SPN 1659 FMI 31 – Thermostat cool down Fault Code (Symptoms, Causes & Fix)
The SPN 1659 FMI 31 fault code is a diesel engine diagnostic trouble code indicating an issue with the thermostat cool down process. This code suggests that the engine control module (ECM) has detected a problem with the cooling system, potentially leading to improper engine temperature regulation.
What SPN 1659 FMI 31 Means
The SPN 1659 FMI 31 fault code specifically relates to the thermostat's ability to regulate engine temperature during the cool down phase. The ECM monitors the coolant temperature and expects it to decrease at a certain rate when the engine is turned off. If the temperature does not drop as expected, the ECM logs this fault, indicating a potential issue with the thermostat or related components.
Common Causes of This Fault Code
- Thermostat stuck open
- Faulty coolant temperature sensor
- Low coolant level
- Air trapped in the cooling system
- Faulty water pump
Symptoms Drivers May Notice
When the SPN 1659 FMI 31 fault code is active, drivers may notice the engine running cooler than normal, especially during idle or low-load conditions. There might also be a noticeable delay in reaching optimal operating temperature, which can affect fuel efficiency and engine performance.
How Technicians Diagnose This Fault
Technicians begin diagnosing the SPN 1659 FMI 31 fault code with a visual inspection of the cooling system components, checking for obvious signs of damage or leaks. Next, they verify the coolant level and inspect for air pockets by bleeding the system if necessary. Using a scan tool, they monitor the coolant temperature sensor readings to ensure they align with expected values. If the thermostat is suspected, it is tested for proper operation, and a coolant system pressure test is conducted to identify any leaks. Multimeters and diagnostic software are essential tools in this process.
Common Misdiagnosis
Technicians often misdiagnose this fault by assuming a faulty coolant temperature sensor without verifying its readings. Another common mistake is replacing the thermostat without checking for trapped air or low coolant levels, which can also cause similar symptoms. Additionally, overlooking a partially blocked radiator or cooling passages can lead to incorrect conclusions.
Can You Keep Driving?
While the truck may continue to operate with the SPN 1659 FMI 31 fault code active, it is not advisable to ignore it. Prolonged driving with this issue can lead to inefficient engine performance and increased fuel consumption. In some cases, it may trigger engine derate or protection modes if the cooling system fails to maintain proper temperature regulation.
Mechanic's Diagnostic Tip
Always verify the coolant level and bleed the system for trapped air before replacing any components. This simple step can often resolve the issue without unnecessary part replacements.
Frequently Asked Questions
Q: Can a faulty thermostat cause overheating?
A: Yes, a thermostat stuck closed can cause overheating, but in the case of SPN 1659 FMI 31, it is more likely stuck open, causing the engine to run cooler.
Q: How often should the coolant be checked?
A: Coolant levels should be checked regularly, ideally during routine maintenance or oil changes, to ensure proper cooling system function.
Q: Is it safe to drive with a low coolant level?
A: Driving with low coolant can lead to engine overheating and potential damage. It is crucial to address low coolant levels promptly.
