Engine Coolant Temperature Above ECM Warning Threshold
The JLG-4351-15 (ADE) diesel fault code means: Engine Coolant Temperature Above ECM Warning Threshold. This is a critical severity code.
- Keep driving?
- No -- stop driving
- DIY difficulty
- moderate
- Estimated cost
- $20-$80 DIY for coolant, thermostat, or sensor replacement. Pro shop diagnosis and repair ranges $150-$600 depending on root cause. Head gasket repair runs $800-$1,800 at a JLG-authorized service shop.
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Common Symptoms
- Platform LCD scrolls SPN 4351 FMI 15 as an active fault
- Engine power drops noticeably as the ADE derate kicks in before shutdown
- All boom and drive functions slow down or stop responding at the platform controls
- Engine shuts down mid-cycle and will not restart until it cools down
- Coolant temperature gauge or bar-graph on the ground control panel is pegged high
- Red engine warning lamp on the ground control panel stays on solid
- Machine may display a STOP or engine fault icon alongside the fault code on the platform LCD
Probable Causes (Ranked by Likelihood)
- Low coolant level due to leak at hose, fitting, or radiator Very Likely
- Clogged or debris-blocked radiator core reducing airflow Very Likely
- Thermostat stuck closed, preventing coolant circulation Likely
- Cooling fan not spinning or spinning at reduced speed due to failed fan motor, clutch, or relay Likely
- Coolant temperature sensor failed or wiring to ECM shorted high, causing false over-temp reading Possible
- Water pump impeller worn or failed, reducing coolant flow Possible
- Internal head gasket failure allowing combustion gases into coolant circuit Less Likely
Step-by-Step Diagnostic Procedure
Before touching anything, let the engine cool for at least 30 minutes. Never open the radiator cap on a hot engine. Check the platform LCD for any additional active codes using JLG Analyzer software via the data link at the ground controls, especially any codes related to fan speed or coolant sensor circuit faults.
Check coolant level in the overflow reservoir and the radiator (when cold). Low coolant is the most common cause. Add the correct premixed coolant if low, then inspect all hoses, clamps, and the radiator core for visible leaks or wet spots.
Inspect the radiator core from the front and rear. Clear any mud, debris, or compacted dust from the fins using compressed air blown from the engine side outward. A partially blocked core can cause over-temp even with a full coolant level.
With the engine at operating temp and the hood open, watch the cooling fan. Confirm it is spinning at full speed. On belt-driven fans, check belt tension and condition. On electric fan circuits, check the fan relay and fuse in the engine bay fuse panel and measure 12V DC supply at the fan motor connector.
Check the thermostat operation. If the upper radiator hose stays cold for more than 5 minutes at idle after the engine warms up, the thermostat is likely stuck closed. Thermostat replacement is straightforward on most Deutz, Perkins, and Kubota engines in JLG equipment. Drain, swap, and refill coolant.
If coolant level and fan operation check out, suspect the coolant temperature sensor or its wiring. Locate the sensor on the engine block or thermostat housing. Disconnect the connector and measure resistance across the sensor terminals. Compare the reading against the engine ECM spec in the service manual for your specific engine. A reading of open circuit (OL on a multimeter) or near zero ohms both indicate a failed sensor. Also inspect the wiring harness from the sensor back to the ECM connector for chafing or corrosion.
If the code clears after repair but returns quickly under normal ambient temperatures, and coolant flow looks correct, pressure-test the cooling system to rule out a head gasket leak. Check the coolant in the reservoir for a milky appearance or oil film, and use a combustion leak test kit to check for exhaust gases in the coolant. This step requires specialty tools and should be escalated to a JLG-authorized service shop.
After any repair, use JLG Analyzer software via the data link at the ground controls to clear the stored fault code and verify the repair by running the engine to operating temperature while monitoring coolant temp live data on the platform LCD or the analyzer.
Frequently Asked Questions
What does JLG fault code 4351/15 mean?
It means the ADE controller received a signal from the engine ECM that coolant temperature climbed above the programmed warning threshold. On diesel-powered JLG machines this triggers a power derate first, then an automatic engine shutdown to prevent serious engine damage. The code will show on the platform LCD and on the ground control display.
Can the machine still operate with this code active?
No. Once 4351/15 is active the ADE will derate engine power and shut the engine down. Boom and drive functions will not work normally. Do not attempt to restart until the root cause is found and corrected. Running an overheating diesel risks warped cylinder heads and blown head gaskets.
How much does it cost to fix JLG code 4351/15?
If it is just low coolant or a blocked radiator you can fix it yourself for under $50 in parts. A thermostat or coolant temperature sensor swap runs $20 to $80 in parts and an hour of labor at a shop. If the water pump or head gasket is the cause, expect $400 to $1,800 at a JLG-authorized service shop depending on engine model and labor rates.
Will the machine start again after it cools down?
It may crank and start after the engine cools, but 4351/15 will reappear as a stored fault and the machine will overheat again if the underlying cause is not fixed. Use JLG Analyzer software via the data link at the ground controls to clear the stored code only after the repair is confirmed. Do not clear and return a machine to service without verifying the coolant system is functioning correctly.