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Predator Generator Won't Start: Combination Switch, Low Oil, CO Sensor and Fuel

Updated 2026-11-02 · Written and checked by the Manual Matters team against the manufacturer documents listed under Sources.

Short answer: most Predator inverter generators that will not start have one of five problems. The Combination Switch is not on START for a cold engine, the gas cap vent is closed (on models that have one, such as the 2000 W), the oil is low or the unit is on a slope (the engine will not start with low or no oil), the gasoline is old or above E10, or the engine is flooded. On newer units, a CO sensor light explains a sudden shutdown. Everything here comes from Harbor Freight's owner's manuals for the Predator 3500 inverter (item 71358) and the Predator 2000 inverter (item 71343).

Predator generator will not start: Combination Switch, gas cap vent, low oil, fuel, flooding, spark arrestor, CO sensor and electric start checks
Predator generator will not start: Combination Switch, gas cap vent, low oil, fuel, flooding, spark arrestor, CO sensor and electric start checks

This page is only about getting a Predator running. Oil type, break-in and the oil-change steps are in our Predator 3500 break-in and oil guide. The general after-storage causes that apply to every brand are in the generator won't start after storage checklist.

Safety first: outdoors only, far from windows, doors and vents. Harbor Freight's manuals say to use the generator at least 20 feet from people, animals and structures, with the exhaust pointed away.

1. The Combination Switch (and the ESC switch)

Predator inverters do not have a separate fuel valve and choke lever. The 3500's control panel figure labels a single Combination Switch as "Fuel Valve + Choke", with OFF / RUN / START positions. Getting it wrong is the first thing to rule out.

The manual's starting order for both models:

  1. Open the vent on the gas cap (2000 W, item 71343; the 3500 manual has no such step).
  2. Move the Economy (ESC) switch to OFF.
  3. Turn the Combination Switch to START.
  4. Pull the starter handle slowly several times so gasoline flows into the carburetor. Then pull gently until you feel resistance, let the cable retract, and pull quickly.
  5. When the OUTPUT light comes on, let it run several seconds, then move the Combination Switch slowly to RUN. Harbor Freight warns that moving it too fast can stall the engine.

Two related lines in the troubleshooting table: "Choke not in START position, cold engine" and "Combination Switch at OFF position". For a warm engine that will not restart, the manual says the opposite: turn the Combination Switch to RUN before trying again.

2. Oil level and the slope trap

Both manuals say it plainly: the engine will not start with low or no engine oil. On the 3500, if the oil drops too low, the LOW OIL light comes on and the engine shuts off automatically. It cannot be restarted until oil is added.

The troubleshooting table adds a second cause: "Engine mounted on slope, triggering low oil shutdown." Move the generator to level ground and check the oil again. Harbor Freight repeats "check engine oil before EVERY use", and the warranty is void if the crankcase is not properly filled before each use. Capacities differ by model. The manuals list 20 fl oz of SAE 10W-30 for the 3500 and 13 fl oz of SAE 10W-30 for the 2000.

3. Fuel: old gas, ethanol, priming

The fuel-related rows of the "Engine will not start" table:

The 3500 manual's after-storage advice: if untreated gas has sat for a month, or the engine does not start, drain it and refill with fresh fuel. For how to store fuel so this does not happen, see Run it dry or stabilize it?.

4. Flooded engine (you can smell gas)

Harbor Freight's fix for too much fuel in the chamber, which it says can be caused by a sticking carburetor needle:

  1. Turn the choke (the Combination Switch) to RUN.
  2. Remove the spark plug and pull the starter handle several times to air out the chamber.
  3. Reinstall the plug, set the switch back to START and try again.

5. Spark, spark arrestor and compression

6. Electric start (3500)

The 3500's electric start uses a 12 VDC battery that must be connected before first use (red cable to the positive terminal first, then black to negative). If the button will not crank the engine, the table lists:

Hold the starter button for no more than 3 seconds. If it does not start, release it and wait at least 10 seconds before trying again.

7. CO sensor light: why it stopped suddenly

Current Predator inverters have a Carbon Monoxide sensor that shuts the engine off. Read the light:

LightMeaning (Harbor Freight manual)What to do
Flashing redDangerous levels of CO have built upLeave the area immediately until it has aired out. Move the generator to a well-ventilated area before running it again
Yellow, flashes once after startingSensor passed its self-checkNormal. Nothing to fix
Yellow, flashing continually shortly after startingSensor malfunctionDo not use the generator until the sensor is working. Call Harbor Freight (1-888-866-5797)
Yellow, flashing continually after a longer runListed with operating conditionsOperate within the rated ambient temperature and keep at least 5 ft of clearance on all sides

Harbor Freight says the sensor must only be serviced by a qualified technician and must never be modified or tampered with. It is an extra layer of protection, not a reason to run the generator anywhere CO can build up. See where to place a generator.

The same "Engine stops suddenly" list also includes a defective fuel tank cap creating a vacuum, which stops fuel flowing. Test or replace the cap.

More generator guides: Portable Generators.

FAQ

Why won't my Predator generator start?

Most often the Combination Switch is not on START for a cold engine, the oil is low (or the unit is on a slope), or the fuel is old. On the 2000 W, also check that the gas cap vent is open.

Where is the choke on a Predator inverter generator?

It is part of the Combination Switch (OFF / RUN / START). START is the choke position for a cold engine. After the OUTPUT light comes on, move it slowly to RUN.

My Predator won't restart when warm. What should I do?

Harbor Freight's manual says to turn the Combination Switch to RUN, not START, before trying to restart a warm engine.

What does the yellow light on a Predator CO sensor mean?

One flash after starting is the normal self-check. Continuous yellow flashing shortly after starting means the sensor needs service: stop using the generator and call Harbor Freight.

Can a Predator generator start with low oil?

No. Both manuals state the engine will not start with low or no engine oil, and a slope can trigger the low oil shutdown even when the level is fine.

Sources