B0204 – Occupant Classification System Sensor Fault

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

Definition source: SAE J2012/J2012DA (industry standard)

DTC B0204 indicates the vehicle has detected an Occupant Classification System (OCS) sensor fault within the body system. The OCS is part of the passenger protection strategy and is used to help determine how certain restraint functions should operate based on sensed seat occupancy conditions. Because OCS designs, sensor types, and diagnostic criteria vary by vehicle, the exact enabling conditions, monitored signals, and response actions can differ even when the DTC text is the same. Use the factory service information for your specific vehicle to confirm sensor location(s), connector views, test points, and the correct diagnostic flow before replacing parts or disturbing seat components.

What Does B0204 Mean?

B0204 means the control module has identified a fault associated with the Occupant Classification System sensor. Based strictly on the official definition, this code does not specify whether the issue is an open circuit, short, high/low input, range/performance concern, or an intermittent condition—only that the OCS sensor is considered faulted by the module’s monitoring logic. SAE J2012 defines how DTCs are structured and categorized, but the specific detection method and thresholds used to decide the sensor is “faulted” are vehicle-dependent. Proper diagnosis requires confirming which OCS sensor or sensing element is implicated (varies by vehicle) and then verifying power, ground, signal integrity, and network/module communication as applicable.

Quick Reference

• Subsystem: Occupant Classification System (OCS) sensor and its related wiring/connection path to the restraint/airbag control electronics (varies by vehicle).
• Common triggers: Sensor signal deemed invalid, missing, implausible, or not updating as expected; loss of communication between the sensor and the controlling module (design-dependent).
• Likely root-cause buckets: Wiring/connector issues, OCS sensor/mat or sensing element failure, poor power/ground feed, connector terminal fit/corrosion, control module input fault or calibration/software issue (as supported by testing).
• Severity: Safety-related; may affect how passenger restraints are enabled/disabled or indicated. Treat as high priority.
• First checks: Scan for related restraint/body DTCs, verify battery voltage health, inspect seat/under-seat connectors and harness routing, check for loose terminals, and confirm the complaint matches scan tool data.
• Common mistakes: Replacing the OCS sensor without verifying power/ground integrity, skipping connector/terminal checks under the seat, or ignoring companion DTCs that point to a shared supply/ground or communication issue.

Theory of Operation

The Occupant Classification System uses one or more sensing elements in the passenger seat area (varies by vehicle) to provide an output that represents occupant status. Depending on design, the sensor may be a pressure/weight-sensing mat, strain-based element, or an electronic module that processes raw sensing data and reports status to the restraint control electronics. The controlling module uses this information along with other inputs to support passenger restraint decisions and to command related warning indicators.

The module continuously monitors the OCS sensor for electrical integrity and credible behavior. A fault may be set if the expected signal is missing, outside a valid operating window, inconsistent with other available inputs, or not updating when conditions change, depending on the platform’s diagnostic strategy. When a fault is detected, the system may store B0204 and may apply a fallback strategy intended to maintain a predictable restraint state.

Symptoms

• Warning lamp: Airbag/SRS or restraint indicator illuminated.
• Passenger indicator: Passenger airbag status indicator shows an unexpected state or does not change as expected.
• Stored DTC: B0204 present as current or history, sometimes with additional related restraint/body codes.
• Intermittent behavior: Warning lamp or passenger status changes intermittently with seat movement or bumps.
• Chime/message: Cluster message or audible alert related to restraint system status (varies by vehicle).
• Readiness/inspection: Restraint system may report a fault status that prevents system self-check from completing normally.

Common Causes

• Loose, damaged, or contaminated connectors at the occupant classification system sensor or its harness (poor pin fit, corrosion, debris)
• Harness damage under or around the seat area (chafed insulation, pinched wiring from seat track movement, strained conductors)
• Open or short between the occupant classification system sensor and the control module (including intermittent opens)
• High resistance in the sensor signal/return circuits due to terminal fretting or partially backed-out pins
• Power supply or ground issue affecting the occupant classification system sensor (shared feeds/grounds vary by vehicle)
• Occupant classification system sensor internal fault (out-of-range or non-responsive output relative to expected behavior)
• Module-side concern for the occupant classification function (control module input fault or internal processing issue; varies by vehicle)
• Previous interior/seat service leaving a connector unseated, a harness misrouted, or a component not recalibrated (when required by service information)

Diagnosis Steps

Tools typically needed include a scan tool capable of reading body DTCs and live data, a digital multimeter, and back-probing or terminal test adapters suitable for the connector type. Access to vehicle-specific service information is important for connector pinouts, wiring routing, and any required setup procedures. Basic hand tools and a good light help for under-seat inspections.

