Generac Error Code 1902: Undervoltage
1902 is an undervoltage alarm. You'll see it called "under speed" on forums — that's wrong. Generac's official code list puts 1900–1916 under Undervoltage and 1600–1603 under Underspeed. Specifically, 1902 means the controller lost both "zero-cross" signals from the alternator for more than 1.5 seconds: the engine was turning, but no usable AC came out. Generac's owner-level checks are fuel selection, propane level, and a clear air intake; if those are fine, the fix is an electrical test at the dealer, because the likely culprits are the excitation winding, the field-boost circuit, or a loose connection inside the unit.
What code 1902 means
The Evolution diagnostic manual's entry for 1902 reads: "Both Zero Crosses missing — Undervoltage due to faulty excitation winding, or zero cross circuit, or circuit in general. Possible cause: loose wiring, field boost hardware failure (both zero cross missing greater than 1.5 sec)." The technician's action is "Perform Preliminary Output Voltage Test."
In plain terms: the controller watches the alternator's output waveform through two "zero-cross" sense lines. When both go silent for 1.5 seconds while the engine is supposed to be producing power, the controller assumes there's nothing to send to the house and shuts down with a red light.
Generac's public support article for the 1900-series is broader: "The generator is not producing proper voltage. This could result from prolonged under voltage or sudden voltage loss." It applies to Guardian 10–26 kW and Next Generation 10–28 kW units built from 2017 on.
| Code | Display | Meaning (Generac wording) | Severity | DIY? |
|---|---|---|---|---|
| 1900 | UNDERVOLTAGE | Loss of voltage — output below 80 % of nominal for 10 seconds or more | Unit will not run | Check, then call |
| 1901 | UNDERVOLTAGE | Undervoltage variant (definition not published in owner-level documents) | Unit will not run | Licensed pro / dealer |
| 1902 | UNDERVOLTAGE | Both zero crosses missing for more than 1.5 s — excitation winding, zero-cross circuit, loose wiring, or field boost hardware | Unit will not run | Check, then call |
| 1906 | UNDERVOLTAGE | Undervoltage variant (definition not published in owner-level documents) | Unit will not run | Licensed pro / dealer |
| 1600 / 1603 | UNDERSPEED | Engine overloaded or never reaches 3,600 rpm — a different family of alarm (see the Generac hub) | Unit will not run | Check, then call |
Older Nexus controllers (pre-2013) don't use numeric codes. Their manual defines Under-Voltage as output below 60 % of nominal for more than five seconds and shows it as a text alarm.
Reset procedure (as the manual states it)
The owner's manual's solution column for UNDERVOLTAGE is one line: "Contact an IASD." The support article is a little more generous — it lets you rule out the obvious before you call. Then:
- Set the external (and, on 13 kW and larger units, the internal) shutdown switch to I.
- Press OFF.
- Press ENTER at the prompt.
- Press AUTO.
- Press the blue MANUAL button and let it run for a couple of minutes with no load, then with load. If 1902 comes back, stop and call.
Don't cycle it more than twice. Repeated attempts on a unit with a wiring fault or a failing rotor do nothing but add hours and heat.
Step-by-step diagnosis
- What else is in the alarm log? From MAIN MENU go to HISTORY → ALARM LOG (Evolution 2.0: the code and message alternate on the display). 1902 following an overcrank (1100) or a rough start suggests the engine stumbled and the output collapsed with it — a fuel problem masquerading as an electrical one. 1902 out of nowhere on a unit that's been running fine points at the alternator side.
- If the log shows 1100 first → work through the 1100 checks before anything below.
- Fuel selection and supply. The support article asks you to verify the fuel source selection (natural gas vs. propane) matches the installation, that the fuel jet is in the correct position, and that a propane tank is above 30 %. Fuel starvation under load drags the engine down and can take the voltage with it.
- If propane is low → fill it and retest.
- If you suspect the selection or jet is wrong → dealer. Don't move the jet yourself.
- Air intake. Clear snow, leaves, and moisture from the intake and check the air filter (0J8478S for 14–26 kW, 0E9371AS for 10 kW). The support article specifically mentions moisture and snow in the filter for this code family.
