SPN 5681 monitors the driver’s explicit activation or deactivation demand directed at the Advanced Emergency Braking System (AEBS), serving as the critical human-machine interface (HMI) command link between the driver and the autonomous braking controller. This parameter is found primarily in commercial vehicles equipped with AEBS technology, including heavy trucks, coaches, and buses from manufacturers such as Volvo, MAN, Mercedes-Benz (Daimler), DAF (PACCAR), Scania, and MAN TGA/TGX platforms. The parameter exists because the Vienna Convention on Road Traffic — adopted internationally and enforced within European Union regulatory frameworks (EU Regulation 347/2012 and its successors) — requires that any mandatory AEBS installation provide the driver with a means to temporarily deactivate the system. SPN 5681 carries that deactivation state as a discrete two-bit value transmitted across the J1939 CAN network from the HMI unit to the AEBS Electronic Control Unit (ECU). Diagnosing faults on this parameter is critical because an incorrect deactivation state can either suppress life-saving autonomous braking interventions or prevent the system from honoring a legitimate driver override, both of which carry severe safety and regulatory consequences.
Technical Overview
SPN 5681 is a 2-bit enumerated parameter encoded within the Parameter Group for Advanced Emergency Braking System 2. The two defined states are binary: 00b indicates the driver has commanded full deactivation — the AEBS shall neither warn nor intervene — while 01b indicates the system is active and authorized to warn or intervene as threat conditions demand. The signal itself is not an analog voltage or a frequency-based input; it is a pure digital CAN message value originating from a dedicated HMI module, which may be a steering column stalk switch assembly, a dashboard control panel, or an integrated driver information display depending on OEM implementation. On Volvo FH and FM platforms, for example, the AEBS deactivation switch is embedded within the driver assistance control cluster on the instrument panel, and its state is processed by the Vehicle Control Unit (VCU) or the Driver Information Module before being forwarded to the AEBS ECU. On Mercedes-Benz Actros MP4 and MP5 trucks, the function is handled through the FleetBoard/MirrorCam integration panel feeding the AEBS node. The signal does not require sensor calibration in the traditional sense — it is a commanded state, not a measured physical variable — but the integrity of the CAN frame transmission, the HMI switch contact quality, and the ECU’s ability to receive and acknowledge the frame are all diagnostically relevant. There is no “normal operating range” expressed in engineering units; rather, the valid operating states are simply 00b and 01b, with any other encoded value or loss of the message being flagged as an error condition.
J1939 Network Behavior
SPN 5681 is contained within the Advanced Emergency Braking System 2 Parameter Group (PG), which uses PGN 64965 (0xFDC5) in the SAE J1939-71 and J1939-73 definitions. This PGN is a peer-to-peer or broadcast message depending on network topology, with a typical transmission rate of 100 milliseconds under active driving conditions. The source address is assigned to the HMI unit or the body controller acting as the HMI proxy, and the destination address is the AEBS ECU node. On most European truck platforms, the AEBS ECU monitors this parameter continuously and will generate a fault if the message goes absent (times out) for more than a manufacturer-defined window — commonly 300 to 500 milliseconds — triggering a communication fault DTC. Other ECUs on the network, including the Electronic Braking System (EBS) controller and the Adaptive Cruise Control (ACC) module, may also subscribe to this PGN to coordinate their behavior with the AEBS state. For instance, on WABCO/ZF EBS systems used widely across MAN, DAF, and Volvo platforms, the EBS module cross-references the AEBS deactivation status when adjudicating brake demand priority during emergency events. Technicians using diagnostic tools such as VCADS Pro (Volvo), DAVIE (DAF), MAN-cats II, or Daimler’s Xentry DISS should be able to observe this parameter live on the J1939 bus through the data monitoring function.
