About Us    Contact Us

Harbor Freight Air Compressor Won’t Start

Published Categorized as Air Compressor Won't Start, Harbor Freight No Comments on Harbor Freight Air Compressor Won’t Start

There have been many folks who wish to inquire about why their Harbor Freight air compressor is not turning on and other questions along those same lines. This article will present some of the reasons why your Harbor Freight compressor won’t start and provide troubleshooting suggestions to resolve those issues, along with existing reader problems.

Table of Contents

Reasons Why Your Harbor Freight Air Compressor Won’t Start

The main reasons as to why your Harbor freight air compressor hums but won’t start are likely to be due to any of the following reasons summarized in the table below:

Power issueAvoid using power bars and power strips.
Faulty pressure switchCheck the unloader valve; if it is faulty, the pressure switch may not work.
Fuse or breaker blowsCircuit breakers will trip off when a power surge blows their fuses. If the fuses are faulty, you will need fuse replacements.
Faulty unloader valveCheck the oil level and inspect the pump area for any potential leaks.
Faulty thermal overload switchCheck the thermal overload switch to ensure it is working correctly.
Insufficient oilCheck the oil level, and inspect the pump area for any potential leaks.
Reasons Why Your Harbor Freight Air Compressor Won’t Start Table

Please visit our Air Compressor Won’t Start – Ways to Fix Air Compressor Won’t Turn On Problems guide for more detailed Harbor Freight air compressor troubleshooting & fixes!

Additional Troubleshooting Pages

Air Compressor Types Troubleshooting

Air Compressor Brand-Specific Troubleshooting

Air Compressor Brand/Problem-Specific Troubleshooting

Air Compressor Part Specific Troubleshooting

Reader Questions & Responses

Harbor Freight Air Compressor Won’t Start – Harbor Freight Air Compressor Troubleshooting


Problem with a Harbor Freight 21 gal. 2.5 HP 125 PSI Cast Iron Vertical Air Compressor.

The unit works great except the compressor doesn’t always start and keep running. This condition is inconsistent.

When turning ON using the switch, it will sound bogged down and attempt to turn over slowly and then blow the thermal breaker on the compressor. If I keep moving the switch from start to stop (to prevent popping the breaker) sometimes it will start and run fine.

Harbor Freight 21 gal. 2.5 HP air compressor
Harbor Freight 21 gal. 2.5 HP air compressor

My question – Could the problem be the capacitor? Are capacitor problems intermittent? I always thought that they either worked or did not but would not vary.

Hope someone can address my issue.



Capacitors can change value depending on temperature, but usually, they are pretty stable.

From your description, it sounds more like an unloader problem.

Try this: next time it acts bad, try draining the tank some. Once you get to zero PSI, if it’s still not starting, it’s likely the cap or wiring to it/them. (the caps are worth checking, regardless)

But if it does start reliably at some lower pressure, then it’s the unloader or check valve – often you can disassemble and clean to fix it.


Thank you for your tips. I will be trying them when the weather warms up. My compressor is more of a summer tool. Use weekly to clean off the mower.


Sure, Rick.

Let us know how you make out if the weather ever warms up…


I had a similar issue, & yes, it was the capacitor. Couldn’t find a 150 µF axial mount replacement for a decent price, so went with 100 µF (I think anything from 100 µF to 150 µF will work) can type CBB 60 or CBB65 start & run capacitor ($5 – $10 on eBay.)

Cut wires where they entered the old capacitor, added female blade connectors and built a new mount.

Used caulking gun ($4) tube as holder, by removing handle & cutting to length.

Removed 8 mm bolt from old capacitor & used to attach to old capacitor mounted on the compressor.

Wrapped black electrical tape around new capacitor to provide firm fit, and added a wire tie to secure it to new mounting.

If you have any questions regarding the Harbor Freight air compressor won’t turn on, please leave a comment below, with a photo if applicable, so that someone can help you!

By Bill Wade

About Air Compressors has been helping folks with their Air Compressor Problems since 2002 online. We're a community of DIY and Compressed Air professionals who are keen to support everyone across the globe with their air compressor issues and troubleshooting. Whether you're trying to identify an old air compressor, or troubleshoot an error code on a sophisticated new industrial air compressor - the community at About-Air-Compressors.com is here to help you

Notify of
Inline Feedbacks
View all comments