Code
B29B1
MITSUBISHI
B — Body
Door lock signal:short to BAT
Views:
UK: 8
EN: 16
RU: 17
AI status
Completed
Completed
100%
Causes
- Damaged or chafed wiring contacting battery feed or other 12V source
- Corroded or bridged connector pins at door harness, BCM, or switches
- Faulty door lock switch or actuator with internal short to battery
- Aftermarket alarm/remote start or wiring improperly tied into lock circuit
- Water intrusion into connector or actuator causing conductive path
- Failing BCM or internal short in module (less common)
Symptoms
- Door locks may operate continuously or behave unpredictably
- Door lock/unlock switches or remote may not function correctly
- Intermittent or permanent DTC B29B1 stored in BCM
- Possible parasitic battery drain when vehicle is off
- No response from central locking or inconsistent lock state indications
What to check
- Scan vehicle and record freeze-frame and related body module codes; note which doors are affected
- Visual inspection of door harness, door jamb boot, connectors, and actuator for damage, corrosion, or water
- Backprobe door lock signal wire at BCM/connector with ignition off and with lock/unlock operated
- Measure voltage on the suspect signal line when switches are OFF — look for unexpected ~12 V
- Disconnect door lock actuator/switch and verify whether the code clears or symptom stops
- Inspect for aftermarket alarm/remote starter connections to lock circuit
Signal parameters
- Normal (rest) voltage on lock signal: typically near 0 V (open/low) or inactive state — should NOT be steady battery voltage
- Active lock/unlock event: signal may be pulled to battery voltage (~11–14 V) or pulsed briefly depending on vehicle design
- Short-to-BAT condition: signal reads constant ~11–14 V even when switches are not operated
- Expected current: low (mA) for signal line; excessive current indicates short or internal failure
Diagnostic algorithm
- Read and record stored codes and freeze-frame data from BCM. Clear codes and attempt to re-create the fault by operating locks.
- Visually inspect door harness, jamb boot, connectors, and actuator for damage, corrosion, or water intrusion. Repair or dry as required.
- With ignition ON (or as required for circuit), backprobe the door lock signal wire at the BCM or connector. Verify voltage with lock/unlock NOT being requested. A steady ~12 V indicates short-to-battery.
- Operate the lock/unlock switches while monitoring the signal. Note changes. If the signal is always high, begin isolating by disconnecting the door connector(s) one at a time until the condition clears.
- If disconnecting a specific door connector clears the fault, inspect/repair wiring and components on that door (switch, actuator, connector). Replace faulty part if confirmed.
- If isolating does not find the short, inspect splice points and aftermarket device connections. Remove or disconnect aftermarket modules and retest.
- If wiring and components are good and short is still present at BCM input, test BCM input circuitry for internal short. If BCM internal fault is suspected after all wiring checks, consult manufacturer procedures for BCM replacement and programming.
- After repairs, reassemble, clear codes, verify proper operation of locks and confirm B29B1 does not return.
Likely causes
- Chafed wiring in door harness where it rubs against hinge or body and touches a battery-fed conductor
- Shorted connector pin at door jamb connector or in door module causing constant 12V feed
- Aftermarket device spliced into lock circuit incorrectly
- Failed door lock switch/actuator that applies battery voltage to the signal line when it should be open
Fault status
Status
B29B1 — Door lock signal shorted to BAT. Inspect wiring, connectors, door actuator/switch, and any aftermarket devices.
Repair difficulty: Medium
Diagnostic time: 0.5-1.5 hours
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