SPN 723, labeled “Engine Speed 2,” monitors the rotational speed of the engine crankshaft as measured by the secondary or backup engine speed sensor. This parameter is critical in heavy-duty applications that employ a dual-speed-sensor architecture for redundancy, such as on-highway trucks equipped with Cummins X15, Detroit Diesel DD15, and PACCAR MX-13 engines, as well as off-highway machinery from Caterpillar and John Deere. The Engine Speed Sensor Information 1 Parameter Group (PGN 65251) transmits this data to the J1939 network, allowing the Engine Control Module (ECM) and other controllers to cross-verify engine speed against the primary sensor (SPN 190). In multiple-sensor configurations, SPN 723 provides an independent measurement that can differ from SPN 190, making it indispensable for detecting sensor drift, mechanical misalignment, or wiring faults before they escalate into performance or emissions compliance issues.
Technical Overview
The secondary engine speed sensor is typically a variable-reluctance (VR) or Hall-effect sensor mounted in close proximity to the engine’s flywheel ring gear or a dedicated tone wheel. On Cummins and Detroit Diesel platforms, the sensor is often located near the rear of the engine block, targeting a 60-2 or 36-1 tooth pattern. The sensor generates an analog AC voltage signal (VR type) or a digital square wave (Hall-effect) whose frequency is directly proportional to engine rotational speed. The ECM conditions this raw signal through a dedicated input circuit, applying filtering and zero-crossing detection to convert the frequency into an RPM value. The normal operating range for SPN 723 spans from 0 rpm (engine stopped) to the governed maximum, typically 2,500–3,000 rpm for heavy-duty diesels. However, because this is a secondary sensor, its output may be intentionally offset or validated against SPN 190 to detect discrepancies. The ECM uses a plausibility check: if the difference between SPN 723 and SPN 190 exceeds a calibrated threshold (often 50–100 rpm), a diagnostic trouble code (DTC) is set, and the system may switch to a limp-home or derate strategy.
J1939 Network Behavior
SPN 723 is broadcast on the J1939 CAN bus as part of Parameter Group Number (PGN) 65251, also known as “Engine Speed Sensor Information 1.” This PGN is typically transmitted by the engine controller at a rate of 10–50 milliseconds, depending on the OEM’s calibration. The source address (SA) is usually the engine’s primary address (0x00 for the engine controller), though on PACCAR and Volvo platforms, a separate electronic control unit (ECU) may relay the data if the sensor is read by a dedicated engine speed module. The data is encoded as a 16-bit unsigned integer representing engine speed in rpm, with a resolution of 0.125 rpm per bit and a range of 0 to 8,031.875 rpm. Other ECUs on the network—such as the transmission controller, aftertreatment system, or instrument cluster—use SPN 723 for load calculation, gear selection strategies, and driver display. In dual-sensor configurations, the transmission controller may compare SPN 723 against SPN 190 to determine if a sensor fault should trigger a shift inhibit or torque reduction. The J1939 Data Link Layer ensures that if the secondary sensor fails, the network continues to receive valid data from the primary sensor, maintaining system functionality.
Diagnostic Importance
Faults associated with SPN 723 are diagnostically critical because they can lead to engine derate, limp-home mode, or complete shutdown if the ECM loses confidence in both speed signals. When the ECM detects an implausible or missing signal from the secondary sensor (e.g., DTC 723-2 for erratic signal, 723-3 for voltage high, or 723-4 for voltage low), it activates engine protection strategies such as reduced power, limited vehicle speed, or forced idle. On Detroit Diesel DD15 engines, for instance, a persistent SPN 723 fault may trigger a 50% torque derate, while on Cummins ISX engines, the ECM may disable cruise control and limit engine speed to 1,500 rpm. Ignoring these faults can result in secondary damage: if the primary sensor fails while the secondary is already compromised, the engine may stall unexpectedly, causing loss of power steering and brake assist in a moving vehicle. Additionally, emissions systems reliant on accurate speed data—such as diesel particulate filter regeneration—may malfunction, leading to increased soot loading and eventual DPF plugging. For off-highway equipment like Caterpillar 336 excavators, a faulty SPN 723 can cause incorrect hydraulic pump compensation, reducing digging force and cycle times.
Common Failure Patterns
The most frequent real-world failures associated with SPN 723 involve wiring harness damage, sensor contamination, and mechanical wear of the tone wheel. On PACCAR MX-13 engines, technicians often encounter chafed or broken wires at the sensor connector due to vibration and heat cycling near the exhaust manifold. Corrosion of the sensor terminals, especially in vehicles operating in salt-laden environments, creates intermittent high-resistance connections that cause erratic SPN 723 readings. Sensor contamination from oil leaks or debris accumulation on the magnetic pickup tip is another common issue, particularly on Volvo D13 engines where the sensor is positioned near the rear main seal—a known oil weep point. Mechanical failures include damaged tone wheel teeth from foreign object ingestion or flywheel flex, which produces a missing or extra pulse per revolution. Calibration drift is rare but possible on Hall-effect sensors; over time, the internal magnet may weaken, causing the sensor to lose detection at low RPM (below 200 rpm). In dual-sensor configurations, a failure in the primary sensor can be masked by SPN 723, but the secondary sensor itself may fail without triggering a derate if the ECM continues to rely on the primary—this is a diagnostic pitfall that technicians must address by monitoring both SPNs simultaneously.
