Symptom
Vehicle sitting low or unevenly
The car has settled — all round, at one corner, or at one end — most often overnight. On an air-sprung vehicle this is expected behaviour when the system stops topping the springs up, because air springs leak slowly by design, so it usually points at the levelling system rather than at broken suspension. A vehicle sitting on its bump stops should not be driven: there is no suspension travel, the tyres can foul the bodywork, and impacts go straight into the structure.
Also described as: car sitting low, suspension collapsed, air suspension down, sagging, on the bump stops, one corner low, car dropped overnight, ماشین یک طرف پایین است, ارتفاع خودرو کم شده, suspension hundida, coche inclinado, no sube suspension, السيارة واطية من جهة, التعليق هابط, المساعدات نازلة
What this usually means
On an air-sprung vehicle this is expected behaviour when the system stops topping the springs up, because air springs leak slowly by design — so it usually points at the levelling system rather than at broken suspension. The car sinking overnight and rising again when started is the signature, and it means the springs are holding less than they should while the compressor is asleep.
A vehicle sitting on its bump stops should not be driven: there is no suspension travel, the tyres can foul the bodywork, and impacts go straight into the structure.
Where it settles tells you where the leak is. One corner is that spring or its valve. One end is the axle's shared valve block or a common line. All round is more often the compressor, its relay, or a leak at the reservoir — because a system that cannot make pressure loses everywhere at once.
Compressors fail from working too hard rather than from age. A small leak makes the compressor run constantly, it overheats, and then it fails — so a failed compressor usually has a leak behind it, and fitting a compressor alone gets a second failure. Find the leak first; soapy water on the springs and fittings finds most of them.
On a conventionally sprung car the causes are different and mostly obvious: a broken coil spring, a collapsed rear spring on a vehicle that carries loads, or sagging springs with age. A broken spring often makes a knock over bumps as well, and the break is usually within a coil or two of the end.
Check the ride height sensors before condemning anything. The system levels to what those sensors report, so one reading wrongly makes the car sit wrongly with nothing pneumatic at fault.
Working through it
| What you find | Likely cause | How to confirm it | What fixes it |
|---|---|---|---|
| Air suspension, sinks overnight, rises when started | A leak the compressor makes up when running — air springs leak slowly by design and the system tops them up | Spray soapy water on the springs, valve block and fittings and look for bubbles. Note which corner sinks first. | Repair the leak. Do this before replacing a compressor, or the new one fails the same way. |
| One corner low, others normal | That spring or its valve | Compare ride heights and the live sensor values corner to corner. A single corner isolates it. | Renew the air spring or repair its valve. Inspect the bellows for perishing and chafing. |
| All corners low, compressor running long or not at all | The compressor, its relay, or a leak large enough that it cannot keep up | Listen for the compressor and check its supply. A compressor that runs continuously has a leak behind it. | Repair the leak first, then renew the compressor and its drier if it has overheated. |
| Sits wrongly but the springs hold pressure | A ride height sensor reading incorrectly — the system levels to what it is told | Compare live sensor values against measured heights. A sensor on a thin link in the road salt is a common failure. | Renew the sensor and run the calibration. No pneumatic work needed. |
| Conventional springs, one corner low with a knock over bumps | A broken coil spring — usually within a coil or two of the end | Inspect each spring fully, rotating the wheel to see the hidden side. Breaks are easy to miss from one angle. | Renew springs in axle pairs. A broken spring can also cut a tyre, so inspect that too. |
| Rear sits low on a vehicle that carries loads | Sagged rear springs, or self-levelling struts that have lost their charge | Measure heights unladen against specification. Establish whether the vehicle has self-levelling at the rear. | Renew the springs or the self-levelling units. Fitting standard struts to a self-levelling car changes its behaviour. |
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Air suspension, sinks overnight, rises when started
- Likely cause
- A leak the compressor makes up when running — air springs leak slowly by design and the system tops them up
- How to confirm it
- Spray soapy water on the springs, valve block and fittings and look for bubbles. Note which corner sinks first.
- What fixes it
- Repair the leak. Do this before replacing a compressor, or the new one fails the same way.
-
One corner low, others normal
- Likely cause
- That spring or its valve
- How to confirm it
- Compare ride heights and the live sensor values corner to corner. A single corner isolates it.
- What fixes it
- Renew the air spring or repair its valve. Inspect the bellows for perishing and chafing.
-
All corners low, compressor running long or not at all
- Likely cause
- The compressor, its relay, or a leak large enough that it cannot keep up
- How to confirm it
- Listen for the compressor and check its supply. A compressor that runs continuously has a leak behind it.
- What fixes it
- Repair the leak first, then renew the compressor and its drier if it has overheated.
-
Sits wrongly but the springs hold pressure
- Likely cause
- A ride height sensor reading incorrectly — the system levels to what it is told
- How to confirm it
- Compare live sensor values against measured heights. A sensor on a thin link in the road salt is a common failure.
- What fixes it
- Renew the sensor and run the calibration. No pneumatic work needed.
-
Conventional springs, one corner low with a knock over bumps
- Likely cause
- A broken coil spring — usually within a coil or two of the end
- How to confirm it
- Inspect each spring fully, rotating the wheel to see the hidden side. Breaks are easy to miss from one angle.
- What fixes it
- Renew springs in axle pairs. A broken spring can also cut a tyre, so inspect that too.
-
Rear sits low on a vehicle that carries loads
- Likely cause
- Sagged rear springs, or self-levelling struts that have lost their charge
- How to confirm it
- Measure heights unladen against specification. Establish whether the vehicle has self-levelling at the rear.
- What fixes it
- Renew the springs or the self-levelling units. Fitting standard struts to a self-levelling car changes its behaviour.
Codes that produce this symptom (6)
Listed with the codes that most typically present this way first — the likeliest place to start.
- U0132 Lost Communication With Suspension Control Module A Typical of this code Moderate
- U0139 Lost Communication With Suspension Control Module B Typical of this code Moderate
- U0321 Software Incompatibility With Suspension Control Module A Typical of this code Moderate
- U0421 Invalid Data Received From Suspension Control Module A Typical of this code Moderate
- U0133 Lost Communication With Active Roll Control Module Moderate
- U0434 Invalid Data Received From Active Roll Control Module Moderate
Information, not instruction
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Have anything you intend to act on checked by a trained person against the manufacturer's own procedure for your vehicle. Nothing here replaces that procedure.
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