B0042 is a Body Diagnostic Trouble Code indicating a detected anomaly in a body subsystem signal related to restraint or occupant-sensing functions. The code flags a signal integrity or plausibility concern reported by a body or restraint control system rather than naming a single failed part. Interpretation can vary by make, model, and year; you must confirm with basic electrical and network testing. Always verify supply voltage, chassis and sensor grounds, reference signals, and communication messages before assuming a sensor, wiring, or module replacement is needed.
What Does B0042 Mean?
This explanation follows SAE J2012 formatting conventions. SAE J2012 defines the structure and classification of diagnostic trouble codes and some standardized descriptions; many body codes and descriptions are cataloged in the SAE J2012-DA digital annex. B0042 is shown here without a hyphen suffix (no Failure Type Byte or FTB). If an FTB were present (for example B0042-1A), it would identify a subtype or failure-mode byte that narrows the condition recorded by the control unit.
There is no single universal SAE component-level definition for B0042 across all makes and years. On most vehicles B0042 describes a plausibility, range, or integrity issue in a body/restraint-related sensor or its signal path. Because implementations differ, confirm the exact source by measuring power, ground, reference, and the sensor or bus signal and by checking stored data such as freeze frame and Mode $06 where available.
Quick Reference
- System: Body — restraint/occupant-sensing signal plausibility
- Format: SAE J2012-style body code (no FTB shown)
- Likely tests: power, ground, reference, signal voltage/waveform, bus messages
- Common symptoms: airbag or restraint warning, occupant classification errors
- Diagnosis: test-driven—do not replace parts without measurement confirmation
Real-World Example / Field Notes
In the shop you’ll often see B0042 set after repairs near seats or when a vehicle has experienced water intrusion. A common pattern is intermittent or inconsistent occupant sensor readings: the control module reports values outside expected range or failing plausibility checks. Technicians commonly associated with this fault check the occupant classification sensor mat, its connector, and the harness for corrosion or chafing, then verify reference voltages and continuity before considering a replacement sensor.
Another frequent field observation is that poor ground or a marginal 5 V reference will produce spurious plausibility faults even though the sensor itself measures within expected resistance when disconnected. A wiggle test of connectors and harness while monitoring live data or using a scope often reproduces the fault. On networked systems, a missing or corrupted CAN/Local Interconnect Network (LIN) message carrying sensor data can also trigger B0042; confirm by monitoring bus traffic and message frequency before condemning a sensor or module.
Freeze frame and Mode $06 data are especially useful—look for inconsistent payload values, timestamps, or related event counters. Document any intermittent behavior and reproduce it with measured evidence: recorded waveforms, scanned voltages, or saved CAN logs. Treat module replacement as a last resort after external power, ground, wiring, connector, and communication tests pass and plausibility checks still fail.
Symptoms of B0042
- Warning Lamp Supplemental Restraint System lamp illuminated or intermittent illumination.
- Incorrect Occupant Status Airbag/seatbelt system shows wrong occupant detected or no-occupant status in diagnostics or dash messaging.
- Service Message Driver information center displays a restraint or airbag system advisory.
- Intermittent Behavior Fault appears or clears with seat movement, loading, or vibration.
- Communication Fault Diagnostic tool shows missing or implausible occupant-sensor data on the network.
- Failed Self-Test Built-in diagnostics report a plausibility or circuit fault for occupant sensing inputs.
Common Causes of B0042
Most Common Causes
Wiring harness damage, connector corrosion, or poor power/ground to the occupant sensing mat or its sensor module are commonly associated with this code. Signal-level faults (voltage out of expected range or intermittent signal) on the occupant detection circuit often trigger plausibility checks. Faulty seat pad sensors or pressure/weight sensors are one possible cause but must be proven by measurements before replacement.
Less Common Causes
Less frequently, this code is associated with network message loss on Controller Area Network (CAN) wiring, improper configuration after module replacement, or internal processing issues inside the restraint control module. Software calibration or mismatched module variants can cause plausibility failures, but those are only likely after wiring and sensor signals test good.
Diagnosis: Step-by-Step Guide
Tools: OBD-II scan tool with live data and Mode $06 support, digital multimeter, lab-grade oscilloscope, wiring diagrams/shop manual, backprobe pins or breakout box, soldering/repair kit, fused jumper, and inspection lamp.
- Confirm the active or stored B0042 and record freeze-frame data and any FTB (failure type byte) subtype. Note whether the code is logged with or without a hyphen suffix; an FTB, if present, refines the subtype.
- Clear the code, then attempt to reproduce under the same conditions while monitoring live data for occupant sensor channels and any related status bits in Mode $06. Reproduction narrows intermittent faults.
- Verify battery voltage and key-on power rails at the occupant-sensing module connector. Measure expected reference voltage (usually 5 V or specified by OEM) and ground integrity—look for voltage drops under load.
- Perform resistance and continuity checks on the sensor harness with connectors disconnected. Compare open-circuit resistance to expected ranges; watch for changing resistance while flexing harness to detect breaks.
- Backprobe the sensor signal with an oscilloscope to view waveform shape and amplitude. A steady, plausible waveform that matches expected pattern indicates a good sensor; noisy, clipped, or absent waveforms indicate wiring or sensor issues.
- Inspect connectors and seat-pad hardware for corrosion, moisture intrusion, or mechanical damage. Clean and reseat connectors, then re-test for changes in live data and DTC status.
- Monitor Controller Area Network (CAN) messages for the occupant sensing module. With a CAN-capable scope or logger, confirm the module transmits expected frames and that other nodes receive them; missing or malformed frames point to bus or module interface faults.
- If all external wiring, power, ground, and sensor signals test good and you still see plausibility failures, consult OEM diagnostics for module input tests or calibration. Only then consider possible internal processing or input-stage issue in the restraint module.
