Transfer Switch Won’t Switch to Generator Power: Diagnosis and Fixes

If the power’s out and your standby generator is running but the house is still dark, the transfer switch is where the handoff is failing — not the generator itself. Here’s how to find which part of that handoff broke:

• Generator runs, but no circuits get power → start with The Transfer Relay Isn’t Engaging
• Switch clicks but only half the house is live → jump to Partial Transfer
• ATS controller screen is blank or flashing an error → see Controller Fault Codes
• Manual interlock kit instead of an ATS → that’s a different setup, covered in transfer switch vs. interlock

The Transfer Relay Isn’t Engaging

Start at the ATS enclosure, not the generator. Open the panel — most standby units use a Generac RXSW, Kohler RDC2/RXT, or similar automatic switch — and look for a lit “utility power” or “generator power” indicator LED. If neither is lit while the generator is clearly running outside, the controller isn’t sensing anything from either source, which usually means a blown control fuse (often a 3–5A fuse on the control board, cheap and easy to swap) or a failed voltage-sensing relay.

Before you touch anything electrical, verify the generator’s own output breaker — the one mounted inside or next to the ATS enclosure, not the transfer switch itself — hasn’t tripped. This breaker exists specifically to protect the ATS from generator output faults, and on Generac units it’s a small black breaker on the right side of the enclosure that’s easy to miss in a quick glance. Flip it firmly off then on. If that resets it and power flows, you’re done — no further diagnosis needed.

If the breaker’s fine and the LEDs stay dark, the fix depends on what you find with a meter. With the enclosure open and the generator running, check for control voltage (usually 12V DC) between the labeled control terminals in the wiring diagram taped inside the enclosure door. No voltage there means the control board itself has failed — a $150–300 part depending on model, and one most homeowners have a licensed electrician swap rather than DIY, since it involves the generator’s main output leads.

Partial Transfer — Some Circuits Dead, Others Live

This is a mechanical problem, not electrical. Standby ATS units use a double-pole contactor — essentially two switches ganged together on one armature — and when only half your house gets power, one pole of that contactor isn’t making contact even though the other is. The most common cause is a burned or pitted contact on one leg, usually from years of switching under load plus a little corrosion.

You can often see this without any tools: with the enclosure open and the unit transferred to generator power, look at the contactor assembly and check whether both sets of contacts are physically closed the same amount. A contact that’s visibly darker, pitted, or sitting slightly open compared to its twin is the failed one. Replacement means the whole contactor assembly ($80–180 depending on switch amperage), since the contacts aren’t sold separately on residential ATS units — and this is a job for a licensed electrician, since you’re working inside an enclosure with both utility and generator leads present at once.

Controller Fault Codes

Generac and Kohler ATS controllers both display fault codes rather than failing silently, and reading that code before opening the enclosure saves a wasted trip. On Generac RXSW controllers, look for codes like “Utility Voltage Sense Fail” or “Transfer Switch Fail” scrolling on the small LCD. Kohler’s RDC2 shows similar plain-language faults rather than numeric codes. Write down the exact wording — it tells you whether the fault is on the sensing side (utility detection) or the transfer side (the physical switch), which is the fork in the road for diagnosis above.

A blank screen with the generator confirmed running and utility power confirmed out points at a dead control board power supply, not a fault code at all — that’s the same control-fuse or control-board path from the first section.