Error Codes: Kohler 4 and 7
What they mean: Kohler Decision-Maker controller code 4 means the engine is running below 90 percent of its rated speed. Code 7 means the generator output voltage is below 90 percent of rated voltage for more than one second.
These two codes are connected. Generator output voltage depends directly on engine RPM. An engine running at 80 percent of rated speed cannot produce full voltage. Underspeed causes undervoltage. When you see code 7, always check for code 4 first.
Kohler residential generators run at either 1800 RPM (for 4-pole alternators on larger units) or 3600 RPM (for 2-pole alternators on smaller units). At either speed, the alternator must spin at the correct RPM to produce 60 Hz output and maintain proper voltage.
Common Causes
- Engine overload — Too much load for the engine to maintain speed. The governor opens the throttle fully, but the engine cannot keep up.
- Fuel supply restriction — Low natural gas pressure, a clogged fuel filter, or a partially closed fuel valve. The engine starves for fuel and cannot reach full RPM.
- Governor actuator failure — The actuator that controls the throttle plate fails mechanically or electrically. The throttle cannot open fully.
- Faulty AVR (Automatic Voltage Regulator) — The AVR fails to supply proper field current to the alternator. Voltage drops even when engine speed is correct.
- Worn or dirty brushes — On brush-type alternators, worn carbon brushes or dirty slip rings reduce field current to the rotor. Output voltage drops.
- Open rotor or stator winding — A breakdown in the alternator winding insulation or a complete open circuit stops voltage production.
- Low excitation (residual magnetism) — The alternator loses its residual magnetism and cannot build voltage on startup.
Step-by-Step Fix
1. Read both error codes from the Decision-Maker controller.
Press the MENU button to scroll to the Alarm screen. The controller displays all active and stored alarms. Code 4 (underspeed) triggers first if RPM drops. Code 7 (undervoltage) follows if the low voltage condition persists past one second.
If code 4 is active, start with the governor and fuel system. If code 7 is active without code 4, the AVR and alternator are the focus.
2. Measure the engine RPM.
Use a contact tachometer on the center of the crankshaft or a non-contact laser tachometer on the fan blades. For a 1800 RPM generator, the target is 1800 RPM at no load and 1800 RPM under full load (a 4-pole alternator). For a 3600 RPM generator (2-pole), target is 3600 RPM.
Most Kohler generators have an RPM measurement test point or display in the controller diagnostic menu. Navigate to the Engine Data screen on a Decision-Maker controller to see live RPM.
Low RPM points to the governor, fuel system, or engine mechanical problem.
3. Test the fuel supply.
Check the fuel valve position. It should be fully open. On natural gas installations, measure pressure at the generator inlet using a manometer. Natural gas target is 5-7 inches WC at full load. LP target is 8-12 inches WC.
A fuel pressure drop under load indicates an undersized gas line, a faulty regulator, or a restriction in the supply line. This is a common problem on residential installations where the gas line is too small for the generator rating.
4. Test the governor actuator.
The governor actuator (Kohler part A-435316 on many models) controls the throttle plate position. The controller sends a signal to the actuator to maintain target RPM.
With the generator running, observe the actuator linkage. It should move smoothly as load changes. A stuck or sluggish actuator points to mechanical binding or an electrical fault.
Test the actuator winding resistance. Disconnect the actuator connector. Measure across the terminals with a multimeter set to ohms. The reading should match the specification from the actuator label. An open or shorted winding means the actuator needs replacement.
5. Measure the output voltage at the main breaker.
Set your multimeter to AC voltage. Measure between L1 and L2 at the main breaker terminals. Target is 240V plus or minus 5 percent (228V to 252V). Measure L1 to neutral and L2 to neutral. Each leg should show 120V plus or minus 5 percent (114V to 126V).
Run the generator at no load first. Then apply a load of at least 50 percent of the generator rating. Voltage should not drop more than 5 percent under load. A larger drop points to a load issue, an AVR problem, or an alternator winding fault.
6. Test the AVR field voltage.
Locate the AVR on the generator. It is typically mounted inside the control panel or on the alternator housing. Identify the F+ and F- terminals on the AVR. These supply DC field current to the rotor.
With the generator running, measure DC voltage between F+ and F-. The reading should fluctuate as the AVR regulates output. Expect 5-30V DC. A steady reading of 0V means the AVR is not supplying field current. A reading that does not change when load changes means the AVR is stuck and likely failed.
7. Test rotor resistance.
Disconnect the brushes from the slip rings or disconnect the brush block. Measure resistance between the two slip rings. An 1800 RPM alternator rotor typically reads 15-25 ohms. A reading of infinity means an open rotor winding. A reading near 0 means a shorted rotor. Both conditions require alternator disassembly for repair.
8. Try a field flash.
If the generator starts but produces no voltage and the AVR appears operational, the rotor may have lost residual magnetism. Apply 12V DC from a battery to the exciter field through a 10-amp inline fuse for 3-5 seconds. Remove the battery connection and restart the generator.
This procedure rebuilds residual magnetism in the rotor poles. You may need to repeat it once or twice. If field flashing does not restore voltage, the problem is in the AVR, rotor, or stator windings.
Error Code Reference Table
| Code | Meaning | Likely Cause |
|---|---|---|
| 4 | Underspeed (RPM below 90% of rated) | Overload, fuel restriction, governor actuator failure |
| 6 | Overvoltage (Voltage above 110% of rated) | AVR failure, loose voltage sense wiring |
| 7 | Undervoltage (Voltage below 90% of rated) | Underspeed, AVR failure, brush wear, rotor fault |
| 13 | Voltage regulator fault | AVR not producing output, wiring fault |
Parts That May Need Replacement
| Part | Typical Cost | Notes |
|---|---|---|
| Governor actuator (A-435316) | $100-$300 | Test winding resistance first |
| AVR (GM71908 residential) | $150-$400 | Verify supply voltage before replacing |
| AVR (GM72405 industrial) | $300-$600 | Test all inputs and outputs |
| Carbon brushes (set) | $20-$60 | Check length; replace if below 3/8 inch |
| Fuel regulator | $50-$150 | Common failure on NG installations |
When to Call a Professional
If you have verified fuel pressure, tested the governor actuator, confirmed proper engine RPM, and the generator still shows code 4 or code 7, call a Kohler service technician. Alternator winding faults, controller board failures, and internal engine governor problems require specialized diagnostic equipment.
A generator that runs at correct RPM but produces low voltage (code 7 without code 4) has an alternator or AVR problem. Do not attempt to bypass the AVR or jury-rig field voltage. Incorrect excitation can damage the alternator windings or create dangerous overvoltage conditions.
Tell the technician: “Generator shows [code 4 / code 7 / both]. I measured [RPM reading] at [load condition]. Voltage reads [L1-L2 reading] at no load and [reading] under [load amount]. Field voltage at the AVR reads [voltage].”