Code
B12A4
LAND ROVER
B — Body
Electrochromatic output of the door mirror Temperature sensor of the left heat exchanger temperature of right heat exchanger Left coolant control valve liquid control of right cooling
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AI status
Completed
Completed
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Causes
- Damaged or corroded wiring or connector in mirror, temperature sensor, or coolant valve circuits
- Open or short to power/ground on sensor or actuator wiring
- Faulty door mirror electrochromatic element or mirror module
- Failed temperature sensor (left or right heat exchanger)
- Stuck or mechanically seized coolant control valve (left or right)
- Failed valve actuator or valve motor
Symptoms
- Warning or information message on dash related to mirror or cooling system
- Reduced or incorrect engine cooling control (temperature regulation inconsistent)
- Left/right cooling performance imbalance or overheating at idle/low speed
- Door mirror auto-dimming not working or dim/always-dim behavior
- One or more related HVAC/cooling or body system functions behave intermittently
- Fault stored that may reappear after driving or on ignition cycles
What to check
- Read all stored codes and freeze frame data from all modules (BCM, HVAC, engine coolant control)
- Check for related network (CAN) communications errors or other B/U codes
- Visual inspection of wiring and connectors at door mirror, heat exchangers, and coolant valve locations for damage, corrosion, water ingress or PIN pushed out
- Check fuses and relays feeding the mirror and coolant valve circuits
- Back-probe connector power and ground with ignition ON to verify supply and ground integrity
- Measure resistance of temperature sensors and compare to expected behavior as ambient temperature changes
Signal parameters
- Temperature sensors: typically a variable resistance (thermistor) or 0–5 V signal; resistance should change smoothly with temperature
- Reference supply for sensors: commonly a stable 5 V reference from the control module (check presence with DVOM)
- Coolant control valve: 12 V supply with ground switching or PWM from control module; expected duty cycle varies with commanded position
- Electrochromatic mirror: low-voltage control or CAN/body-bus message to mirror module; physical mirror element may see small DC voltage or ground switching
- Communication: CAN bus nominal voltages ~2.5 V recessive on CAN_H/CAN_L when idle—check if other modules show CAN faults
Diagnostic algorithm
- Retrieve and record all DTCs from all modules. Note freeze frame and mileage. Check for related body, HVAC or powertrain codes.
- Inspect wire harnesses and connectors for visible damage, corrosion or water entry at mirror, heat exchangers and valve locations. Repair or secure as required.
- Verify fuses/relays for the mirror and coolant valve circuits. Replace suspect items.
- With a DVOM, check for proper reference voltage (typically 5 V) and signal voltage at each temperature sensor connector with ignition ON. Verify sensor resistance changes with ambient/engine temperature.
- Back-probe the coolant control valve connector: check for 12 V supply and control signal (or PWM) while commanding the valve with a scan tool. If no response, check continuity to module.
- Command the electrochromatic mirror function (if supported) via diagnostic tool or toggle auto-dim in vehicle menus; observe whether the mirror module responds. Check for power/ground at mirror connector.
- If wiring and supplies are good but component does not respond, isolate by substituting a known-good component (mirror, sensor or valve) where possible, or test component on bench where feasible.
- If intermittent or network-related symptoms persist, inspect CAN wiring, test for proper termination and check for other modules reporting communication errors; update module software where applicable.
- After repair or replacement, clear codes and perform a road/drive cycle and functional tests. Monitor live data to confirm normal temperatures and valve/mirror operation before closing the job.
Likely causes
- Wiring harness damage at door sill, A-pillar, or behind the mirror
- Corroded connector at heat exchanger temperature sensor or coolant valve
- Failed or water-damaged door mirror module (electrochromatic function)
- Coolant control valve electrically inoperative or mechanically jammed
- Open reference supply (5V) or poor ground for temperature sensors
- BCM or control module reporting inconsistent/missing values due to internal fault
Fault status
Status
Electrical fault affecting door mirror electrochromatic output and/or left/right heat exchanger temperature or coolant control valve circuits. Inspection of wiring, connectors, components and module communication recommended.
Repair difficulty: Medium
Diagnostic time: 1.5-3.0 hours
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