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 Mobile Link, Kohler RX, Cummins PowerCommand) have a display showing current status and fault history. 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. Two wires (typically 16 to 18 AWG) carry 12 to 24 VDC signal:

• Disconnect at the generator side
• With the ATS initiating start, measure voltage at the disconnected wires
• Should read 12 to 24 VDC during start command

If voltage is present, the ATS is sending the signal correctly; the generator is not responding.

If no voltage, the ATS is not sending the signal. 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). Test each:

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.