C0291 – Lost Communications With BCM

System: Chassis | Standard: ISO/SAE Controlled | Fault type: General

Official meaning: Lost Communications With BCM

Definition source: SAE J2012 naming/structure guidance

What Does C0291 Mean?

C0291 – Lost Communications With BCM is a chassis diagnostic trouble code with the official meaning: Lost Communications With BCM. This indicates that at least one controller on the vehicle network is not receiving the expected communication messages from the Body Control Module (BCM) within a required time window, or the received BCM messages are invalid.

This is a General fault type under an ISO/SAE Controlled classification, and it belongs to the Chassis system. C0291 is a communication-loss condition; it does not, by itself, prove that the BCM has failed. The correct approach is to verify module power/ground integrity and network circuit integrity, then confirm whether the BCM is able to communicate consistently.

Quick Reference

• Code: C0291
• Official Title: C0291 – Lost Communications With BCM
• Official Meaning: Lost Communications With BCM
• System: Chassis
• Fault Type: General
• Standard Classification: ISO/SAE Controlled
• What it indicates: BCM messages are missing, delayed beyond the allowable timeout, or invalid on the vehicle communication network
• Primary diagnostic focus: BCM power and ground, network wiring/connectors, and module communication status (online/offline)

Symptoms

Because C0291 is a network communication fault involving the BCM, symptoms depend on which systems require BCM data and how the vehicle responds when that data is unavailable. Possible symptoms include:

• Warning indicators illuminated, especially those related to chassis functions that depend on network data
• Reduced or disabled traction/stability-related functions if required BCM communication is missing
• Intermittent or inconsistent operation of body-related electrical functions controlled or coordinated by the BCM
• Instrument cluster messages or abnormal indicator behavior related to missing network information
• Multiple diagnostic trouble codes stored across modules, particularly other communication or network-related codes
• A scan tool may intermittently fail to communicate with the BCM (depending on whether the BCM is currently offline)

Common Causes

C0291 sets when communication with the BCM is lost. The root cause is typically an interruption of power/ground to the BCM, a network circuit fault, or a connection problem that disrupts message delivery. Common causes include:

• BCM power supply issue (open circuit, voltage drop, fuse/relay/contact problem) causing the BCM to reset or go offline
• BCM ground circuit issue (loose connection, corrosion, excessive resistance) leading to BCM brownout or shutdown
• Open, high resistance, or intermittent wiring on the communication network circuits connected to the BCM
• Short to ground, short to power, or short between network lines affecting the network segment used for BCM communication
• Loose, damaged, contaminated, or corroded terminals/connectors at the BCM or at network splices/junctions
• Low system voltage or unstable charging voltage that causes one or more modules to reset and lose communication
• An internal fault in a control module on the same network that disrupts network communication
• An internal BCM fault that prevents consistent communication (to be considered after circuit integrity is verified)

Diagnosis Steps

Diagnosing C0291 requires confirming whether the BCM is offline, determining whether the issue is constant or intermittent, and verifying the integrity of power, ground, and network circuits. Use a scan tool capable of reading chassis DTCs and accessing network/module status, a digital multimeter for voltage and voltage-drop testing, and the correct wiring information for the vehicle.

1. Run a complete module scan and document all DTCs.
   Record codes from every accessible module. Note any low-voltage codes, network bus codes, or multiple “lost communication” codes that may indicate a shared power/ground problem or a network segment fault.
2. Confirm whether the scan tool can communicate with the BCM.
   If the BCM is not reachable, prioritize BCM power/ground checks and network circuit checks. If it is reachable, treat the issue as intermittent and focus on conditions that cause dropouts.
3. Check battery and system voltage stability.
   Verify battery terminal condition and tightness. Measure system voltage with key on and with the engine running. Unstable voltage can cause module resets and communication loss.
4. Verify BCM power feeds under load.
   Identify BCM B+ and ignition feeds from wiring information. Test for proper voltage and perform voltage-drop testing across fuses, relay contacts, and feed circuits while the circuit is loaded (not only with an ohmmeter).
5. Verify BCM grounds with a voltage-drop test.
   With the BCM powered, measure voltage drop between the BCM ground circuit and the battery negative while loads are active. Excessive drop indicates unwanted resistance that can cause communication failures.
6. Inspect BCM connectors and related harness routing.
   Check for loose connector locks, bent or backed-out terminals, corrosion/contamination, damage from rubbing, and signs of moisture intrusion. Correct any terminal fitment problems before proceeding.
7. Inspect network junctions/splices and module-to-module connections.
   Communication issues may originate at splice packs, junction connectors, or areas where the harness is stressed. Look for poor terminal tension, fretting, or damage.
8. Test network circuits for opens/shorts per wiring information.
   Check continuity of the communication lines and check for shorts to ground, shorts to power, or shorts between communication lines. If the issue is intermittent, gently manipulate the harness while monitoring readings.
9. Use scan tool network information to identify online/offline patterns.
   If multiple modules drop offline together, suspect shared power/ground or a network segment issue. If only BCM communication is lost, prioritize BCM circuits and the immediate network path to the BCM.
10. Clear codes and verify the repair.
    After repairs, clear DTCs, cycle the ignition, and perform a road test (if safe) while monitoring module communication status. Re-scan to confirm C0291 does not return.

Possible Fixes

Fixes for C0291 depend on the specific failure found during testing. Possible repairs include:

• Repair or replace damaged wiring in BCM power, ground, or communication circuits
• Restore proper BCM power supply (repair fuse/relay contacts, power feed wiring, or terminal fit issues causing voltage drop)
• Restore proper BCM ground integrity (clean/secure ground points, repair ground wiring, correct high resistance)
• Clean, repair, or replace BCM and network terminals/connectors affected by looseness, corrosion, or contamination
• Correct network circuit faults (opens, shorts, or high resistance) that prevent reliable BCM message transmission
• Address system voltage issues that cause module resets (battery connections, battery condition, charging system faults)
• Replace the BCM only after verifying that power, ground, and network circuits are correct and communication loss is attributable to BCM malfunction

Can I Still Drive With C0291?

Driving with C0291 present may or may not be advisable because the code indicates a loss of communication with the BCM, and the BCM can affect multiple vehicle functions through network coordination. If chassis-related functions depend on BCM messages, those functions may be limited or disabled. Additionally, intermittent communication loss can cause unpredictable behavior in systems that rely on network data.

If the vehicle displays warning indicators related to braking/traction/stability, has abnormal instrument cluster behavior, shows electrical system instability, or exhibits starting authorization problems, treat the condition as a safety and reliability concern and avoid driving until diagnosis is completed. If the vehicle appears to operate normally, the issue may still be intermittent; prioritize diagnosis because communication faults can worsen with vibration, temperature changes, or increasing circuit resistance.

Repair Costs

Repair cost depends on whether the confirmed root cause is wiring, connector condition, a switch or module issue, or the labor needed to diagnose the fault correctly.

Last updated: March 25, 2026