- Load. Was a large load — well pump, central air, electric dryer — starting when it tripped? An air-cooled unit sized at the edge of its rating can see a momentary voltage collapse on a big motor start. This is where the Generac hub's note on load shedding applies, and where the wattage calculator tells you whether the unit was ever big enough.
- If a motor start lines up with the alarm → try MANUAL with that circuit off. If it runs clean, the fix is load management (a load-shed module or a soft-start on the AC), not a repair.
- Clear and test. Follow the reset above and run in MANUAL. Listen for a steady engine note; a surge or hunt means the engine speed, not the alternator, is wandering (see 1600).
- Still 1902? Stop. The diagnostic manual's next step is the "Preliminary Output Voltage Test," a technician's procedure: measurements on a running generator at 240 V, working through the causes the manual lists (excitation winding, zero-cross circuit, loose wiring, field boost hardware). That belongs to a licensed technician.
Parts, numbers, and consumables
There's no owner-replaceable part for a genuine 1902. The parts that fix it — stator, rotor, brush assembly, field boost components, harness — are model-specific and installed by the dealer. The consumables below are the ones the owner-level checks touch.
| 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 |
| Battery, Group 26R, 540 CCA | 0H3421S | 10–26 kW air-cooled | Owner's manuals A0001846499 / A0000973303 |
Repair or replace?
The causes Generac lists range from cheap (a loose connection) to expensive (a failed excitation winding, which means alternator work). On an out-of-warranty 20 kW unit the expensive case can reach a meaningful fraction of the price of a new generator; the cheap case is an hour of dealer time. Check the install date and your warranty paperwork before you assume the worst — an alternator fault inside the warranty period is Generac's cost, not yours. Replace only if the unit is also out of production (Nexus era) and the alternator parts are unavailable.
When to call a dealer
- 1902 returns after one clean reset with fuel and air confirmed.
- The generator runs but the house has no power, or the lights flicker and dim when it's running — that's the same undervoltage seen from inside.
- You see burn marks, a hot smell, or a tripped main breaker on the generator.
- The alarm log shows 1902 with any 2100-series (overload) or 1600-series (underspeed) codes.
- Generac support: 1-888-436-3722. Warranty repairs go through an Independent Authorized Service Dealer.
Parts and supplies mentioned on this page
The consumables above are the only owner-level parts involved. Confirm fit against your model number.
- Generac air filter 0J8478S (14–26 kW)Check price on Amazon
- Generac 0H3421S Group 26R batteryCheck price on Amazon
Related codes and symptoms
If the engine never reaches speed at all you'll see 1600/1603 Underspeed, not 1902. If the unit stalls and restarts, look for 1505 RPM Sense Loss in the log. A generator that runs but delivers no power to the house without any alarm is a transfer-switch question, covered on the Generac hub.
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
Is Generac 1902 "under speed"?
No. Generac's official code list assigns 1900–1916 to Undervoltage and 1600–1603 to Underspeed. The confusion comes from third-party code lists that mislabel it. The display on the unit says UNDERVOLTAGE.
Can a weak battery cause code 1902?
Indirectly. The controller and the field-boost circuit run off the battery during startup; a battery too weak to hold the controller steady while the engine loads can produce erratic sensing. Check the BATTERY menu reading as part of the process, but a battery alone is not the usual cause.
My generator threw 1902 during a storm and now starts fine. Do I still need service?
Yes, at least a call. A one-time 1902 during a heavy motor start may be a load issue, but the same code with no load change means something in the output circuit is marginal and will fail again — usually during the next outage.
What does "zero cross" mean?
An AC waveform crosses zero volts 120 times a second at 60 Hz. The controller uses those crossings to confirm the alternator is producing power and to measure frequency. When both sense lines stop reporting crossings for 1.5 seconds, the controller logs 1902.
Does Mobile Link report 1902?
Yes. The listed message for 1900–1916 is "The generator has experienced an undervoltage alarm. Please contact a dealer."