Generac Error Code 2780: Charger Missing AC
Code 2780 is a yellow warning, not a shutdown: the battery charger has no AC input. Generac's diagnostic manual: "AC power is missing from the battery charger input." Its support article is more specific — "the T1 wire is not receiving 120 VAC." T1 is the wire that carries 120 V from the transfer switch to the generator's charger. During a power outage this warning is expected: the utility is gone, so T1 is dead. With the grid on, Generac's two owner checks are that the generator's main line circuit breaker (MLCB) is ON and utility power is actually present; if the warning stays, the T1 fuse in the transfer switch or the T1 wiring needs a technician. Generac says the code "will clear automatically" once the input returns. The related yellow codes — 2750 Low Battery, 2760 Battery Problem, 2770 Charger Warning — are covered below.
What code 2780 means
An air-cooled Guardian keeps its starting battery topped up with a charger built into the controller, fed by 120 V on the T1 wire. T1 originates in the transfer switch (behind a fuse on RTS and HomeLink switches) and lands on the customer-connection strip in the generator. Generac's charging-system article explains where that 120 V comes from: from the utility whenever the grid is up — charging continues "even if the controller is OFF and/or the generator's MLCB is OFF" — and, during an outage, from the generator's own output once the switch has transferred, which needs the MLCB ON and the unit running. The charger monitors its own input; when T1 goes dead, the controller lights the yellow LED and posts CHARGER MISSING AC.
Generac's article names three causes: a voltage issue on the T1 wire, a blown T1 fuse in the transfer switch, or a transfer switch that failed to move to the generator position during an outage. Its second owner step adds the fourth and most common: during an outage the warning is expected — with the grid down and the generator not yet running (or not yet transferred), there is no 120 V anywhere to charge from. Generac's starting-issues article says the same: Charger Missing AC during an outage is normal. The lights go out, the unit sits in AUTO for the seconds before it starts, the warning logs. It's noise.
The version that matters is 2780 with the utility on. Then the battery is slowly discharging with nothing to replace it, and in a few weeks you'll have 2750 Low Battery (below) — or a unit that won't crank when the next outage hits.
| Code | Display | Meaning (Generac wording) | Severity | DIY? |
|---|---|---|---|---|
| 2780 | CHARGER MISSING AC | "AC power is missing from the battery charger input" — "the T1 wire is not receiving 120 VAC" (yellow) | Runs with warning | Check, then call |
| 2770 | CHARGER WARNING | "Less than 12.5 volts of battery voltage at the end of an 18-hour charge" (yellow) | Runs with warning | Check, then call |
| 2760 | BATTERY PROBLEM | "More than 16 volts of battery voltage or 600 milliamperes or more of charge current at the end of an 18-hour charge" (yellow) | Runs with warning | Check, then call |
| 2750 | LOW BATTERY | "Battery less than 12.1 volts for 60 seconds" (manual); support article: below 12.5 VDC (yellow) | Runs with warning | DIY-friendly |
Mobile Link messages: 2780 — "The unit is not charging the battery properly. Please contact a dealer." 2760 and 2770 — "The battery is not charging properly. Please replace the battery or contact a dealer." 2750 — "The battery charge level has fallen below acceptable level. Please inspect or replace the battery." The owner's manual's Quick Reference lists all four as yellow — the unit still starts and runs in AUTO with any of them active. (Generac's support index table shows 2750 in red; the owner's manual and dealer table say yellow, and the manual is the authority.)
One number people search alongside these — 2690 — is not a charger code and does not appear in any Generac document for air-cooled Guardians. Dealer material places it on liquid-cooled units as a low-fuel-pressure alarm. If your air-cooled unit shows it, call a dealer.
Reset procedure (as the manual states it)
Generac's 2780 article does not give a clear sequence, because the code clears itself: "When the issue is resolved, the code will clear automatically." The steps are about restoring T1:
- Confirm the generator's main line circuit breaker (MLCB) is ON. With the grid up, the charger runs regardless of the MLCB; during an outage, T1 is fed by the generator through the transfer switch, and an open MLCB leaves it dead.
- Confirm utility power is present at the house. If you're in an outage, the warning is expected and will clear when the grid returns.
- If the grid is on, the MLCB is on, and 2780 persists — the T1 fuse or T1 wiring. Generac: "Generac does NOT recommend end users attempt to modify or service internal generator wiring." The fuse lives in the transfer switch, which is live at 240 V on the utility side; the replacement is a dealer task.
For the other three codes the standard clear applies — OFF, ENTER, AUTO (Next Generation: hold OFF three seconds) — but a charger or battery warning that is cleared without being fixed simply returns at the end of the next 18-hour charge cycle.
Step-by-step diagnosis
- Is the grid on? If not, 2780 is expected. Note the time it appeared in Mobile Link; if it matches the outage, ignore it and confirm it clears once utility returns.
