Can I drive with SPN 2791 FMI 7?

Not recommended. SPN 2791 FMI 7 is marked critical and may escalate into derate or shutdown. Diagnose immediately and confirm whether the fault is ACTIVE vs stored (steps below).

How do I fix SPN 2791 FMI 7?

Start with: Inspect DEF injector for blockage. Then follow the diagnostic steps and scan-tool checks on this page.

What tools help diagnose SPN 2791 FMI 7?

A quality J1939 scan tool and, when available, OEM diagnostic software (examples: Cummins INSITE, Detroit DDDL, PACCAR DAVIE4) for actuator tests and live data.

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Level: Critical

SPN 2791 FMI 7: SCR dosing unit not responding – DEF delivery issue

SPN 2791 FMI 7: SCR dosing unit not responding – DEF delivery issue. Symptoms (3), causes (3), and fixes (3). What to check first to prevent derate and downtime. Critical: don’t ignore.

Code	SPN 2791 FMI 7
Severity	Critical
Applies to	Varies by OEM configuration (confirm your exact calibration)
Can I drive?	Not recommended. SPN 2791 FMI 7 is marked critical and may escalate into derate or shutdown. Diagnose immediately and confirm whether the fault is ACTIVE vs stored (steps below).

Symptoms

Likely causes

Fixes

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What does SPN 2791 FMI 7 mean?

When SPN 2791 FMI 7 sets, the ECM is telling you it detected DEF dosing performance that doesn’t match what it expects for emissions control.

That usually means the system commanded dosing, but the outcome (NOx conversion / feedback) didn’t line up. The root cause can be mechanical (restriction/crystallization), electrical (connector/harness), or fluid-related (quality/contamination).

The exact triggers vary by OEM calibration, but the diagnostic logic is consistent.

The key is to confirm whether the fault is ACTIVE and repeatable right now, or stored from a past event.

How this system works (why SPN 2791 FMI 7 happens)

The aftertreatment system reduces NOx by injecting DEF upstream of the SCR catalyst. The DEF decomposes into ammonia, and the SCR catalyst uses it to convert NOx into harmless nitrogen and water.

If the ECM commands DEF dosing but doesn’t see the expected result (from NOx sensors and model-based checks), it assumes dosing performance is abnormal.

Common disruptors include crystallized DEF around the doser tip, restricted lines, connector corrosion, or poor fluid quality that prevents consistent dosing.

Detailed guide for SPN 2791 FMI 7

When DEF is not injected properly, NOx reduction drops immediately.

This fault commonly appears in winter due to DEF crystallization.

Location hint for SPN 2791 FMI 7

The DEF dosing valve (doser) is typically mounted in the exhaust aftertreatment piping upstream of the SCR catalyst (often on the decomposition pipe). Look for a small injector-style body with an electrical connector and a DEF line attached.

Tip: White crust around DEF fittings or injector body usually indicates crystallization.

Scan tool checks for SPN 2791 FMI 7

Confirm whether SPN 2791 FMI 7 is ACTIVE. If it’s active, don’t guess. Diagnose it.
Look for companion codes: torque derate, inducement, NOx sensor, SCR efficiency, DPF soot, or DEF quality-related faults.
If supported by your tool (OEM diagnostic software (examples: Cummins INSITE, Detroit DDDL, PACCAR DAVIE4) / J1939 scanner), run aftertreatment tests: doser/actuator test, verify commanded vs actual behavior.
Review regen history (if available). Frequent regens and aftertreatment faults together often indicate underlying emissions performance problems.

If your tool can’t run actuator tests, you can still diagnose a lot with careful inspection + repeatability testing.

Step-by-step diagnostic for SPN 2791 FMI 7

Visual inspection (fast wins). Harness routing, connector pins, DEF lines, leaks, and crystallization.
Check fluid sanity. Confirm DEF level and condition. If contamination is suspected, plan service soon.
Confirm the fault is ACTIVE. Stored-only faults can be old events.
Run actuator tests (if supported). Use OEM diagnostic software (examples: Cummins INSITE, Detroit DDDL, PACCAR DAVIE4) (or a capable J1939 tool) to command tests when available.
Verify after repair. Clear codes, road test, confirm it stays inactive.

Decision guide (fast path) for SPN 2791 FMI 7

If SPN 2791 FMI 7 is ACTIVE → do a quick inspection for crystallization/leaks + connector corrosion.
If inspection finds obvious damage/crystals → fix/clean → clear → road test.
If inspection is clean → run doser/actuator test (tool permitting). Weak/inconsistent operation points to restriction, fluid supply issue, or failing doser.
If companion NOx/SCR/inducement codes exist → diagnose those too. Dosing faults often “ride along” with the real root cause.

Common misdiagnosis mistakes for SPN 2791 FMI 7

Replacing the doser immediately without checking the connector and harness routing (rub-through is common near heat shields).
Ignoring companion codes. NOx sensor faults, SCR efficiency faults, or inducement codes can make dosing faults show up as “symptoms” instead of the root cause.
Assuming DEF is fine because the tank is full. Contamination and crystallization can still happen.

Will SPN 2791 FMI 7 cause derate or inducement?

Critical aftertreatment faults can trigger rapid derate depending on calibration. Some trucks reduce torque quickly and may progress to severe speed limiting if the fault stays active.

Exact behavior depends on ECM calibration and which companion faults are active.

“Do this first” checklist for SPN 2791 FMI 7

Scan for companion/derate codes and record freeze-frame data (if available).
Inspect connectors, harness rub points, and pins for corrosion.
Check DEF quality/contamination and look for crystallization near fittings.
If available, run an actuator/doser test with OEM diagnostics.
After repair, clear codes and confirm the fault stays inactive after a drive cycle.

Tools needed to diagnose SPN 2791 FMI 7

Basic hand tools + good lighting for inspection
Multimeter for connector/power/ground checks (if wiring suspected)
OEM diagnostic software (examples: Cummins INSITE, Detroit DDDL, PACCAR DAVIE4) (or equivalent scan tool) for actuator tests and live data
DEF refractometer/test strips (optional but useful for quality checks)

Prevention tips to avoid repeat SPN 2791 FMI 7

Use fresh, sealed DEF from reputable sources (ISO 22241).
Fix small DEF leaks early to prevent crystallization and connector contamination.
Avoid excessive idling when possible (aftertreatment systems hate it).
During service, inspect harness routing and clamp points near exhaust heat.

What to check first for SPN 2791 FMI 7

Inspect connectors and harness near the affected component for corrosion, rubbing, or loose pins.
Check for upstream causes (communications, sensors, air/fuel issues).
Check for other active faults that may be upstream causes (communications, sensors, air/fuel issues).

If the fault repeats after clearing, diagnose it as ACTIVE. Many faults escalate into derate when ignored.

Related fault codes

No related faults found yet. Commonly seen with: SPN 5246, SPN 1569, SPN 4364, SPN 3719.

Mechanic community notes for SPN 2791 FMI 7

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