Diagnostic Importance
A fault on SPN 5681 is treated as a safety-critical system fault by virtually all OEM AEBS implementations. If the AEBS ECU cannot confirm whether the driver has activated or deactivated the system, it must default to a fail-safe state. Depending on OEM strategy, this typically means the AEBS is disabled entirely and a warning lamp is illuminated on the instrument cluster — often the AEBS amber warning indicator — along with a stored DTC. Under EU Regulation 347/2012 and 2015/68, a non-functional AEBS must be reported to the driver, and the vehicle may be flagged as non-compliant during a roadworthiness inspection (MOT/TÜV). Ignoring an active fault code here means the vehicle is operating without a functioning collision mitigation system, which increases liability exposure dramatically in the event of a rear-end collision. Fleet operators subject to EU or FMCSA oversight face regulatory penalties if AEBS faults are not addressed within the defined maintenance window.
Common Failure Patterns
Technicians most frequently encounter SPN 5681 faults in the following real-world scenarios. First, HMI switch assembly failure — the deactivation button or rocker switch develops intermittent contact resistance or complete open-circuit failure due to moisture ingress, vibration fatigue, or contaminated contact surfaces; this is particularly common on vehicles operating in high-humidity or wash-down environments. Second, CAN bus wiring faults between the HMI module and the AEBS ECU — chafed harness segments near the A-pillar or firewall pass-through grommets introduce intermittent high resistance or short-to-ground conditions that corrupt the message frame. Third, HMI module firmware mismatch following a partial software update where the AEBS ECU and the HMI module are running incompatible parameter definitions, causing the receiving ECU to reject the PGN. Fourth, AEBS ECU internal CAN receiver fault where the node itself fails to correctly decode incoming frames despite valid bus signals — this requires ECU replacement and is confirmed only after all wiring and HMI sources are eliminated.
Diagnostic Approach
Begin diagnosis by connecting a J1939-capable diagnostic tool — Volvo VCADS Pro, MAN-cats II, DAF DAVIE, Daimler Xentry, or a universal tool such as Jaltest or Nexiq USB-Link 2 — and reading all active and stored DTCs from both the AEBS ECU and the HMI/Body Control Module simultaneously. Retrieve freeze frame data to identify operating conditions at fault onset. Next, use the live data monitor to observe SPN 5681 value in real time while toggling the AEBS deactivation switch; the value should toggle cleanly between 00b and 01b with each switch actuation. If no change is observed, isolate the HMI switch by back-probing the switch connector and measuring contact continuity with a digital multimeter — expect near-zero resistance in the closed state. Inspect the CAN High and CAN Low lines between the HMI unit and the AEBS ECU with an oscilloscope: look for a clean differential signal of approximately 2.5V ±1V with no ringing or dropout events. Verify CAN termination resistance across the relevant network segment (should read approximately 60 ohms across a properly terminated two-node segment). If wiring and switch integrity are confirmed, perform an ECU-to-ECU communication test using OEM software to verify the HMI module is correctly generating PGN 64965. If the message is present on the bus but the AEBS ECU still reports a fault, escalate to OEM dealer-level software for an ECU parameter reset or software reflash before condemning either module.