- After any repair, clear codes and perform a verification drive or test sequence to confirm the fault no longer appears and that live/datalogged sensor values are within expected ranges.
Professional tip: Treat occupant-sensing circuits as safety-critical. Use measured voltages, resistance, and waveform evidence to justify any part swap. If intermittent faults reproduce only under certain seating positions, use a lab scope and wiggle tests to isolate harness or pad failures before replacing control modules.
Possible Fixes & Repair Costs
Low: $50–$150 — justified by inspection or simple tests such as blown fuse replacement, cleaning a corroded connector, or reseating a loose mating connector after continuity and voltage checks show intermittent contact or an open/low voltage on the circuit. These are low-cost when the fault is external and confirmed by repeatable voltage/continuity readings.
Typical: $150–$500 — justified when wiring repairs or sensor replacement are required. Use measured evidence: a failing sensor or harness short to ground indicated by resistance checks, wiggle tests that reproduce the fault, or a failed voltage/reference signal shown on a lab scope. Replace only the specific sensor or repair the harness section that failed testing.
High: $500–$1,500+ — justified when a control module replacement is required after all external diagnostics pass (power, ground, reference, wiring continuity, signal integrity, and network messaging). Only consider “possible internal processing or input-stage issue” after external inputs test good. Costs rise with module price, programming requirements, and labor to access and calibrate systems.
Factors affecting cost include access labor, parts availability, whether the module requires dealer-level programming, and whether multiple subsystems share wiring that must be traced. Every recommended repair here must be backed by a specific test result: measured open/short, failed sensor voltages, corrupted network frames, or confirmed intermittent connector behavior.
Can I Still Drive With B0042?
You may be able to drive short distances, but risk depends on what B0042 represents in your vehicle. Because B0042 is a body-circuit-level fault and interpretation varies by make/model, it can range from a noncritical sensor circuit to a degraded safety-related system. If the vehicle shows additional warnings, degraded safety features, or unstable electrical behavior (flickering lights, random warnings), limit driving and seek diagnosis. For safety-related uncertainty, have the vehicle towed to a shop.
What Happens If You Ignore B0042?
Ignoring B0042 can let an intermittent electrical fault worsen, lead to unpredictable behavior of a body system, or mask developing faults that affect safety features. The underlying cause may progress from a minor connector issue to a full open or module input failure, increasing repair complexity and cost.
Related Codes
- B0019 – Occupant Restraint Airbag Circuit Fault
- B0018 – Body Circuit Fault — Restraint Sensor Signal
- B0017 – Body Circuit Signal Integrity Fault
- B0016 – Occupant Sensing Circuit Fault (SRS)
- B0014 – Occupant Restraint Circuit Fault
- B0011 – Occupant Restraint Circuit Fault
- B0009 – Restraint System Circuit Fault
- B0008 – Supplemental Restraint System Circuit High
- B0007 – Supplemental Restraint System Circuit Fault
- B0006 – Restraint Deployment Commanded Too Long
Key Takeaways
- SAE J2012-DA: DTC structure and descriptions follow the SAE standard and can vary by vehicle.
- System-level fault: B0042 indicates a body-circuit issue; component meaning often varies by make/model.
- Test first: Always confirm with voltage, continuity, signal, and network message testing before replacing parts.
- Module caution: Suspect internal module issues only after power, ground, wiring, and inputs test good.
Vehicles Commonly Affected by B0042
B0042 is commonly seen and frequently reported on vehicles from Ford, General Motors (Chevrolet/GMC), and Toyota. These manufacturers often use multiplexed body control architectures and multiple occupant or restraint sensors that increase the chance of body circuit diagnostic codes. The code’s appearance is more about system complexity and how a vehicle interprets a circuit fault than a single shared defective part.
FAQ
Can I clear B0042 with a basic code reader?
You can clear B0042 with many basic OBD-II scanners, but clearing does not diagnose the fault. If the underlying wiring, connector, or sensor issue remains, the code will likely return. Use clearing only after making a repair or to verify whether the fault was intermittent. Always record freeze-frame data and live signals before clearing so you have a baseline for testing if the code reappears.
Is B0042 always related to airbags or restraint systems?
Not always. B-codes cover body circuits and can include restraint systems, lighting, doors, or occupant detection, depending on manufacturer implementation. SAE J2012 defines structure but not a universal component for many body codes. Confirm the subsystem on your vehicle with a scan tool, freeze-frame data, and basic electrical checks—voltage, continuity, and signal plausibility—before assuming it is an airbag-related fault.
What tools should I have to diagnose B0042 properly?
Essential tools include a good OBD-II scan tool with live data, a digital multimeter, a lab scope (or known-good signal meter) for waveform checks, wiring diagrams, back-probing leads, a wiring continuity tester, and a reliable power/ground reference. These let you measure reference voltages, verify continuity, capture intermittent faults, and confirm network messages. Prioritize tests that reproduce the fault before replacing parts.
How long does a professional diagnosis of B0042 usually take?
Diagnosis typically takes 1–3 hours, depending on fault complexity and access. Simple issues like a blown fuse or loose connector can be found quickly. Intermittent wiring faults, harness chafing, or network message problems require longer tracing and possibly road-test reproduction. Allow extra time if module bench-testing or manufacturer-level diagnostic data is needed.
Can a weak battery or charging problem cause B0042?
Yes. Low system voltage or poor charging can create spurious body circuit faults and corrupted network frames, which may trigger B0042. Start by checking battery resting voltage, charging voltage with engine running, and voltage stability during accessory loads. If voltage is out of range or drops during tests, resolve the battery/charging issue first, then confirm whether B0042 persists with stable power.