SPN 3363 FMI 3: DEF tank heater voltage high (short to power / wiring fault)

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.

• Confirm whether SPN 3363 FMI 3 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.

1. Visual inspection (fast wins). Harness routing, connector pins, DEF lines, leaks, and crystallization.
2. Check fluid sanity. Confirm DEF level and condition. If contamination is suspected, plan service soon.
3. Confirm the fault is ACTIVE. Stored-only faults can be old events.
4. 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.
5. Verify after repair. Clear codes, road test, confirm it stays inactive.

• If SPN 3363 FMI 3 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.

• 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.

Warning aftertreatment faults often start as an emissions warning, then progress to torque derate if the fault remains active over multiple drive cycles. If ignored, many platforms can eventually enter inducement and severe speed limiting.

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

• 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.

• 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)

• 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.

Real-world tips from technicians. Submissions are moderated to keep spam and “my cousin fixed it with duct tape” content out.