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Home / DTC / U028A — Lost Communication With Transmission Fluid Pump Module B

U028A — Lost Communication With Transmission Fluid Pump Module B

Detailed page for trouble code U028A.

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Code

U028A

Generic U — Network/User

Lost Communication With Transmission Fluid Pump Module B

Brand: Generic
Type: U — Network/User
Views: UK: 14 EN: 29 RU: 15
AI status
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ready
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Page language: EN

Causes

  • Transmission Fluid Pump Module B (TFP-B) powered off or failed
  • Open, shorted, or intermittent wiring/circuit between module and CAN bus
  • Poor or corroded connector pins or grounding at the module
  • CAN bus wiring fault (short to battery/ground, high resistance)
  • Failed CAN transceiver in module or another bus device
  • Missing/incorrect module programming or software fault

Symptoms

  • Transmission warning light or MIL illuminated
  • Transmission may default to a limp/limited mode or fail to shift correctly
  • No status messages from Transmission Fluid Pump Module B on a scan tool
  • Intermittent or constant communication-related fault codes
  • Possible abnormal transmission pressure or shift feel if pump is unresponsive

What to check

  • Use a capable scan tool to verify U028A and any related U-codes; note freeze frame and time stamps
  • Check module presence: confirm Transmission Fluid Pump Module B appears in module list and whether it responds to requests
  • Inspect fuses and power/ground circuits for the pump module
  • Visually inspect connectors and wiring for damage, corrosion, or loose pins at the module and splice points
  • Backprobe module connector and measure CAN H and CAN L voltages with key on, engine off
  • Check CAN bus termination resistance (approx. 60 ohms across CAN H-L for two 120 ohm terminators in parallel)

Signal parameters

  • CAN bus idle voltages: CAN_H and CAN_L common-mode ~2.5 V each; dominant state differential typically ~2 V
  • Expected presence of module-specific messages (manufacturer-defined IDs) at a steady periodic rate — check with manufacturer data or scan tool
  • Message content examples: pump status, pump speed/command, requested pressure, module diagnostic status (periodic bytes)
  • Monitor error counters and bus-off counters reported by gateways/ECUs
  • Check supply voltage to module (nominal battery voltage with key on) and ground resistance

Diagnostic algorithm

  1. Verify code: connect scan tool, confirm U028A and note any additional communication codes. Clear codes and see if U028A returns.
  2. Confirm module visibility: attempt to communicate with Transmission Fluid Pump Module B. If no response, note whether any other modules on the same CAN bus are missing.
  3. Inspect power/ground: with connector disconnected, check for proper battery voltage at the module power terminal(s) and a good ground reference.
  4. Inspect connectors/wiring: open and examine the module connector and nearby splices for corrosion, bent pins, or water intrusion; repair as needed.
  5. Check CAN physical layer: measure CAN_H and CAN_L at the module and at a known-good point on the bus. Verify recessive/dominant voltages and termination resistance (~60 Ω across H-L).
  6. Isolate fault: if CAN voltages are incorrect at the module but correct at a junction, trace and repair wiring between points. If the entire bus shows issues, inspect termination and other modules for faults.
  7. Test for bus-off: check for a device driving the bus into bus-off. Disconnect suspected modules one at a time while monitoring the bus until normal communication returns.
  8. Software/firmware: if wiring and power/ground are good and the module does not respond, check for available module software updates or known technical service bulletins before replacement.
  9. Replace module only after verifying wiring, power/ground, and network integrity. After replacement, reprogram/configure the module per manufacturer procedures and verify communications.
  10. Final verification: road test and monitor scan tool for restoration of normal messages and ensure U028A does not return.

Likely causes

  • Corroded or loose connector at Transmission Fluid Pump Module B
  • Failed module CAN transceiver or internal module failure
  • Open/shorted CAN H or CAN L wires near the transmission harness
  • Loss of module battery power or ground (fuse, splice, cluster)
  • Other module on same bus is pulling network into bus-off state

Fault status

⚠️ Status
Lost Communication With Transmission Fluid Pump Module B — The vehicle network is not receiving expected messages from the Transmission Fluid Pump Module B. Inspect module power/ground, connectors, CAN bus wiring/termination and module presence before replacing hardware.
🟡 Repair difficulty: Medium
⏱️ Diagnostic time: 1-3 hours

Similar codes

U0100 U0073 U0121 U028B U0293
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