Generac Error Code 1900: Undervoltage
Code 1900 means the generator ran but did not produce proper voltage. That is Generac's own framing: "The generator is not producing proper voltage. This could result from prolonged under voltage or sudden voltage loss." One article covers the whole range — 1900 through 1906, 1902 included — and it publishes no separate meaning for the individual numbers. The owner-level checks are all about fuel: fuel selection (NG or LP) correct, fuel jet in the right position, propane above 30 %, fuel supply adequate, and in cold weather the air filter free of moisture and snow. Then clear the alarm and test in MANUAL. If it returns, what follows is a voltage measurement on a running machine, and that is dealer work.
| Code | 1900 (UNDERVOLTAGE) |
|---|---|
| Applies to | Guardian and Next Generation air-cooled standby, Evolution / Evolution 2.0 / Power Zone 200 controllers; Generac publishes 1900–1906 as one condition (1902 has its own page) |
| Severity | Unit will not run |
| DIY | Check, then call — owner-level checks first, then a dealer |
| Last checked |
What code 1900 means
An air-cooled Guardian regulates its output at 240 V by controlling current in the rotor field, and the controller reads that output continuously. When the voltage is wrong for long enough, the controller stops the engine rather than let a brownout reach your furnace board, refrigerator compressor and sump pump. Generac does not publish the threshold or the delay, so no figure is given here — and you do not need one, because nothing an owner can check depends on it.
Two things can make voltage fall while the engine is turning. The engine may be starving — a lean mixture, low propane, or the wrong fuel setting lets it slow under load, and voltage tracks speed. Or the fault is on the electrical side of the machine. Generac names exactly those two families, fuel supply and selection on one hand, circuitry and voltage production on the other, and every owner-level check it lists belongs to the first. The second family is measured, not guessed, which is why this page stops where it does.
| Code | Display | Meaning (Generac wording) | Severity | DIY? |
|---|---|---|---|---|
| 1900–1906 | UNDERVOLTAGE | "The generator is not producing proper voltage. This could result from prolonged under voltage or sudden voltage loss." One definition and one owner procedure for the whole range | Unit will not run | Check, then call |
| 1902 | UNDERVOLTAGE | Same condition, and the number people search most — see the 1902 page for why it is not an underspeed code | Unit will not run | Check, then call |
| 1907–1916 | UNDERVOLTAGE | Inside the range named in the article title, but outside the range its text describes; no definition published for air-cooled units | Unit will not run | Licensed pro / dealer |
The Mobile Link message for every code in the family is "The generator has experienced an undervoltage alarm. Please contact a dealer," and Generac's code list classes the whole range as a red alarm. Its own pages disagree on where the range ends: the article title says 1900–1916, the text of the same article says 1900–1906. If your display shows a number above 1906, treat it as an undervoltage whose sub-type Generac has not published, and go straight to a dealer.
How to clear Generac code 1900
Do the fuel checks first, then clear 1900 and start the unit in MANUAL with no load; Generac's undervoltage article allows one owner attempt. The owner's manuals give UNDERVOLTAGE a one-line solution, "Contact an IASD," so if the alarm returns, stop clearing it and call a dealer. The key presses for each controller are collected on Generac lights and alarms.
The undervoltage article (Aug 26, 2025) sets out the owner's part: "You can attempt to clear the alarm and manually restart the unit," after the fuel checks below, with the warning "Never start a generator while under load." Its troubleshooting actions are three: "Perform checks above," "Clear the alarm at the generator's control panel," and "Attempt to start the unit manually by pressing the blue MANUAL button on the controller." The Quick Reference Guides add nothing an owner can do: UNDERVOLTAGE, "Contact an IASD." (Guardian A0001846499 Rev. C, p. 34; Next Generation A0004332577 Rev. C, p. 38). If the unit "fails to start and run after performing the outlined troubleshooting steps," the article's next step is a dealer.
Fuel checks Generac lists for an undervoltage
- Fuel selection (Guardian units). "Verify that the correct fuel source has been selected in the controller (NG or LP)." Do not change the setting yourself; if it is wrong, that is the installer's correction. On Next Generation units the fuel is set in the installer's Field Pro app.
