ATS Troubleshooting Decision Tree

Symptom

An automatic transfer switch (ATS) is the device that senses utility power loss, signals the generator to start, transfers the home's load from utility to generator within 10 to 30 seconds, and transfers back when utility returns. When the ATS does not perform any of these steps correctly, the customer's complaint is "the generator didn't kick in" or "the power doesn't transfer". The underlying failure can be at the ATS itself, the generator side, the utility-sense circuit, the control signal, the contactors, or the load-side connections. Walking the diagnostic in sequence narrows the fault in 20 to 30 minutes; jumping straight to the most expensive component (the ATS controller board) wastes parts dollars.

How a typical residential ATS works

1. Utility sensing: ATS monitors utility voltage on each phase. When voltage drops below threshold (typically 70 to 80 percent of nominal) for a set delay (typically 3 to 10 seconds), utility-fail is declared.

2. Generator start signal: ATS sends a 12 to 24 VDC signal to the generator's controller via dedicated wires (the "2-wire start" or "remote start" circuit).

3. Generator startup: Generator cranks, runs, reaches stable voltage/frequency. Generator controller signals "available" back to the ATS.

4. Transfer: ATS opens the utility contactor, brief pause (typically 100 to 500 ms), closes the generator contactor. Load is now on generator.

5. Operation on generator: ATS monitors generator output and load.

6. Utility return sensing: ATS monitors utility voltage. When voltage is stable above threshold for a set delay (typically 30 to 300 seconds, intentionally long to verify utility is solidly back), utility-restore is declared.

7. Retransfer: ATS opens the generator contactor, brief pause, closes the utility contactor. Load is back on utility.

8. Cooldown and stop: ATS sends "stop" signal to generator. Generator runs for cool-down period (typically 60 to 300 seconds), then shuts down.

A failure at any step produces specific symptoms.

Causes ranked by frequency

Diagnostic decision tree

Step 1: Verify the failure mode

Get the customer's specific description:

• "Generator doesn't start at all" - generator side, signal side, or battery
• "Generator starts but ATS doesn't transfer" - ATS contactor or control board
• "Transfer happens but power doesn't reach the home" - wiring or load side
• "Transfers but doesn't transfer back" - utility-sense or contactor
• "Test mode works but real outage doesn't" - utility-sense timing or threshold
• "Test mode fails too" - ATS or generator control fault

The failure mode points to the subsystem.

Step 2: Read the ATS display / fault history

Modern ATS units (Generac RTS and RXS series, Kohler RXT, Cummins PowerCommand) have a display showing current status and fault history. Mobile Link is Generac's remote-monitoring accessory, not a transfer switch; if the customer says "Mobile Link", they are describing the app, and you still have to identify the actual switch on the wall. Common displays:

• Utility voltage L1-L2, L1-N, L2-N
• Generator voltage L1-L2, L1-N, L2-N
• Current draw
• Transfer status (utility, generator, transferring)
• Fault history with date stamps

Read the fault history. Recent faults point to the failure path. "Battery low", "Generator failed to start", "Contactor failed to operate" each point to specific subsystems.

Step 3: Verify battery and generator

If the ATS reports "Generator failed to start" or similar:

• Read battery voltage with the generator off: should be 12.5 to 13.0 V (with engine off, fully charged)
• Read battery voltage during crank attempt: should not drop below 10.5 V
• If voltage drops below 10.5 V, battery is failing
• Verify battery cables are tight and clean
• Verify battery charger is operating (if the generator has one - typically yes)

If battery is good but generator does not start, the failure is at the generator (not the ATS). Different diagnostic.

Step 4: Verify the 2-wire start circuit

The "remote start" circuit connects ATS to generator over two wires, typically 16 to 18 AWG.

Know which convention your unit uses BEFORE you interpret a reading, because the two behave opposite each other and a tech who guesses wrong condemns a healthy switch:

• Dry contact (the classic two-wire start). The ATS supplies no power at all. It is just a relay contact. The generator controller puts its own battery voltage on the loop and watches for the contact to CLOSE. Here you read roughly battery voltage across the open wires when the ATS is idle, and near 0 V when it commands start. Voltage disappearing is the good result.
• Powered signal. The ATS itself pushes 12 to 24 VDC down the pair on start command. Here you read 0 V idle and 12 to 24 VDC on command.

