NOx Sensor Heater Circuit Voltage Above / Below Normal / Open
NOx sensor heater circuit faults are confirmed by resistance and commanded voltage tests before sensor replacement.
Fault Overview
Circuit faults on the NOx sensor heater (upstream or downstream). Voltage high, low, or open prevents the sensor from reaching operating temperature and produces NOx data faults or heater-specific codes.
Engine & Platform Coverage
Symptoms
- •NOx sensor heater circuit
- •NOx heater open
- •NOx heater short
- •NOx heater voltage
- •Inlet NOx heater
- •Outlet NOx heater
Possible Causes
- 1.Open or shorted heater element inside the sensor
- 2.Wiring open or short
- 3.Connector corrosion or heat damage
- 4.Failed ECM or ACM driver
- 5.Blown fuse or poor power feed
Wiring & Circuit Notes
Inspect the NOx sensor connector (often multi-pin) for corrosion, bent pins, and heat damage. Check heater power and control wires for continuity and shorts.
DVOM Specs & Sensor Tests
Heater resistance typically in the low ohm range (platform-specific, often 2–10 Ω). Current draw when heating is significant. Voltage at the heater pins when commanded. Open = infinite resistance; short = near zero.
Diagnostic Flow — FSM Sequence
AI Diesel Tech enforces this evidence-gate sequence. Each gate must pass before advancing.
6 steps locked behind Technician Mode- 1Step 1 — Identify Which NOx Sensor and Which FMI: Identify which NOx sensor (inlet or outlet) and which FMI. Heater element open is one of the most common NOx sensor failure modes.
- 2Step 2 — Measure Heater Resistance at the Sensor Connector: Measure heater resistance at the sensor connector. Heater resistance is typically in the low ohm range (platform-specific, often 2–10 Ω). Open = infinite resistance; short = near zero.
- 3Step 3 — Check Voltage at the Heater Pins When the Heater is Commanded: Check voltage at the heater pins when the heater is commanded. No voltage confirms an open in the power or control circuit.
- 4Step 4 — Perform Continuity and Short Tests on the Heater Wires: Perform continuity and short tests on the heater wires. Connector heat damage produces both open and short symptoms.
- 5Step 5 — Inspect Connector Condition Thoroughly: Inspect connector condition thoroughly. Inspect the NOx sensor connector (often multi-pin) for corrosion, bent pins, and heat damage.
- 6Step 6 — Verify Power Feed and Ground: Verify power feed and ground.
- 7Step 7 — Replace the Sensor Only if the Heater Element is Proven Open or Shorted and Wiring is Good: Replace the sensor only if the heater element is proven open or shorted and wiring is good.
Unlock the full evidence-gated diagnostic sequence, DVOM test steps, and repair procedures in Technician Mode.
Full Diagnostic Access
Verification steps, preventative measures, severity guidance, and legacy comparison are available in Technician Mode.
Typical Repair Costs
Repair wiring or connector $100–300; replace NOx sensor with failed heater $300–900.
Start Diagnosing This Fault Now
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Summary
Use the AI Diesel Tech diagnostic engine to run this code through a structured, evidence-gated sequence and get a confirmed root cause before any parts are replaced.