What does it mean?
B1687 is a Body (airbags, seat belts, body electronics) fault code. It indicates: Engine Coolant Temperature Sensor Circuit - CAN Signal .
Symptoms
- Temperature gauge stays cold, reads maximum or does not move at all
- Cooling fan runs constantly, sometimes from the moment the ignition is switched on
- Heater output is poor or the climate control behaves unpredictably
- Engine warning lamp on and several modules storing related codes
- Remote or timed start functions, where fitted, refuse to operate
- Higher fuel consumption and rough running while the engine is cold
Likely causes
- Coolant temperature sensor open, shorted or drifting out of its normal resistance range
- Corroded or loose sensor connector, which sits in a hot and damp area of the engine
- Damaged sensor wiring, often chafed against the cylinder head or a bracket
- The engine control module has its own temperature fault stored and stops broadcasting a usable value
- Break or corrosion in the network wiring between the engine control module and the receiving module
- Low system voltage or a poor module ground causing lost messages
- Software or configuration mismatch after a module replacement
How to fix it
- Read the coolant temperature from the engine control module on live data and compare it with an infrared thermometer reading taken on the engine
- Check whether the engine control module has stored its own coolant temperature codes, which point straight to the sensor or its wiring
- Compare the coolant temperature reported by different modules to confirm that the message is missing rather than wrong
- Inspect the sensor connector for corrosion, melted plastic and loose terminals, and check the wiring for chafing
- Measure the sensor resistance at a known temperature and compare it with the specification
- Inspect the network wiring and connectors between the modules and check module grounds and supply voltage
- Replace the sensor or repair the wiring as proven by the tests, then confirm every module reports the same temperature