SPN 924, labeled as Auxiliary Output #1, is a digital or pulse-width modulated (PWM) command signal generated by the Electronic Control Module (ECM) to control a discrete output device, such as a relay, solenoid, or indicator light. This parameter does not monitor a sensor; rather, it represents the commanded state of a specific output circuit. The systems that utilize this SPN vary widely by OEM and application, but it is commonly found on vocational trucks, agricultural equipment, and industrial power units. For example, on a PACCAR MX-13 engine, this output might control the engine brake enable relay, while on a John Deere 9.0L PowerTech, it could drive the cold start aid solenoid. In a Volvo D13, it may be used to control the auxiliary water pump relay for engine pre-heat. The diagnostic criticality of SPN 924 lies in its binary nature: if the ECM commands the output ON but the circuit fails (open, short, or overload), the intended function (e.g., engine brake activation, pre-heat, or fan clutch engagement) is lost, potentially leading to reduced vehicle performance, increased emissions, or even engine derate. Ignoring a fault on this SPN can cascade into secondary failures, such as overheating due to a non-engaged fan clutch or excessive brake wear from inoperative engine brakes.
Technical Overview
From an engineering standpoint, SPN 924 represents the commanded state of a discrete output driver inside the ECM. The output is typically a low-side or high-side driver, controlling a circuit that supplies battery voltage (12V or 24V) to a load. The ECM measures the actual voltage at the output pin and compares it to the expected voltage based on the commanded state. For a low-side driver, when the output is commanded ON, the driver pulls the pin to ground, allowing current to flow through the load. The ECM monitors the voltage drop across the driver; if the voltage does not drop sufficiently (open circuit) or drops too much (short to ground), a fault is logged. The signal type is a digital ON/OFF command, but in some implementations, it may be a PWM signal for variable control (e.g., fan speed modulation). The normal operating range for the output voltage is near battery voltage when OFF (high impedance) and near 0V when ON (low impedance), with current typically limited to 2-5 amps depending on the driver design. On a Detroit Diesel DD15, for instance, the ECM uses a smart high-side driver that reports back diagnostics via a feedback line, allowing the ECM to distinguish between open load, short to battery, and short to ground. The actual current draw is compared to a calibrated threshold; if the current exceeds the limit for more than a few milliseconds, the driver enters protection mode (pulsed or latched off) and sets a fault for SPN 924.
J1939 Network Behavior
SPN 924 is transmitted on the J1939 CAN bus as part of a dedicated Parameter Group Number (PGN) that varies by manufacturer. The most common PGNs for auxiliary output status are PGN 65133 (Electronic Engine Controller 3) or PGN 65252 (Auxiliary I/O), but OEM-specific PGNs are also used. For example, on a Caterpillar C13, SPN 924 may be included in PGN 65217 (Engine Auxiliary Output Status). The transmission rate is generally event-driven or periodic, typically once per second or upon a state change. The source address is always the ECM that controls the output—usually address 0 (Engine #1) on a heavy-duty truck, but on a machine with multiple ECUs, a body controller or transmission ECU may also transmit this SPN. Other ECUs on the network, such as the instrument cluster or a body control module, use this data to display the status of the output (e.g., “Engine Brake Active” indicator) or to coordinate actions (e.g., the transmission ECU may inhibit a shift if the engine brake is active). On a MAN D26, the aftertreatment control module may monitor SPN 924 to verify that the auxiliary coolant pump is running before initiating a regeneration cycle. The data length is typically 1 byte, with the status encoded as a bit-mapped field where bit 0 represents the ON/OFF state of Auxiliary Output #1.
