If you’ve read this site’s Generac error-code guide [→ /maintenance-repair/generac-error-codes/] and own a Kohler instead, the first thing to know is that Kohler’s system works a little differently. Rather than leading with a four-digit numeric code, Kohler’s residential controller (RDC2 on current RCA/RCAL and RESV/RESVL model lines) displays plain-text fault descriptions directly on its LCD screen — “Low Battery,” “Overcrank,” “High Coolant Temperature” — which is a genuine usability advantage: you’re reading what’s wrong in words, not decoding a lookup table first [→ /standby-generators/best-whole-house-generators/].
This guide covers how the RDC2’s fault and warning system works, the faults reported most consistently across owners and technicians, what you can check yourself, and how Kohler’s automatic cranking sequence behaves — since understanding that sequence explains a lot of what you’ll see on the display during a failed start.
How Kohler’s Fault System Works
The RDC2 controller (Kohler’s standard residential/commercial controller on current model lines) uses:
- A 2-line LCD screen displaying plain-text status messages and fault descriptions — readable without a code lookup, a deliberate design choice
- LED indicators for utility power status, generator availability, and transfer switch position
- Two severity levels, conceptually similar to Generac’s yellow/red system: warnings that don’t necessarily stop the unit, and fault shutdowns that do
- An Overview menu (Select button) that shows the active fault message directly, and a clear-fault procedure accessed the same way
The cranking sequence worth understanding on its own: current RDC2 firmware on many models attempts three starting tries (15 seconds of cranking, 15 seconds off, repeated three times), then pauses for 45 seconds before attempting another full set of three tries. If the engine still hasn’t started after these cycles, the controller shuts down and reports an overcrank-type fault. Knowing this sequence matters practically: hearing the generator crank, pause, and crank again during a failed start attempt is the system working as designed, not a separate malfunction — the eventual fault message is what tells you it gave up after exhausting its programmed attempts.
The Faults Reported Most Consistently
Low Battery Voltage
What it means: the controller checks battery voltage before both self-tests and real starts specifically to verify the battery can deliver the cranking power needed — this pre-check is a deliberate design feature, not an afterthought. A low reading here prevents a start attempt that would likely fail anyway.
What to check: battery terminals for corrosion and tightness; battery age (2–3 years is the standard replacement window regardless of a passing simple voltage check [→ /maintenance-repair/generator-battery-replacement/]); a proper load test, since — as with Generac units — a battery can show acceptable resting voltage and still fail under real starting load.
Overcrank
What it means: the engine failed to start after the full programmed cranking sequence described above (three sets of three 15-second attempts, with 45-second pauses between sets on current firmware).
What to check: fuel supply first — propane tank level [→ /fuel-running-costs/propane-tank-size-standby-generator/] or natural gas valve/pressure [→ /installation-costs/natural-gas-line-sizing-standby-generators/]; battery health as above; and, if fuel and battery both check out, spark plug condition, which Kohler’s own troubleshooting guidance flags as a common contributor to starting failures [→ /maintenance-repair/generator-wont-start-after-sitting/].
High Coolant Temperature (liquid-cooled models)
What it means: on Kohler’s liquid-cooled units, the engine exceeded a safe coolant temperature threshold. Notably, Kohler’s controller runs cooling fans for a set cooldown period (commonly around 2 minutes) after shutdown specifically to evacuate heat from the engine compartment before fully powering down — a detail worth knowing so you don’t mistake continued fan operation after a shutdown for a separate problem.
What to check: coolant level; clearances and airflow around the unit [→ /installation-costs/generator-distance-from-house/]; for a unit that has been running near its rated capacity for an extended period in hot weather, this may simply be the system doing its job under genuinely demanding conditions [→ /standby-generators/can-a-generator-power-central-air-conditioning/].
Low Oil Pressure / Oil Level
What it means: the same safety-critical shutdown every standby generator brand implements — oil pressure or level outside the safe operating range, checked both before starts and during operation.
What to check: oil level against the dipstick, topped up with the correct grade if low [→ /maintenance-repair/generator-oil-guide/]; if the level is correct and the fault persists, the sensor itself may need professional attention. As with any oil-pressure fault on any brand, this isn’t a code to keep pushing through with repeated restart attempts.
ATS Communication Errors
What it means: the RDC2 controller lost communication with the Kohler RXT transfer switch it’s designed to interface with — a fault category specific to Kohler’s integrated controller-and-transfer-switch architecture [→ /installation-costs/transfer-switch-cost/]. Kohler’s own service bulletins note this can follow improper handling of the communication wiring (commonly called RBUS) during installation or service work — for instance, connecting or disconnecting those wires while the controller is still powered.
What to check: this is squarely a technician-level fault — RBUS wiring should only be connected or corrected with the controller de-energized, and diagnosing a communication fault typically requires verifying continuity across multiple wires with proper equipment. Not a homeowner DIY item.
