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DTC codes / U01xx · Lost communication with a control module / U01AF

U01AFLost Communication With Hybrid/EV Battery Interface Control Module P

U01AF is a generic communication fault indicating that the engine control module or another primary controller has lost the CAN (Controller Area Network) data stream from the hybrid/EV battery interface control module designated 'P'. The computer detects this when expected periodic messages from that module stop arriving within the assigned timeout window, setting the code and typically illuminating the warning light.

SystemNetwork
Code typeGeneric (all makes)
SeverityMedium
Fix difficultyProfessional
AreaLost communication with a control module

Symptoms you may notice

  • Illumination of the check engine light or hybrid system warning light
  • Often no symptoms besides the warning light, with the vehicle continuing to drive normally
  • Reduced power or a 'limp home' mode if the hybrid battery interface module is critical to torque management
  • Failure of the high-voltage battery state-of-charge display or hybrid energy flow monitor
  • Inability to start the vehicle or a 'ready' light that fails to come on in severe cases
  • Multiple unrelated warning lights appearing simultaneously due to a shared communication bus fault

Common causes, most likely first

  1. A discharged or failing 12-volt auxiliary battery causing low-voltage communication dropouts
  2. Corroded, loose, or damaged terminals at the hybrid/EV battery interface control module connector
  3. Open or shorted CAN bus wiring between the interface module and the rest of the network
  4. Failed hybrid/EV battery interface control module itself, often due to internal circuit faults
  5. Poor ground connection for the interface control module or the battery pack assembly
  6. Water intrusion or contamination in the connector or module housing
  7. A separate module on the same CAN segment holding the bus low and disrupting all communication

How to diagnose and fix it

  1. Verify the 12-volt battery state of charge and perform a load test; recharge or replace if marginal
  2. Inspect and clean the hybrid/EV battery interface control module connector for corrosion, tightness, and pin damage
  3. Check the module's ground circuit for excessive resistance and repair any loose or corroded ground points
  4. Measure CAN bus resistance and voltage at the interface module connector to confirm bus integrity and rule out a shorted segment
  5. Use a scan tool to check for communication with the interface module directly; if it does not respond, perform a wiggle test on the harness while monitoring
  6. If wiring and power/ground are confirmed good, substitute a known-good module or perform a module relearn/configuration as required
  7. Clear codes and verify communication restores and remains stable through a complete drive cycle

Hidden causes of hybrid battery module communication loss

  • A weak 12-volt battery is the most commonly overlooked cause; the hybrid battery interface module may boot but drop off the CAN bus under cranking voltage sag.
  • This code does not automatically mean the hybrid battery pack or the interface module is defective; a single chafed CAN wire elsewhere on the bus can silence multiple modules.
  • After any module replacement or battery service, some vehicles require a communication relearn or initialization procedure that, if skipped, will immediately reset U01AF.
  • Water intrusion into the battery pack enclosure or connector seals often causes intermittent communication loss that only appears in damp weather or after a car wash.
  • A failed module on the same CAN segment can pull the bus voltage out of spec and set U01AF along with U-codes for other modules, so always check for companion codes before condemning the interface module.

Can I keep driving with U01AF?

The vehicle may remain drivable with only a warning light, but loss of hybrid battery interface communication can disable high-voltage battery management and lead to reduced power or a no-start condition. Avoid long trips and high-load driving until the fault is diagnosed, and do not continue driving if the hybrid system warning or reduced power message appears.

Tools that help

Tools we recommend are in the Auto Veteran shop on Amazon. As an Amazon Associate we earn from qualifying purchases.

Guidance is general and written for all OBD-II vehicles. Exact tests and values vary by model, so confirm the fault before replacing parts.

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