SPN 1659 FMI 1 is the Detroit DD-series code for a thermostat error detected on the coolant inlet temperature monitor, and this sheet lists the enabling conditions and the warm-up test that confirms it. Read it before replacing the thermostat.
Official document details
| Brand | Detroit |
| Model coverage | DD13, DD15, DD16 (EPA 2007-current) |
| System / group | Engine cooling — thermostat monitor |
| Fault code | SPN 1659 / FMI 1 |
| Publication no. | Not printed on sheet |
| Year / revision | Reference dated 2017 |
| Pages | 3 |
| File size | 0.3 MB |
| Format | |
| Language | English |
Document table of contents
- Fault header: SPN 1659 / FMI 1 — thermostat error detected
- Monitored parameter and detailed enabling conditions
- Verification drive cycle and operating temperature check
- Steps 1-4 — cooling-system check and 85C warm-up test
- Steps 5-7 — inspect and resistance-check the coolant sensors
SPN 1659 FMI 1 troubleshooting steps
The monitor watches the coolant inlet temperature sensor once per drive cycle under tight enabling conditions — engine speed 500-2500 RPM, coolant over 65C, battery above 11 VDC, average load 25-55%, and a cold coolant inlet at key-on. Step 1 clears any cooling-system issues first, then Step 2 brings the engine to 85C (185F). If it will not reach operating temperature, the thermostat is replaced per Power Service literature. If it does, the inlet and outlet coolant temperature sensors are disconnected, inspected for bent, spread or corroded pins, and resistance-checked against the specification chart, replacing any sensor out of range.
Applicable truck models
This procedure covers Detroit DD-series engines — DD13, DD15 and DD16 on the EPA 2007-current platform — and shares its SPN/FMI structure with related Series 60 DDEC VI applications. The verification drive is best done for 30 minutes with a trailer; confirm the emissions calibration (EPA07, EPA10 or GHG14) first. For engine serial-number and calibration confirmation, use the Detroit Diesel official support site.
Who needs the SPN 1659 FMI 1 manual
Fleet and shop technicians use this sheet to confirm a thermostat error without guesswork, meet the exact drive-cycle conditions the monitor needs, and decide between a thermostat and a coolant temperature sensor.
The full PDF is available in the DiagTechPro manuals library; a free account is required to download. Source: Detroit SPN/FMI fault-code reference (© 2017); coolant temperatures are logged with the Detroit Diesel Diagnostic Link (DDDL).
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