U028A

Code

Generic U — Network/User

Lost Communication With Transmission Fluid Pump Module B

Brand: Generic

Type: U — Network/User

Views: UK: 25 EN: 134 RU: 24

AI status

Completed

ready

Completed 100%

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

Verify code: connect scan tool, confirm U028A and note any additional communication codes. Clear codes and see if U028A returns.
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.
Inspect power/ground: with connector disconnected, check for proper battery voltage at the module power terminal(s) and a good ground reference.
Inspect connectors/wiring: open and examine the module connector and nearby splices for corrosion, bent pins, or water intrusion; repair as needed.
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).
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.
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.
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.
Replace module only after verifying wiring, power/ground, and network integrity. After replacement, reprogram/configure the module per manufacturer procedures and verify communications.
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