This is typically a false/intermittent code. It can occur if:
Someone hooks up a scanner to the OBDII port while the ignition is on or while the engine is idling.
A trailer brake system is installed improperly/incorrectly on a truck with ABS (anti-lock brakes).
Check the 'Possible Causes' listed above. Visually inspect the related wiring harness and connectors. Check for damaged components and look for broken, bent, pushed out, or corroded connector pins.
The cost to diagnose the U1900 Mercury code is 1.0 hour of labor. The diagnosis time and labor rates at auto repair shops vary depending on the location, make and model of the vehicle, and even the engine type. Most auto repair shops charge between $75 and $150 per hour.
When the PCM, Brake Controller and Instrument Cluster are unable to communicate with each other at any time while the ignition switch is turned to the “Run” position, code U1900 sets in memory.
The Anti-ock Brakes System (ABS) module, the Powertrain Control Module (PCM), and the instrument cluster communicate over the Controller Area Network (CAN) Bus. This is a high-speed bus communication system and is not fault tolerant, which means any type of intermittent/loose connection between these 3 controllers will cause communication to stop. When the PCM, ABS module and instrument cluster are unable to communicate with each other at any time while the ignition switch is turned on, code U1900 will set.
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