- MLCB. Open the generator's lid; the main breaker is under the controller. It should be ON. It doesn't affect charging while the utility is up, but a unit that ran an outage with the MLCB off charged nothing the whole time — and didn't power the house either.
- House-side 120 V. Generac's article stops at the two checks above, but some installations feed the charger through a separate 120 V breaker in the house panel (usually labelled "generator" or "gen charger"). This is our note, not Generac's step: look for that breaker and confirm it isn't tripped. If you can't identify one, don't guess — go to step 5.
- Battery voltage. Check the BATTERY menu on the controller. Above 12.6 V, you have time. Below 12.5 V with 2780 active, the battery has been running down; get the charger fixed before it becomes a no-start.
- T1 fuse and wiring — dealer. The technician measures 120 V on T1 at the customer-connection strip; nothing there means the fuse in the switch or the run between. This is a short visit and it's the correct end of the owner's part.
- 2770 instead of 2780? The charger has AC but the battery won't come up — Generac's causes are a blown T1 fuse, a charging-circuit fault, or a battery at end of life. Owner checks from the article: battery health on the controller or Mobile Link, terminal connections tight, corrosion cleaned, T1 fuse status. Generac's reset for a lingering Mobile Link warning: run in MANUAL for at least one minute, then AUTO. Most 2770s end with a new battery.
- 2760 Battery Problem? The opposite fault — over 16 V or still drawing 600 mA after 18 hours. That usually points at the battery (a shorted cell) or at the charger overcharging. Replace the battery first (cheap, and the usual answer); if the new one posts 2760 too, the charger is the problem — dealer.
- 2750 Low Battery? Generac's steps: check DC voltage at the battery and in the controller menu; below 12.5 V, allow 24 hours to charge; above 12.5 V, look at the charger. New installations with charger warnings: verify the neutral is landed in the charger circuit (installer). A hard reset may clear it. Persisting after 24 hours on charge → dealer, or a new battery.
Parts, numbers, and consumables
| Part | Part number | Fits | Where verified |
|---|---|---|---|
| Battery, Group 26R, 540 CCA (or Group 35 AGM 650 CCA) | 0H3421S | 10–26 kW air-cooled | Owner's manuals A0001846499 / A0000973303 |
| Control panel fuse, 7.5 A | 0D7178T | 10–24 kW air-cooled | Owner's manual A0000973303 |
The T1 fuse is part of the transfer switch, and its rating and holder differ between RTS and HomeLink models; no part number is listed because none could be confirmed with fitment. The battery charger is integrated into the controller on Evolution units and is not sold as an owner-replaceable part.
Repair or replace?
A 2780 caused by an outage costs nothing. A T1 fuse is a trivial part and a service call. A battery is the one purchase most owners make from this family, and Generac expects it: its starting-issues article says to replace batteries every three to five years whether or not a warning ever appears. A charger fault inside the controller is the expensive end — but confirm it with a new battery and a measured T1 before believing that diagnosis, because the controller is the last thing to fail and the first thing to be blamed.
When to call a dealer
- 2780 with utility on and the MLCB confirmed ON — T1 fuse or wiring, inside the transfer switch.
- 2770 or 2760 returns after a new battery.
- 2750 persists after 24 hours on charge.
- New installation with any charger warning — installer, to check the charger-circuit neutral.
- Generac support: 1-888-436-3722.
Parts and supplies mentioned on this page
Keep a replacement battery number on hand; a multimeter across the terminals tells you in ten seconds whether the charger is doing its job.
- Generac replacement battery 0H3421S (Group 26R)Check price on Amazon
- Digital multimeter (AC/DC, auto-ranging)Check price on Amazon
Related battery and charger codes
The four codes on this page describe the same circuit from four angles: 2780 — no AC into the charger; 2770 — AC present, battery still low after 18 hours; 2760 — battery overcharging or won't stop taking current; 2750 — battery voltage low right now. A dead battery with no code at all is the most common reason a Guardian won't start. A miswired T1 at installation can also show up as a 2098/2099 wiring error.
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 code 2780 serious?
It's a yellow warning — the unit still starts and runs. It matters only when it stays on with the grid up, because the battery is then discharging with no charger. Fix it before the next outage.
Why does 2780 appear every time the power goes out?
Because the charger's 120 V comes from the utility through the T1 wire. When the grid drops, T1 goes dead for the seconds before the generator starts, and the warning logs. Generac says it clears automatically when the input returns.
Can I replace the T1 fuse myself?
Generac says end users should not service the internal generator or transfer-switch wiring, and the fuse sits in the transfer switch next to live utility conductors. It's a short dealer visit.
What is the difference between Generac 2770 and 2780?
2780: the charger has no AC input (T1 dead). 2770: the charger has AC but the battery is still under 12.5 V after an 18-hour charge — usually a battery at end of life.
Is Generac code 2750 red or yellow?
Yellow. The owner's manual and Generac's dealer table both list Low Battery as a yellow warning; the unit still runs. Check the voltage, charge 24 hours, and replace the battery if it doesn't recover.