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Home / DTC / B1018 — Heater water TEMP. sensor open

B1018 — Heater water TEMP. sensor open

Detailed page for trouble code B1018.

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Code

B1018

MITSUBISHI B — Body

Heater water TEMP. sensor open

Brand: MITSUBISHI
Type: B — Body
AI status
Completed
ready
Completed 100%
Page language: EN

Causes

  • Open or broken sensor wiring or connector
  • Disconnected or corroded connector at the sensor
  • Failed temperature sensor (open internal element)
  • Water ingress/corrosion at sensor or connector
  • Damaged harness from chafing, repair, or rodent damage
  • Control module input fault (less common)

Symptoms

  • HVAC/heater temperature control behaves incorrectly or defaults to safe mode
  • Insufficient or no cabin heat from heater core
  • Stored B1018 fault present in HVAC or body control module
  • Possible related warning/message on climate control display
  • In some vehicles engine temperature gauge/logic may be affected if same circuit is used

What to check

  • Visually inspect sensor and connector for corrosion, water, damage or disconnection
  • Check harness routing for chafing, breaks, or repairs near firewall and heater core area
  • Use scan tool to read live heater water temp sensor parameter and monitor for changes while warming
  • Measure continuity between sensor connector and control module connector
  • Measure sensor resistance at the sensor and compare while warming/cooling (in-situ or after removal)
  • Wiggle the harness and connectors while monitoring live data or DTCs to find intermittent opens

Signal parameters

  • Sensor type: typically NTC thermistor (resistance changes with temperature)
  • Open-circuit reading: infinite resistance / OL (no continuity)
  • Expected behavior: resistance should change smoothly with temperature (cold = higher resistance, hot = lower resistance for typical NTC)
  • Voltage at module input (if the circuit is a voltage divider): generally within 0–5 V and should vary with temperature; open circuit may show battery voltage or near 0 V depending on circuit design

Diagnostic algorithm

  1. Safety first: allow engine/coolant to cool before working near heater core; disconnect battery if required by the vehicle service manual.
  2. Retrieve freeze frame and live data with a scan tool and confirm B1018 is current or stored.
  3. Visually inspect sensor, connector, and nearby harness for damage, corrosion, or disconnection.
  4. Disconnect the sensor connector, inspect pins for corrosion/heat damage, then reconnect securely. Clear code and retest.
  5. Measure sensor resistance with multimeter at the sensor terminals. If resistance reads OL/infinite, replace sensor. If resistance changes with temperature, sensor is likely OK.
  6. If sensor resistance checks good, back-probe harness at connector with connector connected and monitor voltage/resistance at the module side to verify continuity to control module.
  7. Perform continuity check between sensor connector and control module connector. Repair any open or high-resistance runs (splice, replace wiring harness section, or repair connector)
  8. If wiring and sensor check good, inspect/replace connector terminals or pigtail. Re-check live data and clear code.
  9. If all wiring and sensor tests pass and fault persists, consider control module input fault — consult manufacturer service information before replacing module.
  10. After repairs, clear codes and perform operation / road test to verify issue does not return.

Likely causes

  • Connector unplugged at heater water temp sensor
  • Broken conductor in harness between sensor and HVAC/ECU
  • Corroded/bent pins causing intermittent/open connection
  • Failed NTC thermistor inside sensor

Fault status

⚠️ Status
Heater water temperature sensor circuit open detected. The HVAC/body control module reports no continuity or out-of-range input from the heater coolant temperature sensor circuit.
🟡 Repair difficulty: Medium
⏱️ Diagnostic time: 0.5-1.5 hours

Similar codes

P0117 P0118 P0119 P0128 B1019
406

Browse 406 MITSUBISHI manuals: repair procedures, diagnostics, wiring diagrams, component locations, service data and Labor Times by year, model and trim.

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