1. Confirm the DTC is present and current. Record freeze-frame or snapshot data (if available) and note whether B0204 is stored as current, pending, or history. Clear the DTC and perform a short recheck to see if it resets immediately or only after movement/occupancy changes.
2. Check for related body/airbag system codes and address them in the order recommended by service information. Multiple codes can indicate shared power/ground or network issues rather than a single sensor problem.
3. Perform a careful visual inspection of the seat area and under-seat harness routing. Look for pinched sections, abrasion points, crushed conduit, or signs the harness contacts the seat track or frame during adjustment.
4. Inspect all accessible connectors in the occupant classification system sensor circuit. Verify connectors are fully seated, secondary locks engaged, and terminals are not bent, spread, pushed back, or contaminated. If contamination is present, correct it using approved practices from service information.
5. With the scan tool, review occupant classification-related live data and status indicators (names vary by vehicle). Log data while changing conditions that should normally change readings (for example, seat empty versus applying light pressure), without forcing the system beyond normal use. If the value is fixed, implausible, or drops out, note the exact condition when it occurs.
6. Perform a wiggle test while monitoring live data and the DTC status. Gently move the harness near connectors and along known flex points (especially where the harness passes under the seat). If the signal/status changes or the fault sets during the wiggle test, isolate the segment that provokes the change.
7. Key off and follow safety procedures from service information before any circuit testing. Then verify power and ground integrity for the occupant classification system sensor circuits (as applicable). Use voltage-drop testing across the ground path and power feed under load where possible to identify hidden resistance and terminal issues.
8. Check circuit continuity and for shorts between relevant circuits only as directed by service information (for example, signal-to-ground or signal-to-power). Flex the harness during checks to catch intermittent opens. If terminal drag or pin fit is questionable, perform a terminal tension check per approved methods.
9. If wiring, power, and ground test good, evaluate the sensor and module inputs per service information. This may include checking that the module sees a plausible sensor signal, verifying reference/return circuits (if used), and confirming no intermittent dropouts during a monitored road test or seat movement test.
10. If the platform requires initialization, zeroing, or calibration after repairs or after seat/component removal, verify the procedure was completed successfully. Then clear codes and run the confirmation test/drive cycle to ensure B0204 does not return.

Professional tip: Intermittent faults in under-seat circuits are often movement-related. To avoid replacing parts unnecessarily, prioritize reproducing the failure with controlled seat movement and harness manipulation while logging live data. When a voltage-drop test indicates resistance, focus on connector terminal condition and pin fit before condemning the sensor or module.

Possible Fixes & Repair Costs

Repair cost for B0204 varies widely by vehicle and depends on what testing finds, how accessible the components are, and whether the issue is in wiring, the sensor, or a control module. Diagnose first to avoid replacing parts unnecessarily.

• Reseat and secure connectors: Inspect and fully seat the occupant classification system sensor and module connectors; confirm locking tabs are intact and terminals are not loose.
• Repair wiring faults: Fix opens, shorts, or high-resistance wiring found during continuity and voltage-drop testing; restore proper routing and strain relief.
• Clean/replace damaged terminals: Address backed-out pins, corrosion, or poor terminal tension using approved terminal repair methods; replace terminals or pigtails as needed.
• Restore power/ground integrity: Repair poor grounds, loose ground fasteners, or power feed issues verified by voltage-drop testing under load.
• Replace the occupant classification system sensor: Replace only if tests confirm the sensor output/response is faulty and wiring/power/ground are verified good.
• Replace or reconfigure the control module: If supported by service information and testing confirms module-side failure, replace and perform any required setup, configuration, or calibration procedures.

Can I Still Drive With B0204?

B0204 relates to an occupant classification system sensor fault, which can affect how the restraint system makes deployment decisions. Driving may be possible, but it may not be safe to assume normal airbag/pretensioner operation. If the airbag warning indicator is on, treat it as a safety concern and minimize driving; verify operation using service information. If additional warnings appear or any safety systems report reduced function, do not drive and have the vehicle inspected.

What Happens If You Ignore B0204?

Ignoring B0204 can leave the occupant classification system unable to correctly classify the seat occupant, potentially leading to incorrect restraint behavior, a persistent warning light, and inspection/roadworthiness issues. The fault may also progress from intermittent to hard-fault as wiring/connector conditions worsen, making repairs more time-consuming later.

Last updated: March 14, 2026

Vehicles Commonly Affected by B0204

• Vehicles with passenger occupant classification: Systems that detect occupancy/weight for front passenger restraint decisions.
• Models with under-seat sensor harnessing: Designs where sensor wiring runs beneath seats and is exposed to movement.
• Vehicles with frequent seat position changes: Repeated fore/aft movement can stress connectors and harness routing.
• Vehicles used in harsh interior environments: Dirt, moisture, or debris intrusion around seat connectors can contribute to terminal issues.
• High-mileage vehicles: Aging terminals, harness insulation wear, and intermittent contact issues become more likely.
• Vehicles with recent interior/seat service: Removed/reinstalled seats and trim can lead to misrouted harnesses or partially seated connectors.
• Vehicles with low-battery events: Repeated low-voltage conditions can expose marginal connections or module communication sensitivities.

After completing repairs, clear codes, perform any required occupant classification system initialization steps per service information, and recheck for pending/stored faults to confirm the fix.