Coolant Temperature Sensor High Voltage or Open Circuit
What does KOHLER-COMMAND-P0118 mean?
The KOHLER-COMMAND-P0118 (Kohler Command) diesel fault code means: Coolant Temperature Sensor High Voltage or Open Circuit. This is a moderate severity code.
Common Symptoms
- Engine starts hard in cold weather because ECM cannot read actual coolant temp
- Check engine light or fault LED blinks the P0118 pattern on the equipment dash
- Engine may run rich or with a heavy choke for longer than normal during warmup
- Fuel economy seems worse than usual, especially on short cuts
- Engine overheating warning does not trigger even under heavy load
- Kohler PCM diagnostic tool shows ECT sensor voltage stuck near 5 volts or above spec
- Engine may idle rough at startup because ECM is using a fixed substitute temperature value instead of a real reading
Probable Causes (Ranked by Likelihood)
- Coolant temperature sensor failed internally, causing an open circuit and pulling the signal line to full reference voltage Very Likely
- Wiring connector at the coolant temperature sensor is corroded, loose, or pushed back in the housing, breaking the circuit Very Likely
- Broken or chafed wire in the ECT sensor harness between the sensor and the ECM, creating an open in the signal or ground wire Likely
- ECT sensor connector O-ring or seal missing, allowing moisture intrusion into the connector and corroding the terminals Likely
- ECM reference voltage circuit issue causing false high voltage reading at the sensor input pin Possible
- Wrong replacement sensor installed with incorrect resistance range for the Kohler Command ECM calibration Possible
- ECM internal failure affecting the coolant temperature sensor input circuit Less Likely
Step-by-Step Diagnostic Procedure
Start with the Kohler PCM diagnostic tool if you have access to it. Connect it and check live ECT sensor voltage. A reading of 4.8 to 5.0 volts at normal ambient temperature confirms an open circuit or failed sensor. A healthy sensor at 70 degrees F should read roughly 2.0 to 2.5 volts.
Turn the key off and locate the coolant temperature sensor on the engine block or cylinder head. On most ECV and EFI Command engines it is a single two-wire sensor threaded into the block. Disconnect the connector and inspect both terminals for green corrosion, bent pins, or moisture inside the housing.
With the sensor connector unplugged and the key ON, measure voltage between the signal wire (usually the dark green or gray wire) and engine ground. You should see close to 5 volts, which is the ECM reference voltage. No voltage here means a wiring or ECM problem, not the sensor itself.
With the connector still unplugged, use a multimeter set to resistance (ohms) and measure directly across the two sensor terminals. At room temperature (around 70 degrees F) a good Kohler ECT sensor should read approximately 2,000 to 3,000 ohms. An open circuit reading (OL or infinite) confirms the sensor has failed internally.
If the sensor resistance checks out, inspect the full harness from the sensor back toward the ECM. Look for spots where the wiring runs near hot exhaust components, sharp metal edges, or zip-tie pinch points. Flex the harness while watching the ohm reading. A spike to OL when you move the harness confirms a broken wire.
Reconnect the sensor connector and measure resistance between the sensor ground wire at the connector and a known good chassis ground. You should see less than 0.5 ohms. Higher resistance means a bad ground path that can cause false high-voltage readings at the ECM input.
If the sensor and harness both check out and the code returns, connect the Kohler PCM diagnostic tool and check for any other active codes, especially P0117 (ECT low voltage) or P0125 (insufficient coolant temp for closed-loop). Multiple ECT codes together can point toward the ECM input circuit. At that point escalate to a dealer technician.
Frequently Asked Questions
What does Kohler Command code P0118 mean?
It means the ECM is seeing a voltage signal from the coolant temperature sensor that is higher than the normal operating range. Usually this means the sensor signal wire has gone open circuit, either because the sensor itself failed or because the wiring connector has a break or corrosion. The ECM cannot read actual coolant temperature, so it substitutes a fixed default value and sets this code.
Can I still mow with a P0118 active?
You can run the mower, but with caution. The ECM is flying blind on coolant temperature, which means it cannot trigger an overheat warning and may run a richer-than-needed fuel mixture. On hot days or under heavy load, you risk not getting an early overheat alert. Keep cuts shorter, watch for any smoke or unusual smells, and get the sensor sorted before your next full day of work.
How much does it cost to fix P0118 on a Kohler Command EFI engine?
If it is just the sensor, the part runs about $15 to $40 depending on the specific Kohler part number for your engine. Most landscapers can swap the sensor in under 20 minutes with basic hand tools. If the problem is a wiring repair or connector replacement, add another $10 to $20 in parts. A dealer tech will typically charge $80 to $180 for diagnosis and the repair combined, depending on shop rates in your area.
Will the mower overheat without me knowing if P0118 is active?
Yes, that is the real risk with this code. Because the ECM cannot read the actual coolant temperature, the overtemperature protection logic that would normally cut power or trigger a warning is compromised. The engine can overheat before you see any sign of it. Do not ignore this code on hot days or when mowing thick, wet grass.