I have a 2007 Envoy Denali with a sagging/lowered rear stabilizing suspension. Compressor would come on randomly, didnt matter if it was running, parked, on or off, loaded or unloaded. Sometimes lasted a few seconds other times a few minutes. We had the airbags replaced and were told the compressor is fine. I just read this on the ALLDATdiy.com site: Suspension - Rear Suspension Is Low/Sagging/Uneven ENGINEERING INFORMATION Bulletin No.: 06-03-09-006B Date: December 15, 2008 Subject: EI06006 - Rear Suspension Uneven, Sagging, Low (Recalibrate Air Spring Sensor) Models: 2006-2007 Buick Rainier 2006-2007 Chevrolet TrailBlazer SS 2006-2007 GMC Envoy, Envoy XL 2006-2007 Saab 9-7X with Suspension Pkg - Rear Load Leveling (RPO G67) Supercede: This bulletin is being revised to update the labor operation in the warranty information. Please discard Corporate Bulletin Number 06-03-09-006A (Section 03 - Suspension). Condition Some customers may comment on the rear suspension being uneven or sagging. Correction DO NOT REPLACE THE COMPRESSOR. Based on technician feedback, engineering has determined that the customer's concern can be corrected by recalibrating the air spring sensor. Refer to the Trim Height Uneven or Low diagnostics and Air Spring Sensor Calibration procedure found in SI. SO.. my question is. Does this match up? Could it be that it just needs to be calibrated, and if so, how much does that normally cost if it only takes a few moments to do? (I read that part here on fixya.com)

Question edited for clarity.
Question moved to model category.

Here are all your fuses

GMC Envoy (2002-2009) fuses and relays

https://fuse-box.info > gmc > gmc-envoy-2002-2009-f...

Here you will find fuse box diagrams of GMC Envoy 2002, 2003, 2004, 2005, 2006, 2007, 2008 and 2009, get information about the location of the fuse panels ...

There is a level sensor connected to the lower control arm on the rear wheel on the drivers side. This operates the pump and lets air out if it is too high in the back.It can fail. Most likely the air pump has gone bad. Expect 800 + to fix it.

air compressors do not operate below 28 degrees you have to get them warm first I assume that your compressor is under your hood as long as you allow your truck to get cold or keep the air suspension your going to have this problem

I’m happy to assist further over the phone at https://www.6ya.com/expert/brock_d67526d2a834c47e

Would need to know what DTC'S - diagnostic trouble codes are stored in the module for the autoride suspension . Hooking up a professional type scan tool ,looking at input data to the module etc... Looking at a wiring diagram an testing electrical circuit's using a DMM - digital multi-meter . Reading how the system works ,what all involved in making the system work . This is how a professional Tech would go about diagnosing this .

The automatic level control (ALC) system maintains a desired rear suspension position under all types of towing, hauling and loading conditions. The following components are involved in the operation of the ALC system:
• Air line tubing--Pressurized air from the ALC compressor is pumped to each of the rear shocks via air line tubing.
• Air drier--Pressurized air from the compressor is run through a drier containing a moisture absorbing chemical preventing water accumulation in the rear shocks.
• Compressor--Supplies pressurized air to the rear shocks
• Compressor motor relay--The relay supplies battery positive voltage to the ALC compressor motor.
• Pressure sensor--The electronic suspension control module (ESCM) module provides a 5-volt reference and low reference to the ALC pressure sensor. The module receives the signal voltage that is relative to the air pressure applied to the rear shocks.
• ESCM--Controls the ALC system and electronic suspension control (ESC) system and detects failures in both. The module monitors inputs from the position sensors, and pressure sensor to determine when to raise and lower the vehicle to trim height as the vehicle is loaded or unloaded. The module limits pump activation to 255 seconds to prevent thermal damage.
• Exhaust valve--An electric solenoid activated by the ESCM to vent air pressure from the rear shocks.
• Left and right rear suspension position sensors--The module provides a 5-volt reference and low reference to all 4 of the body-to-wheel suspension position sensors. The sensors send the ESCM a signal voltage that is relative to the rear suspension ride height
• Left and right rear shock absorbers with internal air chambers and dampers--Dampens the rear suspension and maintains trim height depending on the air pressure applied.

well I will tell you my problem it maybe the same happening to you, every time I park the car for more than 4 hours it will lower the rear and after I start it will go to normal high, well the car have a air leak on one of the air bags in the rear, the compressor was pumping air almost all the time because of that to the point that one time just stop working. I have to buy new air compressor and 2 air bags. I bought them at www.arnottindustries.com not difficult to replace either.

There has been a service bulletin for that type of concern. Here is what it states.









Suspension - Rear Suspension Is Low/Sagging/Uneven

Notes




ENGINEERING INFORMATION

Bulletin No.: 06-03-09-006B

Date: December 15, 2008
Subject:
EI06006 - Rear Suspension Uneven, Sagging, Low (Recalibrate Air Spring Sensor)

Models:
2006-2007 Buick Rainier
2006-2007 Chevrolet TrailBlazer SS
2006-2007 GMC Envoy, Envoy XL
2006-2007 Saab 9-7X


with Suspension Pkg - Rear Load Leveling (RPO G67)

Supercede:

This bulletin is being revised to update the labor operation in the warranty information. Please discard Corporate Bulletin Number 06-03-09-006A (Section 03 - Suspension).

Condition

Some customers may comment on the rear suspension being uneven or sagging.

Correction

DO NOT REPLACE THE COMPRESSOR.

Based on technician feedback, engineering has determined that the customer's concern can be corrected by recalibrating the air spring sensor. Refer to the Trim Height Uneven or Low diagnostics and Air Spring Sensor Calibration procedure found in SI.

Check your rear suspension. Load-leveling air suspension struts could be bad or leaking. Air suspension compressor could be inoperative. Air suspension compressor relay could be bad. Air lines from compressor to struts could be leaking, from cuts, cracks, or having rubbed through against the frame. Transverse leaf spring could be sagging.
And it's Toronado, not Tornado. Sacrilege!

check for leak at the suspension air bag

Just behind right rear tire up in the wheel well. Air inlet is inside right rear quarter near the accessory inflator. Compressor runs 3 lines one for the accessory inflator and one for each air bag.
THere are also 2 sensors, one on each trailing arm that senses load and then raises and lowers accordingly.

check youer air shocks for leakes and rear hight sensor