Diagnostic Importance
Faults on SPN 924 are considered critical because they directly affect the operation of a controlled device that is often tied to engine protection strategies. For example, if the output drives the fan clutch solenoid, a failure can cause the engine to overheat, triggering a progressive derate or shutdown. On a Cummins ISX15, the ECM will initiate a “Reduced Power” mode (derate to 50% torque) if the fan output is stuck OFF and coolant temperature exceeds a threshold. Similarly, if the output controls the engine brake, a fault can lead to a loss of braking power, increasing the risk of brake fade on long descents. The ECM may also log a “Derate in Progress” event if the output failure is linked to a critical system. Ignoring active fault codes for this SPN can result in repeated driver warnings, increased fuel consumption (e.g., fan running continuously), and potential secondary damage—such as a failed engine brake valve from continuous engagement due to a shorted circuit. On a Mercedes-Benz OM471, the ECM will disable the start sequence if the Auxiliary Output #1 is detected as shorted to battery, preventing potential fire hazards from an energized load at ignition.
Common Failure Patterns
Technicians encounter several real-world failure patterns for SPN 924. The most frequent is wiring harness damage: chafed insulation causing a short to ground or an open circuit at the connector pins near the load device (e.g., fan solenoid or brake relay). On Volvo D13 engines, corrosion in the 2-pin connector at the fan clutch is a known issue, leading to intermittent open circuits. The second pattern is sensor or actuator degradation: solenoids or relays can fail internally, causing a coil winding short or open, which the ECM detects as a current fault. For example, on a PACCAR MX-13, the engine brake solenoid coil resistance should be 10-15 ohms; a reading outside this range indicates a degraded solenoid. The third pattern is contamination: oil or coolant ingress into the output driver circuit on the ECM connector (often due to a leaking injector or fuel line) can cause high resistance or intermittent shorts. On John Deere 6.8L engines, technicians often find the auxiliary output connector packed with dirt and moisture, especially on agricultural equipment. Calibration drift is rare for a discrete output, but on some Bosch ECU platforms, the current sense resistor can drift over time, causing false “overload” faults. Mechanical failures include a seized fan clutch that draws excessive current, tripping the driver protection, or a stuck engine brake actuator that causes the solenoid to remain energized beyond the duty cycle limit.
Diagnostic Approach
When diagnosing any fault code involving SPN 924, a systematic approach is essential. Begin with a multimeter and a breakout box (or back-probe pins) to verify the circuit integrity. First, disconnect the load device and measure the resistance of the solenoid or relay coil—compare to OEM specifications (typically 5-50 ohms). Next, check for shorts: measure resistance from the output pin (at the ECM connector) to ground and to battery positive; it should be greater than 10k ohms when the ECM is unpowered. Then, with the ignition ON and the ECM commanding the output OFF, verify that the voltage at the output pin is within 0.5V of battery voltage. Command the output ON (using a diagnostic tool like Cummins INSITE, Detroit Diesel Diagnostic Link, or Volvo Tech Tool) and confirm the voltage drops to below 0.5V. If the voltage does not change, suspect an open circuit in the harness or a failed ECM driver. If the voltage drops but the load does not operate, check the ground side of the load (for a low-side driver) or the power supply (for a high-side driver). Use a current clamp to measure actual current draw; if it exceeds the driver’s rated limit (e.g., 5A for a typical J1939 output), the ECM will shut down the driver and set a fault. If all circuit checks pass, escalate to OEM software to perform an output actuation test and verify the driver’s feedback signal. On a Deutz TCD 7.8, the diagnostic tool can display the raw current sense value in milliamps, allowing you to confirm if the ECM sees the correct load. When the issue is intermittent, perform a wiggle test on the harness while monitoring the fault status; if the fault reappears, focus on the connector pins and wire routing near the load device. Only replace the ECM after exhausting all circuit and load diagnostics, as driver failures are rare compared to wiring and actuator faults.
