Detroit Diesel SPN 1322 FMI 31: Multiple Cylinder Misfire Detected
SPN 1322 FMI 31 (FMI 31) — Engine Misfire for Multiple Cylinders - Condition Exists
Quick Answer
SPN 1322 FMI 31 means the ECM has detected misfire on more than one cylinder. It's a High severity fault - the truck is drivable but running rough, and the cause is usually a shared fuel, air, or wiring issue rather than one bad injector.
Overview: Detroit Diesel SPN 1322 FMI 31
This is exactly the kind of code ProfessionalDieselRepair.com warns techs about most - the temptation to swap injectors one at a time on a code that's telling you the problem is shared, not isolated.
Repair Snapshot
Time
2 - 6 Hours
Difficulty
Advanced
Parts Cost
$100 - $2,500
Dealer Labor
$900+
Tools Required
- Detroit Diesel Diagnostic Link (DDDL)
- Diesel Laptops JPRO
- Multimeter
- Fuel Pressure Gauge
What SPN 1322 FMI 31 Means
SPN 1322 FMI 31 means the ECM's misfire monitor has detected misfire activity across more than one cylinder rather than a single injector problem. This points toward a shared cause - fuel supply, air intake, or a wiring harness issue affecting multiple cylinders at once - rather than one bad injector.
Symptoms Drivers Notice
- Rough idle or a noticeable shake felt through the cab
- Check engine light, often with individual cylinder misfire codes alongside it
- Loss of power and poor fuel economy
- Excessive white or black smoke depending on which cylinders are affected
Most Likely Root Causes
Restricted or contaminated fuel supply
CommonA mechanical fuel delivery issue - low or fluctuating supply pressure can starve several injectors at once, especially under load.
Chafed or corroded shared harness ground or power feed
CommonAn electrical fault. A single damaged wire on a shared circuit can cause intermittent misfire signatures across multiple cylinders it feeds.
Marginal crankshaft or camshaft position sensor signal
OccasionalA sensor/signal-quality issue rather than a true combustion problem - a noisy timing signal can cause the ECM to log false misfire across cylinders.
Air intake restriction or boost leak
OccasionalA mechanical airflow issue that can cause lean misfire conditions spread across multiple cylinders rather than one.
Common Misdiagnoses (Don't Waste Parts)
Replacing individual injectors one at a time based on which cylinder shows the most misfire counts
Why it happens: It feels like a direct, isolated fix.
Do this instead: That's a parts-cannon approach for a multi-cylinder code - verify shared fuel supply, harness integrity, and timing signal quality first. Swap injectors only after ruling those out.
Assuming a boost leak can't cause misfire on a diesel
Why it happens: Misfire feels like a purely fuel or ignition problem to many techs.
Do this instead: Run a boost leak test - a significant intake leak can cause lean misfire conditions that mimic fuel or injector problems.
Master Tech Commentary
Mechanic's Notes
Note from ProfessionalDieselRepair.com's shop floor: when misfire shows up on multiple cylinders at once, resist the urge to start swapping injectors. Work the shared systems - fuel supply, harness grounds, cam/crank signal quality - before you ever isolate down to a single injector.
Step-by-Step Diagnostic Procedure
- Step 1: Pull the full fault list in Detroit Diesel Diagnostic Link (DDDL) - look for which specific cylinder SPNs (651-666 range) are logging alongside this multi-cylinder code.
- Step 2: Check fuel supply pressure and filter condition first, since a starved or restricted fuel supply commonly affects multiple cylinders simultaneously rather than just one.
- Step 3: Inspect the injector harness and connectors for a shared ground or power wire that could affect several cylinders if chafed or corroded.
- Step 4: Run individual injector cutout tests in DDDL to see if misfires cluster on one bank or side, which often points to a harness or fuel rail issue on that side.
- Step 5: If cylinders are scattered with no pattern, check crankshaft and camshaft position sensor signal quality, since a marginal timing signal can register as false misfire across multiple cylinders.
Professional service / OEM-level scan tools are recommended before clearing this fault for good.
Live Data & Freeze-Frame Tips
In DDDL, log individual cylinder misfire counts side by side with fuel rail pressure and boost pressure during a road test. Misfires that spike when rail pressure dips under load point to a supply issue rather than an injector-specific fault.
Safety Warnings
- Relieve fuel system pressure before disconnecting injector lines or rail fittings.
- Allow the engine to cool before working near the exhaust manifold and turbo.
When to Limp In vs Tow
Not usually necessary - the truck remains drivable, though expect rough running and reduced fuel economy until the shared cause is identified and repaired.
Prevention Tips for Fleets & Owner-Operators
- Keep fuel filters and water separators on schedule to protect supply pressure to all cylinders.
- Inspect injector harnesses for chafing at routine PM intervals, especially where they cross moving components.
- Address boost leaks and intake restrictions promptly rather than letting them linger.
Related Fault Codes
SPN 651-656 FMI 5
Individual cylinder injector circuit faults that can accompany or explain a multi-cylinder misfire pattern.
SPN 157 FMI 1
Low fuel rail pressure, a common shared-system cause of misfire across multiple cylinders.
Frequently Asked Questions
Can I keep driving with this code active?
Yes, but expect a rough idle and reduced power. Continued driving with unresolved misfire can lead to unburned fuel washing cylinder walls and accelerated wear over time.
How do I reset this code without a scan tool?
A battery disconnect may clear it temporarily, but it returns on the next misfire event if the shared cause isn't fixed. Confirm the clear with DDDL after the repair.
What triggers a derate from this code?
Severe or sustained multi-cylinder misfire can trigger a protective power reduction to prevent unburned fuel from damaging the aftertreatment system.
Is this always a fuel problem?
No - shared wiring faults, a weak timing sensor signal, or a boost leak can all produce the same multi-cylinder pattern.
How much does this typically cost to fix?
A harness repair or fuel filter service runs $100-$400. If it comes down to multiple injectors, costs can climb toward $2,000-$2,500.
Should I replace all the injectors at once?
Not without confirming which cylinders are truly at fault mechanically - replacing injectors that were never bad wastes money on a shared-system problem.