AC Output Voltage Below Minimum Threshold
The KOHLER-13 (Kohler RDC2) diesel fault code means: AC Output Voltage Below Minimum Threshold. This is a serious severity code.
- Keep driving?
- No -- stop driving
- DIY difficulty
- moderate
- Estimated cost
- DIY checks (fuel, connections, load shedding) cost nothing to $30 in supplies. AVR board replacement runs $150-$400 in parts if you are comfortable doing it yourself. Professional diagnosis and AVR replacement typically runs $300-$700. Stator or rotor repair or replacement can reach $1,000-$2,500 or more with labor.
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Common Symptoms
- Generator shuts down and displays code 13 on the RDC2 controller display panel
- RED fault light illuminates on the generator panel
- Lights or appliances connected to the generator dim noticeably before shutdown
- Generator cranks and starts but trips off within seconds of reaching run speed
- ATS transfers load back to utility or drops load entirely after generator fault
- Weekly exercise cycle starts but the unit shuts itself down before completing
- OnCue Plus monitor or Mobile Link app shows a fault alert with undervoltage description
Probable Causes (Ranked by Likelihood)
- AVR (automatic voltage regulator) failure or degraded AVR output, preventing proper voltage buildup Very Likely
- Generator overloaded by too many appliances at startup, pulling voltage below the lower limit Very Likely
- Weak or shorted excitation winding in the stator, reducing field current and collapsing output voltage Likely
- Residual magnetism loss in the rotor (loss of residual field), causing the generator to fail to self-excite Likely
- Loose or corroded connection on the AC output leads or generator terminals introducing voltage drop under load Possible
- Engine running below rated RPM due to governor issue, fuel restriction, or LP/NG supply pressure problem, lowering output frequency and voltage Possible
- RDC2 controller sensing circuit fault causing a false undervoltage reading when actual voltage is within range Less Likely
Step-by-Step Diagnostic Procedure
Check the RDC2 controller display panel for the exact fault code and note whether any companion codes appear alongside code 13. Multiple codes together can narrow the root cause faster.
Inspect the fuel supply first. For NG (natural gas), verify the gas meter is not in bypass or shut off and that other gas appliances in the home are working. For LP (liquid propane), check tank level and regulator pressure. Low fuel pressure causes low RPM which causes low voltage.
Set the panel to MANUAL and attempt a start while watching the RDC2 display for AC output voltage. A healthy Kohler RES-series unit should stabilize at 240V AC plus or minus 2 percent at no load. If the voltage never builds above roughly 180-200V, suspect AVR or excitation loss.
With the generator shut OFF and the fuel valve closed, use a multimeter to measure DC voltage across the AVR terminals (label varies by model, but typically F+ and F-). You should see a small DC signal from the controller. No voltage here points to a failed AVR or broken wiring. This step requires opening the generator access panel -- proceed only if you are comfortable doing so safely.
Disconnect non-essential loads from the ATS or transfer switch and attempt a restart in MANUAL mode. If the generator holds voltage without tripping, the unit was simply overloaded. Shed large motor loads (well pumps, HVAC compressors, sump pumps) and restart them one at a time after the generator has stabilized.
Inspect all AC output wiring connections at the generator output lugs and at the ATS for corrosion, loose terminations, or heat damage. Tighten any loose connections to manufacturer torque spec. Corroded terminals can cause enough resistance to drop voltage under load.
If the generator produces zero AC voltage at startup rather than low voltage, the rotor may have lost residual magnetism. A certified technician can perform a flash procedure to restore residual field. Do not attempt to flash the field yourself without proper training and a DC power source -- incorrect polarity can permanently damage the AVR.
If all external checks pass and the fault persists, the AVR board itself likely needs replacement or the stator windings need an ohm and megohm test. At this point, call a Kohler-certified technician. Stator testing requires specialty equipment and the repair cost can be significant.
Frequently Asked Questions
What does Kohler code 13 mean?
Code 13 means the RDC2 controller detected that the AC output voltage dropped below the minimum acceptable limit. The generator shut itself down to protect your home appliances and its own internal components from running on unstable low voltage. The most common reasons are an overloaded circuit, a failing AVR, or an excitation problem inside the alternator.
Can my generator still run with code 13?
No. Code 13 is a hard shutdown fault on Kohler RDC2 controllers. The generator will not stay running and will not automatically power your home until the fault is cleared. You need to identify and fix the underlying cause, then press OFF and restart in AUTO mode to resume normal standby operation.
How much does it cost to fix Kohler code 13?
It depends entirely on the cause. If overload is the problem, fixing it costs nothing. A replacement AVR board typically costs $150-$400 in parts, and a technician may charge $300-$700 total for diagnosis and replacement. If the stator or rotor has failed, expect $1,000-$2,500 or more for a professional repair.
Will my generator start the next time the power goes out?
Not reliably. Once code 13 is logged, the RDC2 controller will attempt a restart the next time utility power fails, but if the root cause has not been fixed, the generator will start briefly and then shut down on the same fault. Your home will lose backup power. Prioritize diagnosing and clearing this fault before the next outage.