Overvoltage / Undervoltage
What it means: the generator’s output voltage moved outside its safe operating band — potentially a load issue, a regulation problem, or a controller/sensing fault.
What to check: total connected load against the unit’s rating [→ /standby-generators/what-size-generator-do-i-need/]; beyond a basic load sanity-check, this fault category generally warrants professional diagnosis.
How to Clear a Fault on the RDC2 Controller
The general procedure (verify against your specific model’s manual, as exact menu steps can vary by firmware version):
- Resolve the underlying cause first — clearing a fault without fixing what triggered it typically just reproduces the same fault on the next start attempt.
- Access the Overview menu via the Select button — the active fault message displays here.
- Press Select again to step toward the clear-fault option; the display will prompt for confirmation (commonly a YES/NO selection) to clear the active alarm.
- Return the unit to AUTO and monitor the next self-test or start attempt to confirm the fault doesn’t return.
What the Exerciser Checks Before Every Run (Useful Diagnostic Context)
Kohler’s programmable exerciser doesn’t just spin the engine — current RDC2 systems run a genuine pre-flight check before and during each exercise cycle, verifying: ATS connection to the controller, battery voltage (specifically checked as a health measurement, not just a go/no-go gate), communication integrity across the controller’s internal buses, engine speed, and oil pressure. Understanding this is useful context for owners: a unit that has been passing its weekly exercise cycles is a reasonably good sign, but the exerciser’s checks are still a lighter-duty verification than a real full-load start — the same caution this site gives Generac owners about self-tests not fully ruling out a marginal battery [→ /maintenance-repair/generac-error-code-1505/] applies here too.
Preventing Kohler Faults Before They Happen
The pattern holds across brands: most faults trace back to a short list of maintenance items, not manufacturing defects:
- Battery load-tested and replaced on a 2–3 year schedule — not just judged by whether recent self-tests succeeded [→ /maintenance-repair/generator-battery-replacement/]
- Spark plugs inspected per Kohler’s own maintenance guidance, which specifically flags them as a common contributor to starting problems
- Confirmed fuel supply and quality — propane level, natural gas pressure, and fresh fuel if applicable [→ /fuel-running-costs/how-much-gas-does-a-generator-use/]
- Coolant level and cooling-path clearance checked on liquid-cooled models as part of routine service
- Annual professional service — RBUS wiring, ATS communication, and controller-level diagnostics are dealer territory, and Kohler’s own literature recommends factory-authorized distributors/dealers for installation and service [→ /maintenance-repair/standby-generator-service-cost/]
Frequently Asked Questions
Does Kohler use numeric error codes like Generac? Not primarily — Kohler’s residential RDC2 controller displays plain-text fault descriptions (“Low Battery,” “Overcrank,” “High Coolant Temperature”) directly on its LCD rather than leading with numeric codes, which is a genuine usability difference from Generac’s four-digit code system.
What does “Overcrank” mean on a Kohler generator? The engine failed to start after the controller’s full programmed cranking sequence — commonly three sets of three 15-second crank attempts with pauses between sets. Fuel supply and battery health are the most common underlying causes.
Why does my Kohler generator crank, pause, and crank again before showing a fault? That’s the controller’s designed cranking sequence working normally — multiple attempt sets with pauses between them — not a separate malfunction. The fault message only appears after the full sequence is exhausted without a successful start.
My Kohler generator fan keeps running after shutdown — is that a problem? Generally no — Kohler’s liquid-cooled units run cooling fans for a set period (commonly around 2 minutes) after shutdown specifically to evacuate heat from the engine compartment. This is designed behavior, not a fault.
What does an ATS Communication Error mean on a Kohler system? The RDC2 controller lost communication with the Kohler RXT transfer switch — often related to the RBUS communication wiring, which should only be handled with the controller de-energized. This is a technician-level diagnosis and repair, not a homeowner fix.
Can I clear a Kohler fault code myself? For faults you’ve resolved yourself (like a battery you’ve since replaced or fuel you’ve since restored), yes — the Overview menu provides a clear-fault option. For ATS communication, wiring, or persistent oil-pressure faults, resolve the underlying issue with a technician before clearing.
The Bottom Line
Kohler’s plain-text fault system tells you what’s wrong in words rather than a code you have to look up — but the underlying causes rhyme with every other brand covered on this site: battery health leads the list, followed by fuel supply, with ATS/wiring and persistent oil-pressure or coolant faults belonging to a certified technician. Understand the cranking sequence so a normal multi-attempt start doesn’t read as alarming, keep the battery on a proactive replacement schedule, and let Kohler’s own recommendation — factory-authorized service — handle anything involving the transfer switch or internal controller diagnostics [→ /maintenance-repair/standby-generator-service-cost/].