SPN 3661 FMI 6 is the Detroit DD Series fault code for the cylinder #3 fuel injector amplifier control valve, and this six-page guide walks a technician through the diagnostic sequence. Sourced from TruckFaultCodes.com and built around Detroit Diesel Power Service Literature, it covers connector inspection, resistance measurements and Motor Control Module (MCM) checks. The EPA10 / GHG14 document applies to DD-series engines and to Series 60 (DDEC VI) platforms from EPA 2007 onward.
What SPN 3661 FMI 6 means
Under FMI 6, the Motor Control Module records the cylinder #3 injector amplifier control valve as a shorted valve circuit. The typical enabling condition is fis_nop_act above 1.85 with a two-second duration. When the code sets, the MIL and CEL dash lamps illuminate and the engine applies a 25% derate; verification runs at 1400-1500 engine RPM for one minute.
Official document details
| Brand | Detroit |
| Model coverage | DD Series (EPA 2007-Current); Series 60 (DDEC VI – EPA 2007) |
| System / Group | DTCs — fuel injector amplifier control valve |
| Fault code | SPN 3661 / FMI 6 (EPA10 / GHG14) |
| Source | TruckFaultCodes.com (© 2017) |
| Pages | 6 |
| File size | 0.6 MB |
| Format | |
| Language | English |
Troubleshooting steps covered
- Check for multiple codes (SPN 168 or SPN 723) and service those first
- Start and warm the engine above 71°C (160°F)
- Inspect the rear fuel injector harness 14-pin connector for bent pins and water or oil intrusion
- Measure and record resistance on the valve-cover side of the injector harness
- Remove the rocker cover and rear injector harness, then measure injector pin resistance
- Check the engine-side harness to the MCM 120-pin connector
- Install a test MCM and confirm whether the fault stays active
Applicable Detroit engines
This guide applies to Detroit DD-series engines and Series 60 (DDEC VI) units meeting EPA 2007 (EPA10 / GHG14) emissions rules. Removal steps for the two-piece fuel injector wiring harness, the injector and the MCM reference Detroit Diesel Power Service Literature. For OEM powertrain data and dealer support, see the official Detroit powertrain website.
Who needs this SPN 3661 FMI 6 guide
Diesel technicians, fleet maintenance teams and workshop owners diagnosing a cylinder #3 injector fault on a Detroit DD engine use this guide to confirm whether the fuel injector harness, the injector or the Motor Control Module is at fault before replacing a part.
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