Fault Codes for SPN 5681
FMI 0: Data valid but above normal operational range (most severe)
SPN 5681 FMI 0 indicates the driver activation demand signal for AEBS exceeds normal operational parameters, violating J1939 protocol standards. This fault commonly appears when technicians encounter corrupted HMI communication after dashboard replacement or during CAN bus electrical interference. T
View SPN 5681 FMI 0 Diagnostic Guide →
FMI 1: Data valid but below normal operational range (most severe)
SPN 5681 FMI 1 indicates the driver activation demand signal for the Advanced Emergency Braking System (AEBS) is below the normal operational range, meaning the ECM reads a continuous deactivation request (00b). This fault commonly appears after a driver accidentally leaves the AEBS disable switch p
View SPN 5681 FMI 1 Diagnostic Guide →
FMI 2: Data erratic, intermittent or incorrect
SPN 5681 FMI 2 indicates erratic data related to the driver activation demand of the Advanced Emergency Braking System (AEBS). This fault often appears when there is a malfunction in the Human-Machine Interface (HMI) unit that communicates with the AEBS. Technicians frequently encounter this issue a
View SPN 5681 FMI 2 Diagnostic Guide →
FMI 3: Voltage above normal or shorted high
SPN 5681 FMI 3 indicates excessive voltage in the driver activation demand circuit for Advanced Emergency Braking System deactivation. This fault commonly appears when drivers repeatedly toggle the AEBS deactivation switch during highway merging scenarios, causing voltage spikes in the HMI-to-AEBS c
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FMI 4: Voltage below normal or shorted low
SPN 5681 FMI 4 indicates the ECM detects a voltage below the normal operating range on the circuit monitoring the driver’s demand for deactivation of the Advanced Emergency Braking System. This fault typically appears after a collision repair where the HMI switch harness was pinched, or when moistur
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FMI 5: Current below normal or open circuit
SPN 5681 FMI 5 is commonly encountered in heavy-duty vehicles with an Advanced Emergency Braking System (AEBS) when the driver activation demand signal experiences a current below normal or open circuit condition. This fault typically arises after a system update or ECM replacement, where the HMI un
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FMI 6: Current above normal or grounded circuit
This fault indicates excessive current or ground fault in the AEBS driver activation demand circuit, preventing proper communication between the HMI unit and AEBS controller. Technicians commonly encounter this code after dashboard replacement or when moisture infiltrates the AEBS deactivation switc
View SPN 5681 FMI 6 Diagnostic Guide →
FMI 7: Mechanical system not responding properly
SPN 5681 FMI 7 indicates the AEBS driver activation demand switch (HMI unit) is not responding properly to the ECM command. Per SAE J1939, the switch must transmit 00b (deactivation) or 01b (activation). When the ECM detects a stuck or intermittent signal, it sets FMI 7. Technicians often see this c
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FMI 9: Abnormal update rate
SPN 5681 FMI 9 indicates that the Advanced Emergency Braking System (AEBS) control unit is receiving the driver activation demand signal at an abnormal update rate, outside the expected periodic transmission window (typically 100 ms). This commonly occurs after an HMI module replacement or a softwar
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FMI 11: Root cause not known
SPN 5681 FMI 11 indicates a fault in the driver’s ability to deactivate the Advanced Emergency Braking System. This issue arises when the system fails to receive a clear command from the Human-Machine Interface (HMI) unit. Technicians often encounter this fault after an HMI software update or when v
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FMI 12: Bad intelligent device or component
This fault indicates failure of the AEBS driver activation demand system’s intelligent control component. The ECM detects malfunction in processing driver deactivation commands transmitted from HMI to AEBS unit. Technicians commonly encounter this after dashboard replacement or following software up
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FMI 13: Out of calibration
SPN 5681 FMI 13 indicates the driver activation demand signal for the Advanced Emergency Braking System is out of calibration. This typically occurs after a forced system reset or HMI unit replacement, where the voltage or PWM signal from the driver deactivation switch does not match the expected ca
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FMI 14: Special instructions
SPN 5681 FMI 14 indicates the driver’s choice regarding the activation of the Advanced Emergency Braking System (AEBS). This fault often appears when there is a mismatch between the driver’s input and the system’s response, such as after an AEBS module replacement or following an HMI unit update. Te
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FMI 18: Data valid but below normal operating range (moderately severe)
SPN 5681 FMI 18 indicates the driver activation demand signal for AEBS deactivation is registering below normal operating parameters. This fault commonly appears when technicians investigate dashboard warning lights after drivers report AEBS malfunction indicators following HMI module software updat
View SPN 5681 FMI 18 Diagnostic Guide →
FMI 31: Condition exists
This fault code is triggered when the AEBS HMI unit transmits a continuous deactivation signal (00b) to the electronic control unit, indicating the driver does not want warnings or interventions. In practice, this commonly appears after a technician inadvertently leaves a diagnostic tool simulating