Check the ATS and generator wiring diagrams for which one you have, then:

• Disconnect at the generator side
• With the ATS commanding start, measure across the disconnected wires
• Compare against the convention above

If the signal changes state correctly, the ATS is doing its job and the generator is not responding.

If the signal never changes state, the ATS is not sending the command. Check:

• The fuse or breaker on the start circuit (typically a small inline fuse)
• The relay on the ATS control board that drives the start signal
• The wiring continuity between the ATS and the generator

Step 5: Verify utility sensing

To verify the ATS is reading utility correctly:

• Read the displayed utility voltage on the ATS display
• Compare to actual utility voltage (measured with a multimeter at the utility terminals)
• Match within 2 to 3 V

If the ATS reads significantly different than the actual utility voltage, the sensing circuit is failing. Check:

• The sensing wires from the utility breaker to the ATS control board
• The sensing transformer on the ATS control board (if equipped)
• The ATS controller board (rarer)

For utility sensing that triggers too easily (false outages):

• Verify the sensing threshold is set correctly (typically 70 to 80 percent)
• Verify the sensing delay (typically 3 to 10 seconds; longer ignores brief utility blips)
• Check for utility-side voltage fluctuation issues

Step 6: Test the contactors

The ATS has two contactors (utility-side and generator-side, mechanically interlocked so both cannot close simultaneously). The line side of an ATS is live even with the generator off and the main open on the load side. Kill both sources at their disconnects, lock them out, and test dead before you put a hand or a tool inside the enclosure.

Test each:

1. Inspect first, with everything dead. Look at the contacts for pitting, blueing, and carbon tracking. Look for discolored or heat-damaged lugs and insulation. Burned contacts and a hot-lug smell tell you the answer before any meter does.
2. Check the mechanical interlock by hand. Operate the mechanism manually per the manufacturer's procedure using the manual operating handle. It should move cleanly from utility to generator and back, and it must be physically impossible to seat both sides at once. Binding, grinding, or a mechanism that needs force means the transfer mechanism is failing, and that is a replace, not an adjust.
3. Measure contact resistance across each pole with the switch dead and in the closed position for that side, using a low-resistance meter. Compare all poles to each other. One pole reading noticeably higher than its siblings is a burned contact set.
4. Check the coil or actuator. Measure coil resistance and compare to the manufacturer's spec, or check it against the other side's coil. Open or badly out-of-spec means a failed coil.
5. Verify the control signal actually reaches the contactor. If the controller commands transfer and the contactor does not move, put a meter on the coil terminals during the command. Voltage present with no movement is the contactor; no voltage is upstream at the controller or the wiring.
6. Run a live transfer test once everything is reassembled. Simulate a utility outage at the utility disconnect and watch the sequence: sense, start, warm-up, transfer to generator, then restore utility and watch retransfer and cooldown. Time each stage and compare to the controller settings.
7. Listen and watch during the transfer. A clean transfer is one decisive snap. Chattering, a slow drag, or a contactor that seats partway is a failing mechanism even if it completed this time.
8. Measure load-side voltage on both sources after transfer, phase to phase and phase to neutral, and confirm all poles are passing full voltage.

A pitted or welded contact set is not field-repairable on most residential units. Replace the transfer mechanism or the switch. On larger commercial gear, contact kits exist; check with the manufacturer before you quote a whole switch.

References

• NFPA 70 (NEC) 2023, Article 702 (Optional Standby Systems).
• NFPA 110 (Standard for Emergency and Standby Power Systems).
• UL 1008 (Transfer Switch Equipment).
• IEEE 446 (Recommended Practice for Emergency and Standby Power Systems).
• OSHA 29 CFR 1910.269 (Electrical hazards including backfeed).
• Manufacturer literature: Generac Mobile Link / RTS, Kohler RXT / RX, Cummins PowerCommand, Briggs and Stratton.
• Manuall internal: Generator Transfer Switch Installation, Generator Diagnose Generator No Start, Generator Generator Interlock vs Transfer Switch.