P3107 – Lost communication with airbag system control module circuit short to auxiliary battery or open (Toyota)

Toyota-specific code — factory diagnostic data

P3107 means your Toyota C-HR has lost communication with the airbag control module, and the vehicle has flagged the communication circuit as open or shorted to the auxiliary battery. In plain terms, the car cannot reliably “talk” to the SRS system. That can disable airbag system functions and turn on warning indicators. According to Toyota factory diagnostic data, this code indicates a lost-communication condition caused by a circuit open or a short to battery on the communication path. Treat this as safety-critical. Follow Toyota SRS depowering procedures before touching related wiring or connectors.

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P3107 Quick Answer

P3107 on Toyota points to a lost-communication fault with the airbag (SRS) control module due to an open circuit or a short to the auxiliary battery on the communication circuit. Diagnose the circuit and network integrity first, using OEM-approved SRS methods.

What Does P3107 Mean?

Official definition (Toyota): “Lost communication with airbag system control module circuit short to auxiliary battery or open.” In practice, the Hybrid Control module cannot consistently exchange required messages with the SRS control module. When that communication drops out, the vehicle can log faults and may limit safety system coordination.

What the module checks and why it matters: The Hybrid Control module monitors expected SRS module presence and message activity over the vehicle network. It also evaluates whether the suspected communication circuit behaves like an open or a short to battery. That distinction matters because it changes the test plan. An open often points to a broken wire, backed-out terminal, or unplugged connector. A short to auxiliary battery often points to harness damage, incorrect pin fit, or water intrusion bridging voltage into a network or signal line. Use OEM-approved test methods around SRS circuits. Do not probe SRS wiring with standard test leads.

Theory of Operation

On Toyota vehicles, the airbag system control module (SRS ECU) communicates with other modules over the in-vehicle network. The Hybrid Control module expects the SRS ECU to appear on the network and to exchange routine status messages. The modules share information for warning logic, system coordination, and diagnostic reporting.

P3107 sets when the Hybrid Control module stops seeing valid SRS communication and it identifies the likely electrical failure mode as open or short to auxiliary battery. An open breaks the signal path, so messages never arrive. A short to battery forces voltage onto a line that should carry controlled network signals, which corrupts data and causes dropouts. Because this involves the SRS ECU, depower the SRS system per Toyota procedure before disconnecting SRS-related connectors or handling harness sections near SRS components.

Symptoms

You will usually notice scan-tool access issues and safety warning indicators first.

• Scan tool behavior SRS module may not respond, may drop out, or may disappear from the ECU list intermittently
• Airbag warning SRS/airbag warning lamp illuminated
• Master warning Master warning message or general warning indicator may appear
• Stored communication DTCs Multiple modules may log related “lost communication” or network codes
• Intermittent faults Symptoms may change with vibration, steering column movement, or temperature
• Reduced diagnostic capability Scan tool may fail to clear SRS-related codes due to missing module communication
• Post-repair history Issue may appear after battery service, interior work, or collision-related repairs near harness routing

Common Causes

• Open in the SRS communication circuit: A broken wire or separated terminal stops the Hybrid control module from exchanging messages with the airbag control module.
• Short to auxiliary battery voltage on the communication line: A rubbed-through harness can feed B+ into the line and collapse data signaling, matching the “short to battery” fault direction.
• Connector terminal spread, push-out, or poor pin fit: Low contact pressure creates intermittent opens that look like “lost communication,” especially with vibration or temperature changes.
• Corrosion or moisture intrusion at an inline junction or module connector: Corrosion adds resistance and can also bridge adjacent terminals, creating an open-like drop or a voltage short.
• Power or ground fault affecting the airbag control module: If the SRS ECU loses clean power or ground, it can drop off the network and trigger a communication loss seen by Hybrid control.
• Power or ground fault affecting the Hybrid control module network interface: Weak module supply or a high-resistance ground can prevent accurate network biasing and message decoding.
• Harness damage near high-movement or pinch points: Areas near the dash structure, kick panels, or under-seat routing can chafe and create opens or shorts to B+.
• Network wiring fault shared with other modules: A short or open on a shared communication segment can take multiple modules offline and make the SRS module appear “lost.”
• Aftermarket accessory wiring interference: Poorly spliced add-ons can introduce battery voltage into a communication circuit or disturb grounds used by multiple modules.

Diagnosis Steps

Use a scan tool that can run a full Toyota network scan and access SRS data. Have OEM wiring diagrams, back-probing tools, and a quality DVOM for voltage-drop tests. Follow Toyota SRS depowering procedures before touching SRS-related connectors. Do not probe SRS connectors with standard test leads or powered test lights.