- Fuel jet position. "Verify the fuel jet position is correct." Generac adds that this is "unlikely to be an issue in existing units since any problems would have likely surfaced already."
- Propane level. "If operating on Liquid Propane (LP), verify fuel levels are above 30%."
- Fuel supply. "This error may also occur if the generator does not have sufficient fuel supply." On natural gas during a widespread outage, Generac says to "contact your gas company to check on the stability of the natural gas grid." Measuring fuel pressure is a technician's job.
- In cold weather, the air filter. "During cold weather, moisture can freeze in the air filter, preventing adequate airflow to the engine," and "Snow must be cleared from around the unit."
- Clear and test in MANUAL with no load. If it runs, put it back in AUTO.
- The number is 1901, 1903, 1904, 1905 or 1906. Generac publishes no separate definition for any of them — the same article, the same checks, the same reset. Write the number down before you clear it; the dealer will ask.
- It came back after one clean reset. Stop clearing. Generac's clearing article: "If the alarm returns after setting the unit to AUTO, contact a Generac authorized service dealer for help."
- Repeats after the fuel checks? Generac's line: "contacting a Generac IASD for diagnosis is strongly recommended." The diagnosis is the dealer's.
Air filter numbers for a cold-weather undervoltage
| Part | Part number | Fits | Where verified |
|---|---|---|---|
| Air filter | 0J8478S | 14–26 kW air-cooled | Owner's manual A0001846499 |
| Air filter | 0E9371AS | 10 kW air-cooled | Owner's manual A0001846499 |
Brushes, the rotor and stator, and the excitation circuit components vary by engine and controller generation. No part numbers are listed for them because none could be confirmed with fitment from a Generac document.
When undervoltage needs a dealer
- 1900 returns after the fuel selection, propane level and fuel valve are confirmed.
- The alarm returns after one clean reset with fuel selection, jet position and propane level confirmed.
- You suspect the fuel jet, regulator or fuel selection is wrong — that is gas-train work for the installer.
- The engine runs steady but the house voltage reads low on a plug-in meter — output side, not fuel.
- Generac support: 1-888-436-3722.
Codes that overlap with undervoltage
1902 is inside this range and has its own page, because it is the number searched most. 1600 Underspeed shares the fuel-supply cause: Generac gives it as "The unit is overloaded or has fuel supply issues." 2100 Overload is the load-side alarm, "Load demand exceeds the generator's rated power capacity." If the unit runs, posts no alarm, and the house is still dark, the problem is on the transfer-switch side — see runs but no power. A red light with UNDERVOLTAGE on the display is an alarm; the other colours are explained on Generac lights and alarms.
Not sure the generator can carry the load?Generator Wattage Calculator Running watts, the largest starting surge, and a generator class from manufacturer charts.Frequently asked questions
What is the difference between Generac 1900 and 1902?
In Generac's owner-facing documentation, nothing: one article covers 1900–1906 as a single condition with one set of checks. 1902 has its own page here because it is the number people search and the one most often mislabelled as an underspeed code, not because Generac publishes a separate meaning for it.
Can I reset a 1900 code myself?
Once, after the fuel checks: Generac's undervoltage article lets you clear it and try a MANUAL start. The owner's manual's own answer for UNDERVOLTAGE is "Contact an IASD," and if the alarm returns after AUTO, that is where it goes.
Which codes does Generac's undervoltage article cover?
Its title says 1900–1916 and its text describes 1900–1906, with one definition and one owner procedure for all of them. Generac does not publish separate meanings for the individual numbers in air-cooled documentation, so the checks below apply whichever one your panel shows.
Can low propane cause code 1900?
Yes. Generac's article asks for the LP tank above 30 %. A low tank on a cold day can't supply enough vapor under load; the engine leans out and the output voltage drops with it.
What is Generac code 1901?
An undervoltage inside the same range, with no separate owner-facing definition published. Generac's article distinguishes only between a prolonged sag and a sudden loss, and gives one set of checks for both. Clear once, test in MANUAL, and if it repeats, call a dealer.