What this code means
U0129 can mean the PCM is missing the electric brake booster signal or receiving information that remains abnormal.
What the vehicle may do
- The PCM may not receive the electric brake booster signal.
- Information from the electric brake booster may remain abnormal.
Possible fault areas
- The electric brake booster power/ground circuit may have a fault.
- The communication circuit or electric brake booster may have a problem.
Diagnostic path
What U0129 means
On this 2025 Honda Accord 1.5, U0129 can indicate a PF-CAN A or PD-CAN B communication problem between the Powertrain Control Module (PCM) and the Electric Brake Booster. Let us walk through the communication and wiring checks for this code. Here is what this code tells us. The PCM may not receive the electric brake booster signal. Information from the electric brake booster may remain abnormal. Here are the main areas to check. The electric brake booster power/ground circuit may have a fault. The communication circuit or electric brake booster may have a problem.
Check booster power and ground first
Before you troubleshoot U0129, review How to Troubleshoot. This code may be stored if the electric brake booster does not start normally. A fault in its power/ground circuit may also store the code. Before starting the troubleshooting checks, check the electric brake booster power/ground circuit first.
The power/ground check comes first because U0129 can be stored when the booster does not start normally or when its power/ground circuit fails. The code alone therefore does not distinguish a communication-wiring problem from these other conditions.
Before the communication checks
Troubleshoot any DTCs listed or indicated systems below first. Network Communications \[CAN gateway\]: Go to the indicated DTC's troubleshooting
Before you troubleshoot, review How to Troubleshoot. These shared checks cover lost communication in the F-CAN circuit connected to the CAN gateway. If the failure comes and goes or is difficult to reproduce, review How to Troubleshoot (Handling Intermittent and Low Reproducibility Failures). Troubleshoot any indicated Network Communications \[CAN gateway\] DTCs first, using the troubleshooting for the indicated code.
Check communication status
Use the diagnostic scanner (HDS) for the Communication Status Check of F-CAN Connected Units. For details on using this function, review How to Troubleshoot (Handling Ongoing Failures). Run Communication Status Check of F-CAN Connected Units. Base the following inspections on the control unit whose result is NG. Check the power/ground circuit of that transmitting control unit.
The HDS result selects which transmitting control unit's circuits to inspect; NG is not, by itself, a diagnosis of a failed unit. An Electric Brake Booster NG result leads to its power/ground and specified wiring inspections, whose results distinguish the circuit branch from the booster branch.
Shared confirmation routine
The separate shared confirmation routine calls for turning the vehicle to the ON mode. For the details that apply here, check the U0129 confirmation procedure.
Prepare for the booster wiring checks
The listed scanner function path is CAN Gateway > Function Test > Communication Status Check of F-CAN Connected Units. If Electric Brake Booster indicates NG in that check, carry out the following VF-CAN A_H and VF-CAN A_L open-wire inspections between the CAN gateway and the booster. Before disconnecting the electric brake booster connector, turn the vehicle to OFF (LOCK) mode with the driver's door open. Wait for 3 minutes. Do not close the driver's door. Disconnect CAN gateway connector A (24P). Disconnect the electric brake booster connector (26P). Then make the following two wiring checks.
Check VF-CAN A_H
For the VF-CAN A_H open-wire check, IG Mode must be OFF. CAN gateway connector A (24P) and the electric brake booster connector (26P) must both be disconnected. Check resistance at the female terminals. Place one probe on CAN gateway connector A (24P) terminal 21. Place the other probe on electric brake booster connector (26P) terminal 12. The reading should be less than 1 ohm. That is the normal result.
Check VF-CAN A_L
For the VF-CAN A_L open-wire check, IG Mode must be OFF. CAN gateway connector A (24P) and the electric brake booster connector (26P) must both be disconnected. Check resistance at the female terminals. Place one probe on CAN gateway connector A (24P) terminal 9. Place the other probe on electric brake booster connector (26P) terminal 7. The reading should be less than 1 ohm. That is the normal result. If these wiring checks are abnormal, there is a problem in the circuit. If they are normal, there is a problem with the electric brake booster.
The normal branch depends on both open-wire checks, not just one passing reading. With IG Mode OFF and both specified connectors disconnected, each female-terminal pair must measure less than 1 ohm; exactly 1 ohm does not meet that limit. In this Electric Brake Booster NG path, abnormal wiring results identify a circuit problem, while normal results for both checks point to the electric brake booster.
U0129 diagnostic path
The U0129 diagnostic path calls for the F-CAN lost-communication checks for the CAN bus connected to the CAN gateway.
When the PCM stores U0129
The PCM stores U0129 if it cannot receive a signal from the electric brake booster through the PF-CAN A line or the PD-CAN B line. It also stores U0129 if information sent by the booster remains abnormal continuously for a specified cycle.
U0129 confirmation routine
Here is the separate confirmation routine for U0129. Turn the vehicle to the ON mode. Wait 6 seconds.
The shared confirmation routine refers to the individual DTC for its details; for U0129, that detail is the 6-second wait in ON mode. This is separate from the 3-minute OFF (LOCK), driver's-door-open preparation before disconnecting the booster connector.
Additional scanner reference
Another listed scanner function path is PGM-FI > Data List.
Final check
Possible fault areas include the electric brake booster power/ground circuit, communication wiring, and the booster itself.
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