Why Is My Air Compressor Not Building Pressure?

When a compressor can’t build pressure, it will usually shut itself down almost straight away.

That can make it seem like the machine itself has failed. In reality, the issue is more often downstream – the compressed air is escaping or being used faster than the system can supply it.

Generally, there are two main reasons for a loss of pressure. 

Either the compressor has a mechanical fault and cannot produce air (a supply-side issue), or your facility is consuming air faster than the machine can generate it (a demand-side issue).

Control Gear is an Atlas Copco Premier Distributor. 

With over 50 years of engineering experience across South Wales and the West, we help businesses identify whether they have a faulty machine or a system-wide inefficiency.

Is It the Machine or the Factory?

Before calling for a repair, it’s worth checking whether the fault is actually inside the compressor at all.

You can do this with a simple check called an isolation test.

This involves closing the valve located between the compressor and your air receiver or factory pipework. When the factory stops getting air, you turn on the compressor and keep an eye on the pressure gauge on the machine.

The machine is probably working right if the compressor quickly builds up pressure and then stops (unloads). This means your pressure loss is caused by issues out in the factory, such as leaks or high demand. 

However, if the compressor cannot build pressure even when it is isolated, the fault is internal to the machine.

The Problem of “Artificial Demand”

If your compressor passed the isolation test, the problem is with your pipes or how you are using them. 

Many people try to compensate by turning the compressor’s pressure up.

In practice, that usually makes the situation worse. This is known as artificial demand.

When air leaks are present, increasing the pressure simply drives more air out through those leaks. That means higher losses – and an even wider gap between what the compressor can supply and what the system is actually using.

The Cost of Air Leaks

Air leaks are silent energy wasters. 

They run continuously – even when nothing is being produced. A small hole in a pipe can seem trivial, but over time the cost builds up far faster than most people expect.

The following table shows the estimated cost of air leaks at a standard 7 bar pressure.

Leak Diameter	Air Loss (l/s)	Estimated Annual Cost
1.6 mm	2.1	£500 – £700
3.0 mm	7.5	£1,800 – £2,200
6.4 mm	29.8	£7,000 – £9,000
9.5 mm	67.0	£15,000 – £18,000

The real fix is to find the leaks and put them right, instead of spending money on a larger compressor.

Ultrasonic detection makes this far easier – it lets you pinpoint exactly where compressed air is escaping, so pressure can be recovered instead of continually lost.

Is Your System Undersized?

Sometimes the issue is simply workload. If the compressor runs constantly and the pressure still drifts down over the course of the day, the system may not be large enough for what’s being asked of it.

What’s happening is straightforward – the compressor is trying to meet demand all the time, but it never quite catches up. 

Adding a larger air receiver won’t solve that. It will only slow the pressure drop for a short while, before the same problem returns. You need to know what flow and pressure are. 

You can be confident you have the correct machine for the job if you know how to read flow (CFM) and pressure (PSI).

Restrictions in Your Pipework

Ideally, the pressure at your tools should be almost the same as the pressure leaving the compressor. If there is a big drop, your pipework might be the cause.

Rust in Steel Pipes

Older galvanised steel pipes are prone to internal rust. 

This rust creates a rough surface and narrows the pipe, acting like a bottleneck. This slows down the air and causes pressure to plummet before it reaches your equipment. 

Upgrading to a modern system like Atlas Copco AIRnet piping solves this by providing a smooth, corrosion-free path for the air.

Blocked Dryers and Filters

Air treatment equipment can also become a blockage. 

When a filter is dirty, air has a hard time getting through. If a refrigerated dryer breaks down and freezes, it can also completely stop the flow of air. The compressor might be fine, but the air can’t get to the factory.

Common Mechanical Faults (Rotary Screw)

If your isolation test showed that the machine itself is at fault, there are a few common components to check. For Atlas Copco GA and GX series machines, these are the usual suspects.

Inlet Valve Issues

The inlet valve acts as the “throat” of the compressor. 

It opens up so that air can get in and be compressed. The machine will work if this valve stays closed, but it won’t take in any air. Because of this, there is no pressure.

Solenoid Valve Failure

A solenoid valve is a small electrical switch that controls the inlet valve. 

This part won’t tell the inlet valve to open if it breaks down due to an electrical problem or because it is clogged with old oil varnish. The controller might say that the machine is “Loaded,” but it is still running.

Minimum Pressure Valve (MPV)

This valve plays a critical role – it holds back a minimum level of pressure inside the compressor so oil can circulate properly.

If the valve sticks open, that pressure is lost. 

Oil flow drops away, and air is sent straight into the system before any pressure can build. The compressor can’t settle into normal operation. If pressure inside the machine then climbs too high, the safety valve opens to protect it.

Blocked Oil Separator

The oil separator cleans the air before it goes out of the machine. 

This filter will stop working if it gets clogged. The compressor stops pumping when it sees high pressure before the filter, even though the pressure in your factory is still low.

Common Mechanical Faults (Piston)

Smaller piston compressors rely on simple mechanics to build pressure – valves opening and closing, seals doing their job, everything working in step. When one part starts to fail, the symptoms show up quickly.

Valve Failure
Inside the cylinder are small metal valves that control airflow. If one cracks or no longer seals, the air never makes it into the tank. It just pulses back and forth. A strong hissing noise from the intake filter is often the giveaway.

Worn Piston Rings
Piston rings gradually thin as hours build up. Once the seal is compromised, compressed air leaks into the crankcase instead of the receiver. The machine may still reach a few bar, but pressure rises very slowly beyond that – if it rises at all.

Check Valve Issues
The check valve’s job is to hold tank pressure back. When it sticks open, the motor has to start against a full tank. That extra resistance is usually enough to make the compressor struggle, stall, or trip out during start-up.

Electrical and Control Issues

Smart controllers are used in modern compressors, such as the Atlas Copco GA series. Sometimes the problem is with the logic, not the mechanics.

Sensor Errors

Over time, sensors can move. The system might only be sitting at 3 bar, yet a faulty pressure sensor can tell the controller it’s already at 7 bar.

The computer thinks the job is done and turns off the compressor. It is always worth checking the digital reading against a standard mechanical gauge.

Cold Start Issues

In the UK, plant rooms can get very cold in winter. 

Atlas Copco compressors have a safety feature that stops them from starting if it is too cold (usually below 2°C). This protects the motor, as cold oil is very thick and hard to turn.

Compliance and Safety

When fixing pressure issues, you must follow UK safety rules. The pressure systems safety regulations 2000 (PSSR) say that safety devices must work properly.

You should never raise the set pressure on a safety valve to prevent it from lifting. If a valve opens before it should, it’s failed and needs to be replaced with a certified component. 

Sticking to your service and maintenance plan helps keep the system compliant – and avoids introducing unnecessary risk.

Conclusion

A compressor that won’t build pressure is a sign something isn’t right – but the fault doesn’t always sit with the compressor itself.

Sometimes it’s a blocked filter. Other times it’s a valve that isn’t moving as it should, or air leaks quietly draining energy and pressure from the system.

Accurate diagnosis prevents the consequences of an undersized compressor and avoids buying equipment you don’t need. From worn parts to old pipework, the issue requires a clear eye to resolve.

Control Gear provides full system audits and leak detection to restore stable pressure. With over 50 years of experience, we can help you get your system back to peak performance.

Restore your system pressure today.

Contact Control Gear for an audit or emergency repair.