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Home / DTC / B122D — Left rear bank of the fan speed sensor

B122D — Left rear bank of the fan speed sensor

Detailed page for trouble code B122D.

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Code

B122D

LAND ROVER B — Body

Left rear bank of the fan speed sensor

Brand: LAND ROVER
Type: B — Body
Views: UK: 2 EN: 8 RU: 3
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Page language: EN

Causes

  • Open, short or intermittent wiring between the fan speed sensor and control module
  • Corroded, damaged or poorly seated connector at the sensor or module
  • Failed or degraded fan speed sensor (Hall effect, magnetic pickup or encoder)
  • Failed fan motor or internal encoder (sensor integrated into motor)
  • Poor ground or low supply voltage to the sensor
  • Faulty control module or software/calibration issue

Symptoms

  • Cooling/auxiliary fan(s) not operating, running at wrong speed, or running intermittently
  • Engine overheating or reduced cooling performance under load or in traffic
  • Warning lamp or message on dash related to cooling or HVAC
  • Reduced air conditioning/AC performance (if fans are part of AC system)
  • DTC stored and possibly limp or reduced fan control

What to check

  • Read fault memory with a diagnostic scanner; note freeze frame and live data for fan speed and related sensors
  • Visual inspection of left rear fan area: connector condition, wiring chafe, signs of water or corrosion
  • Backprobe the sensor connector with ignition on and measure supply voltage, ground continuity and signal output
  • Wiggle test harness/connectors while monitoring live data to look for intermittent faults
  • Apply known good 12V directly to the fan motor (if safe and serviceable) to verify motor operation
  • Compare left rear fan speed signal to other fan(s) or expected values using a scope or scanner

Signal parameters

  • Sensor supply: typically switched 12V or regulated 5V depending on design (verify on vehicle)
  • Sensor signal: typically a pulsed/square wave or analog 0–5 V signal proportional to speed (varies by model)
  • Expected idle/power frequencies: signal frequency increases with fan speed (verify specific Hz/RPM in factory data)
  • PWM command from control module to fan motor: 0–100% duty cycle (for controlled fans)
  • Normal continuity: low ohms between sensor ground and chassis ground; no short to battery positive

Diagnostic algorithm

  1. Connect a factory-grade scan tool. Read and record codes, freeze frame and live data (fan speeds, temperatures, battery voltage).
  2. Reproduce condition if possible (engine at operating temp or A/C on) and observe left rear fan speed PID or sensor output.
  3. Perform a visual inspection of the left rear fan, connector and wiring harness for damage, corrosion or contamination.
  4. With ignition on (engine off unless instructed), backprobe the sensor connector: check reference supply, signal and ground. Compare to expected ranges.
  5. Perform a wiggle test on harness and connector while monitoring live data to identify intermittent connections.
  6. If the sensor has a removable connector, disconnect, inspect pins, clean and reconnect; clear codes and retest.
  7. If wiring and connectors appear OK, bench or on‑vehicle test the fan motor/sensor: apply 12V to verify motor runs and use an oscilloscope or multimeter to confirm sensor output while motor turns.
  8. If sensor output is absent or incorrect while motor turns, suspect sensor or integrated encoder failure—replace sensor or fan assembly as per repair manual.
  9. If sensor output is good but control module reports fault, check module grounds, supply voltages and wiring to control module. Compare to other fans' behaviour.
  10. Consult vehicle wiring diagrams and manufacturer service information for component locations, pinouts and repair instructions before replacing modules.
  11. After repairs, clear codes and perform a functional test under the same conditions that originally set the code to confirm the fault is resolved.

Likely causes

  • Connector corrosion or water intrusion at the left rear fan
  • Broken or chafed wiring harness near left rear wheel arch or radiator area
  • Failed fan speed sensor or internal encoder in the left rear fan motor
  • Poor ground at fan module or engine/body ground point

Fault status

⚠️ Status
Left rear fan speed sensor circuit — signal out of range, missing or implausible. Control module has logged a sensor fault for the left‑rear fan bank.
🟡 Repair difficulty: Medium
⏱️ Diagnostic time: 0.5 - 2.0 hours (model and access dependent)

Similar codes

B122C B122E B123D P0480 P0481 P0482
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