Plain English: Your Kohler generator’s Advanced Diagnostic Active shutdown is a controlled test cycle—the controller deliberately stops the engine during exercise mode, restarts it at full speed for 2 minutes, then shuts down and returns to AUTO. This is normal behavior, not a failure.
What Is Advanced Diagnostic Active?
If you own a Kohler standby generator with an RDC2 controller (found on 14RCA, 20RCA, 26RCA, and RCAL models), you may see “Advanced Diagnostic Active” logged in the event history after a shutdown. This message indicates that your generator has just completed a built-in diagnostic test cycle.
Here’s what happens during this cycle: The controller triggers a shutdown while the engine is running at reduced speed during a routine exercise period. This is intentional. After the shutdown, the generator automatically restarts and runs at full speed for approximately 2 minutes. Once that 2-minute window closes, the engine stops again, and the controller returns to AUTO mode, ready to respond to a power outage or the next scheduled exercise.
The purpose of this test is to verify that your generator can reliably shut down and restart under controlled conditions. It’s a preventive health check built into the RDC2 controller firmware.
Why Does This Happen?
Standby generators sit idle for long stretches, sometimes weeks or months between actual power outages. During that downtime, fuel can degrade, spark plugs can foul, and mechanical components can develop issues that only show up under load. The Advanced Diagnostic Active cycle is designed to catch these problems before they matter—before you actually need the generator during a real emergency.
By forcing a shutdown and restart sequence, the controller tests several critical functions at once: fuel delivery, ignition timing, load acceptance, and engine stability. If something is wrong, the diagnostic will often reveal it, and you’ll see a fault code in the event history that points to the real issue.
Is This Normal?
Yes. If your generator completed the Advanced Diagnostic Active cycle and then returned to AUTO mode without displaying any other fault codes, everything worked as intended. You may see this message appear in your event history periodically—the exact frequency depends on your controller’s settings and exercise schedule.
However, if the generator does not restart after the diagnostic shutdown, or if it fails to return to AUTO mode, then you have a secondary problem that needs attention.
Diagnostic Checklist
If you see Advanced Diagnostic Active in your event history but the generator is running normally, follow these steps in order:
- Review the complete event history. Check if any other fault codes appear alongside or after the Advanced Diagnostic Active message. Common codes might reference fuel, ignition, battery, or load issues. Write down any codes you see.
- Verify the generator returned to AUTO mode. Look at your controller display. The mode should show AUTO, not MANUAL or EXERCISE. If it’s stuck in another mode, that’s your next clue.
- Check fuel level and quality. Open the fuel cap and look inside. The tank should be at least three-quarters full. If the fuel has been sitting for more than 6 months, it may have degraded. Stale fuel is a common cause of restart failures. Consider draining and replacing it if it smells varnished or looks discolored.
- Inspect the spark plug. Turn off the generator and let it cool for 10 minutes. Remove the spark plug wire and unscrew the plug. The electrode should be light tan or gray. If it’s black and sooty, or if the gap looks too wide, replace the spark plug with the correct type for your model.
- Check the battery voltage. If your generator has a 12V battery (used for starting and controller operation), measure its voltage with a multimeter. It should read at least 12.5 volts at rest. If it’s below 12 volts, the battery may not have enough power to turn the starter motor reliably.
- Review your exercise schedule. If the Advanced Diagnostic Active cycle is running more frequently than you’d expect, check your controller settings. Some setups run diagnostics weekly; others run them monthly. Adjust the schedule if needed to match your preference.
- Run a manual exercise cycle. If the generator hasn’t been exercised recently, manually start it and let it run under load for 10–15 minutes. This clears out fuel lines and confirms the engine is responsive.
When to Call a Pro
Contact a qualified generator technician if any of the following occur:
- The generator shuts down during the Advanced Diagnostic Active cycle but does not restart within a few seconds.
- The generator restarts but cannot reach full speed or runs rough and unstable.
- The controller displays a fault code in addition to Advanced Diagnostic Active (such as a low-oil code, overspeed code, or battery code).
- The generator does not return to AUTO mode after the diagnostic completes.
- You see Advanced Diagnostic Active appearing multiple times in a single day or the event history shows repeated restart failures.
- The engine cranks but does not fire when the diagnostic attempts to restart it.
These scenarios indicate a secondary fault that requires professional diagnosis and repair. Attempting to force-start the generator or bypass the controller’s safety shutdowns can cause engine damage.
Parts You May Need
If your diagnostic checklist reveals a specific issue, you may need one or more of these common replacement items:
- Spark plug (correct type and heat range for your model)
- Oil filter
- Air filter
- Fuel filter
- 12V battery (if applicable to your model)
- Fresh fuel stabilizer or fuel system cleaner
Always consult your owner’s manual for the correct part numbers and specifications before ordering.
Frequently Asked Questions
Does Advanced Diagnostic Active mean my generator is broken?
No. Advanced Diagnostic Active is a normal, scheduled test cycle. It only becomes a concern if the generator fails to restart afterward or if other fault codes appear in the event history. A successful diagnostic cycle is actually a good sign—it means your controller is working and monitoring the system.
How often should Advanced Diagnostic Active run?
The frequency depends on your RDC2 controller’s configuration and your exercise schedule. Some units run a diagnostic every week, others monthly. You can usually adjust this setting in the controller menu. Check your owner’s manual for the specific procedure for your model.
Can I disable Advanced Diagnostic Active?
Disabling the diagnostic cycle is not recommended. This test helps catch fuel degradation, spark plug fouling, and battery issues before they prevent your generator from starting during an actual power outage. If the diagnostic is interfering with your schedule, adjust the exercise time instead of turning off the test.
What should I do if the generator doesn’t restart after the shutdown?
First, wait 30 seconds and observe whether the controller attempts another restart cycle automatically. If the generator remains off and no restart occurs within 2 minutes, check the event history for fault codes. If you see codes related to fuel, ignition, or battery, address those issues first. If the history is clear but the generator still won’t start, contact a technician—the problem may be a weak starter motor or a fuel delivery issue that requires professional service.
Disclaimer
This article provides general information about Kohler RDC2 controller behavior and is not a substitute for your generator’s owner’s manual or manufacturer’s service documentation. Always consult the manual specific to your model before performing maintenance or troubleshooting. If you are unsure about any procedure, contact a qualified generator technician or Kohler customer support. Improper maintenance or repair can void your warranty and create safety hazards.
Source: Information adapted from official manufacturer documentation (reference). Always consult your generator owner’s manual for model-specific procedures.