SPN 3242 FMI 3 — Aftertreatment DOC Temperature Circuit Voltage Above Normal
DOC temperature high-voltage codes are open-circuit heat-zone electrical diagnostics. Failed regen diagnosis driven by an open DOC temp circuit is the most documented misdiagnosis — the DOC was not failing to light off; the sensor circuit was open and producing a false cold reading.
Fault Overview
SPN 3242 FMI 3 indicates the aftertreatment DOC temperature sensor circuit voltage is above the normal maximum — an open circuit or high-signal class fault. Open circuit, failed EGT sensor, unplugged connector, and brittle heat-zone harness are the primary causes. Failed regen diagnosis driven by an open DOC temp circuit is the most documented misdiagnosis.
Engine & Platform Coverage
Possible Causes
- 1.Open circuit from heat-sleeve damage near DOC housing
- 2.Failed DOC EGT sensor — open internal element
- 3.Unplugged connector after AT service
- 4.Brittle heat-zone insulation causing intermittent or permanent open
- 5.Burnt connector pins near DOC housing
Wiring & Circuit Notes
Inspect heat-sleeve damage and burnt connectors near the DOC housing — these are the highest-probability open locations for the DOC EGT circuit. Confirm connector is fully locked after any recent AT service. Check for brittle insulation along the harness run near the DOC.
DVOM Specs & Sensor Tests
An open DOC EGT circuit often reads an extreme cold value — the ECM interprets the open circuit as a very low temperature. Correlate with adjacent EGT sensors after the circuit is repaired to confirm the reading is plausible. Regen light-off evaluation is only appropriate after the DOC EGT circuit is proven valid.
Diagnostic Flow — FSM Sequence
AI Diesel Tech enforces this evidence-gate sequence. Each gate must pass before advancing.
5 steps locked behind Technician Mode- 1Inspect connector near DOC housing — confirm fully locked after any recent AT service
- 2Perform circuit open checks — continuity from sensor to ECM
- 3Perform OEM sensor circuit resistance test
- 4Replace DOC EGT sensor after circuit is proven clean
- 5Correlate DOC EGT against adjacent sensors after repair for plausibility
- 6Regen light-off evaluation only after sensing is confirmed valid
Unlock the full evidence-gated diagnostic sequence, DVOM test steps, and repair procedures in Technician Mode.
Repair Steps
4 steps locked- 1Inspect connector near DOC housing — confirm fully locked after any recent AT service
- 2Perform circuit open checks — continuity from sensor to ECM
- 3Perform OEM sensor circuit resistance test
- 4Replace DOC EGT sensor after circuit is proven clean
- 5Correlate DOC EGT against adjacent sensors after repair for plausibility
- 6Regen light-off evaluation only after sensing is confirmed valid
Unlock full diagnostic steps, DVOM tests, and repair procedures.
Full Diagnostic Access
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Summary
SPN 3242 FMI 3 is an open-circuit heat-zone electrical diagnosis. The DOC EGT circuit must be proven valid before any regen light-off or DOC conclusions are drawn.