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There have been many folks that wish to inquire about why their Harbor Freight air compressor not turning on and other questions along those same lines. This article will present some of the reasons why your Harbor Freight air compressor won’t start may be happening and provide Harbor Freight air compressor 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:

IssueAdvice
Power issueAvoid using power bars and power strips.
Faulty pressure switchUnplug the Husky compressor from the electrical outlet before opening the drain valve releasing the air from the tank down to zero and then closing the drain. 
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 unloader valve, if it is faulty, the pressure switch may not work.
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

Question

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.

Thanks

Response

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.

Questioner

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.

Response

Sure, Rick.

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

Response

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!