aren't told which is which) test drive/ride can not tell the difference between either.
The choice comes down to if you want to 'stay stock,' and have to replace parts every few years, and 'converting,' and having to replace the shocks when they're supposed to be replaced if you want it to ride smoothly. Some shops will only replace the factory parts, some will strongly recommend the conversion (if they even know about it), and if you do your own work, you'll need an alignment from a shop after you get done with either setup.
It isn't necessarily what is 'cheapest,' but what you prefer. When it comes to suspensions, I'm always more concerned with safety, and driveablilty. Is it really worth saving a couple of bucks if you have an accident due to faulty components?