1. Confirm P3107 in the Hybrid control module and record DTC status as pending, confirmed, or history. Save freeze frame data for ignition state, battery voltage, vehicle speed, and any related network or SRS DTCs. Freeze frame shows conditions when the code set. A scan tool snapshot is different and helps catch intermittents during testing.
2. Run a complete network scan and verify whether the airbag control module appears in the module list. If the SRS ECU does not respond, treat this as a network or power issue first. If it responds, pull SRS codes and note any bus or power supply faults.
3. Inspect power distribution and fuses that feed the SRS system and the Hybrid control module. Check for incorrect fuse type, loose fit, heat damage, or a fuse that tests good but has poor contact tension. Confirm the fuse has power on both sides with ignition in the same state shown in freeze frame.
4. Verify Hybrid control module powers and grounds with voltage-drop testing under load. Command an appropriate load with the scan tool when possible. Keep ground drop under 0.1V with the circuit operating. Do not rely on continuity alone.
5. Depower the SRS system using Toyota OEM procedures and wait the specified time before disconnecting any SRS-related connectors. Then inspect SRS ECU power and ground paths for loose fasteners, water tracks, or repaired splices. After reconnecting per OEM steps, repeat voltage-drop tests under load.
6. Perform a careful harness and connector inspection along the communication circuit between Hybrid control and the airbag control module. Look for pinch points, abrasion, crushed conduit, and evidence of prior interior work. Pay close attention to terminal push-out, spread terminals, and partial connector seating.
7. If the code description includes “short to auxiliary battery,” isolate the circuit sections and check for unintended B+ on the communication line. Do this with the ignition ON when checking for line bias behavior, because bias voltage is only valid when powered. If you find B+ where it does not belong, locate the chafe or incorrect splice before reconnecting modules.
8. Check for an open circuit by measuring end-to-end continuity only after you isolate the circuit and depower per Toyota procedures. Then load-test the suspect conductor with a small approved load method per OEM guidance. Continuity alone can miss high resistance that drops data integrity.
9. If other modules also show communication faults, diagnose the shared network segment next. Disconnect non-critical modules one at a time per OEM strategy to see when the network recovers. Avoid disconnecting SRS connectors unless the system is fully depowered.
10. After repairs, reconnect all connectors, restore SRS power per OEM procedure, and clear DTCs using a scan tool with SRS capability. Re-run a network scan and confirm the airbag control module communicates normally. Road-test under similar ignition state and operating conditions, then confirm P3107 does not return.

Professional tip: Treat P3107 with FTB suffix -15 as a highly diagnostic clue. SAE J2012-DA maps -15 to “short to battery or open” direction. Use that to drive your testing toward an open or a B+ intrusion. Do not default to module replacement. Prove line integrity and module power and ground first.

Possible Fixes

• Repair an open or high-resistance section of the communication circuit: Restore conductor integrity and confirm stable communication with a post-repair network scan.
• Correct a short to auxiliary battery voltage: Repair chafed insulation, remove incorrect splices, and secure the harness to prevent repeat contact with B+ sources.
• Clean, dry, and correct terminal fit issues at affected connectors: Address corrosion, terminal spread, and pin push-out, then verify connector lock and strain relief.
• Restore proper power and ground to the SRS ECU or Hybrid control module: Repair loose grounds, damaged power feeds, or poor fuse block contacts proven by voltage-drop testing.
• Remove or rewire aftermarket accessories that interfere with the network: Eliminate backfeeds and ground noise that disrupt module communication.
• Replace a control module only after verification: Consider an SRS ECU or related module fault only after you prove wiring integrity, correct powers/grounds, and proper network behavior.

Can I Still Drive With P3107?

You can usually drive a Toyota C-HR with P3107, but you should not treat it as “normal.” This code means the Hybrid Control ECU lost communication with the airbag system control module because the communication circuit looks open or shorted to the auxiliary battery. That condition can leave the SRS status unknown to other modules. Assume the SRS may not respond correctly in a crash. Do not touch any SRS connectors or wiring without following Toyota depowering procedures and OEM-approved test methods. Arrange professional diagnosis soon, especially if the airbag warning lamp stays on or communication faults appear across multiple modules.

How Serious Is This Code?

P3107 ranges from an inconvenience to a serious safety concern. It may feel like a “no symptoms” fault when the vehicle drives normally and only the MIL appears. However, the code points at SRS communication integrity, not engine performance. Treat the SRS system as potentially compromised until proven otherwise. The FTB subtype -15 aligns with SAE J2012DA guidance for an open circuit or short-to-battery direction, which often traces to harness damage, connector issues, or power/ground integrity problems that disrupt data lines. Diagnosis requires SRS-capable scan access and SRS-safe tooling. This is not a DIY repair area.

Common Misdiagnoses

Technicians often replace the airbag ECU or Hybrid Control ECU too early. P3107 does not prove a module failure. SAE J2012 Section 6.1 applies here: the DTC message points to a suspected area, not a root cause. Another common miss involves chasing CAN bus faults without confirming basic power and ground integrity at the SRS module first. Shops also waste time when they use a generic scan tool that cannot access SRS data, freeze frame, or health checks. Finally, many people probe SRS connectors with standard meter leads. That creates terminal spread and adds resistance, which can create repeat communication drops.

Most Likely Fix

The most common confirmed repair direction involves restoring circuit integrity between the airbag system control module and the network the Hybrid Control ECU monitors. In practice, that usually means locating and repairing an open in the communication line, or correcting a short-to-auxiliary-battery condition caused by chafing, moisture intrusion, or a misrouted harness. A second frequent fix addresses poor power or ground at the SRS module that makes it drop off the network. Verify the cause with OEM procedures and SRS-safe backprobing methods before any repair. Re-test with a scan tool that can run Toyota network health checks.

Repair Costs

SRS/airbag repair costs vary significantly by component. Diagnosis must be performed by a qualified technician with SRS-capable equipment. Do not attempt airbag system repairs without proper training and safety procedures.

Last updated: April 9, 2026

Definition source: Toyota factory description · Autel MaxiSys Ultra & EV. Diagnostic guidance is based on factory-defined fault logic for this code.