Fault Codes for SPN 924
FMI 0: Data valid but above normal operational range (most severe)
SPN 924 FMI 0 indicates Auxiliary Output #1 is operating above normal parameters, typically manifesting as excessive current draw or voltage spikes. This fault commonly appears during hydraulic system activation on construction equipment when auxiliary relays experience internal failures or when aft
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FMI 1: Data valid but below normal operational range (most severe)
SPN 924 FMI 1 indicates the ECM has detected that the voltage on Auxiliary Output #1 is below the normal operational range, typically less than 2.5 V for more than 0.5 seconds. This fault commonly appears after a technician replaces the ECM without verifying the output load or after a wiring harness
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FMI 2: Data erratic, intermittent or incorrect
The SPN 924 FMI 2 fault indicates erratic or incorrect data from the Auxiliary Output #1. This is often seen in heavy-duty vehicles after ECM replacements or software updates. The fault can lead to sporadic system performance, which may affect the overall vehicle operation. For instance, technicians
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FMI 3: Voltage above normal or shorted high
SPN 924 FMI 3 indicates Auxiliary Output #1 experiencing voltage above normal or shorted high conditions. This fault commonly appears when technicians connect high-amperage accessories to auxiliary outputs without proper relays, causing ECM protection circuits to activate. The auxiliary output chann
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FMI 4: Voltage below normal or shorted low
SPN 924 FMI 4 signals that the ECM detected the voltage on Auxiliary Output #1 circuit is below the normal operating range, typically a short-to-ground condition. This output often controls a relay for a cooling fan or a pre-lubrication pump. Technicians commonly encounter this fault after a wiring
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FMI 5: Current below normal or open circuit
SPN 924 FMI 5 indicates insufficient current flow or open circuit in auxiliary output #1, commonly controlling hydraulic pumps, fans, or solenoids. This fault frequently appears after equipment modifications or harness repairs when technicians encounter intermittent auxiliary system failures. The EC
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FMI 6: Current above normal or grounded circuit
The SPN 924 FMI 6 fault code indicates an abnormal current condition in Auxiliary Output #1, typically caused by a grounded circuit or an over-current situation. This is a common issue after ECM replacements or wiring modifications, particularly in systems where auxiliary outputs handle substantial
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FMI 7: Mechanical system not responding properly
SPN 924 FMI 7 indicates Auxiliary Output #1 mechanical system failure, commonly controlling hydraulic pumps, cooling fans, or PTO drives. This fault frequently appears during cold weather startup when hydraulic systems fail to reach operating pressure despite electrical signals being present. The EC
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FMI 9: Abnormal update rate
SPN 924 FMI 9 indicates the Engine Control Module (ECM) has detected an abnormal update rate from Auxiliary Output #1, meaning the signal did not transition within the expected time window. This fault commonly appears after a forced DPF regeneration when the output driver overheats or after replacin
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FMI 11: Root cause not known
SPN 924 FMI 11 indicates an unknown root cause in the Auxiliary Output #1 system. This code can appear after ECM replacement or software updates. Technicians often encounter it when dealing with complex electrical systems in heavy machinery where auxiliary functions are critical. It requires a detai
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FMI 12: Bad intelligent device or component
SPN 924 FMI 12 indicates a bad intelligent device or component in Auxiliary Output #1 circuit. This fault commonly occurs when ECM-controlled auxiliary devices like hydraulic pumps, PTO units, or cooling fans fail internally. Technicians frequently encounter this code after auxiliary system modifica
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FMI 13: Out of calibration
SPN 924 FMI 13 indicates the ECM has detected that the calibration data for Auxiliary Output #1 is out of the acceptable range. This commonly occurs after an ECM software update or replacement when the calibration parameters for the output driver are not correctly programmed. Technicians frequently
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FMI 14: Special instructions
The SPN 924 FMI 14 fault code refers to an issue with Auxiliary Output #1, requiring special instructions to address the problem. This code can often appear after electrical upgrades or modifications to the vehicle’s auxiliary systems. In practice, technicians may encounter this fault after installi
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FMI 18: Data valid but below normal operating range (moderately severe)
SPN 924 FMI 18 indicates Auxiliary Output #1 operates below normal range with moderately severe impact. This fault commonly occurs during hydraulic attachment operation when work lights or auxiliary hydraulics receive insufficient voltage. Technicians frequently encounter this code after ECM replace
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FMI 31: Condition exists
SPN 924 FMI 31 indicates that Auxiliary Output #1 has an active condition present, meaning the ECM detects a continuous signal or load where it expects a controlled off-state. This code commonly appears after a forced DPF regeneration when the exhaust brake output remains latched. Technicians freque