Diagnostic Approach
When diagnosing a fault code involving SPN 723, the technician should begin with a J1939 diagnostic tool capable of reading live data from PGN 65251, such as Cummins INSITE, Detroit Diesel Diagnostic Link (DDDL), or a generic CAN bus analyzer. First, compare SPN 723 against SPN 190 at idle and under load (e.g., 1,000–2,000 rpm). A discrepancy greater than 50 rpm warrants further investigation. Next, perform a circuit check: measure the sensor’s resistance (typically 800–1,200 ohms for VR sensors) between the signal and return pins at the ECM connector. For Hall-effect sensors, verify a 5V supply and a clean square wave output using an oscilloscope—a missing tooth or asymmetric pulse indicates a tone wheel problem. Inspect the sensor air gap: on Cummins engines, the gap should be 0.020–0.045 inches; excessive clearance attenuates the signal. Check for continuity and insulation resistance in the harness (minimum 1 MΩ to ground). If circuit values are within specification, swap the secondary sensor with the primary (if identical) to isolate the fault. If the fault moves with the sensor, replace it; if it remains, the ECM input circuit may be damaged. For intermittent faults, use a data logger to capture the SPN 723 value during road tests or load cycles. Escalate to OEM software when the fault proves elusive—manufacturers like Bosch and MAN provide proprietary diagnostic routines that can trigger a “sensor plausibility test” to force the ECM to evaluate both speed channels under controlled conditions. Always reference the factory service manual for the specific engine model, as sensor pinouts and ECM calibration thresholds vary widely between OEMs.
Fault Codes for SPN 723
FMI 0: Data valid but above normal operational range (most severe)
SPN 723 FMI 0 indicates that the engine speed as measured by the second speed sensor is above the normal operational range. This fault is often encountered after the replacement of the ECM or during post-maintenance test drives. It can lead to engine performance issues if not addressed promptly. Tec
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FMI 1: Data valid but below normal operational range (most severe)
SPN 723 FMI 1 indicates the secondary engine speed sensor reports valid data below normal operational threshold, typically under 400 RPM during active engine operation. This fault commonly appears during cold start sequences when primary speed sensor correlation fails, triggering ECM redundancy prot
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FMI 2: Data erratic, intermittent or incorrect
SPN 723 FMI 2 indicates erratic or intermittent data from the secondary engine speed sensor in dual-sensor configurations. This fault commonly occurs after ECM replacement when sensor calibration parameters aren’t properly transferred, or during cold startup conditions when magnetic reluctance senso
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FMI 3: Voltage above normal or shorted high
This fault indicates that the Engine Speed Sensor 2 (secondary speed input) circuit has detected a voltage level exceeding the normal operating range, typically above 4.8V with a pull-up to 5V. In practice, this code often appears after a sensor connector is damaged during engine removal or when wir
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FMI 4: Voltage below normal or shorted low
SPN 723 FMI 4 occurs when the voltage from Engine Speed Sensor 2 is below normal or shorted low. This fault often arises after the replacement of the engine control module (ECM), particularly when the ECM is not properly calibrated for multiple engine speed sensors. In practice, technicians may noti
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FMI 5: Current below normal or open circuit
SPN 723 FMI 5 indicates the secondary engine speed sensor circuit has current below normal or open circuit conditions. This fault commonly appears in dual-sensor configurations when technicians encounter intermittent speed readings during diagnostic testing. The ECM detects insufficient current flow
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FMI 6: Current above normal or grounded circuit
SPN 723 FMI 6 indicates the ECM has detected a current above normal or a grounded circuit on Engine Speed Sensor 2. This fault commonly appears after a forced DPF regeneration or a jump-start event that causes a voltage spike, shorting the sensor signal wire to chassis ground. The ECM immediately fl
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FMI 7: Mechanical system not responding properly
SPN 723 FMI 7 is a fault indicating that the engine speed measured by the second speed sensor is not responding properly. This fault typically arises in applications with multiple engine speed sensors and can lead to discrepancies between the expected and actual engine speed readings. For instance,
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FMI 8: Abnormal frequency, pulse width or period
SPN 723 FMI 8 indicates the secondary engine speed sensor (Sensor 2) is transmitting a signal with abnormal frequency, pulse width, or period. The ECM detects a waveform outside the expected 50–150 Hz range at idle. This code commonly appears after a forced DPF regeneration when excessive vibration
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FMI 9: Abnormal update rate
SPN 723 FMI 9 indicates an abnormal update rate from the second engine speed sensor, crucial for maintaining precise engine control. This fault often appears after sensor replacement or wiring interventions. In applications with multiple sensors, discrepancies between them can lead to this code. Tec
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FMI 11: Root cause not known
SPN 723 FMI 11 indicates that the Engine Speed Sensor 2 signal has failed with an undetermined root cause. This code often appears after a forced DPF regeneration when thermal stress causes intermittent sensor output. Technicians may find the engine running roughly or stalling without a clear electr
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FMI 12: Bad intelligent device or component
SPN 723 FMI 12 indicates failure of the secondary engine speed sensor intelligence or internal processing capability. This fault commonly appears in dual-sensor configurations when the backup crankshaft position sensor develops internal circuit failures. Technicians frequently encounter this code af
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FMI 13: Out of calibration
SPN 723 FMI 13 indicates that Engine Speed Sensor 2 has an output signal outside the expected calibration range, often due to mechanical misalignment or sensor gap issues. This code commonly appears after a forced DPF regeneration when thermal expansion alters the sensor-to-reluctor air gap. Technic
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FMI 14: Special instructions
SPN 723 FMI 14 relates to the engine speed as measured by the second engine speed sensor. It is critical in applications with multiple engine speed sensors, where discrepancies can lead to driveability issues. This fault code often emerges following an ECM replacement or reprogramming, where the sen
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FMI 18: Data valid but below normal operating range (moderately severe)
SPN 723 FMI 18 indicates the secondary engine speed sensor is providing valid data below normal operating range. This fault commonly appears during cold start conditions when cranking speed remains below expected thresholds, or after ECM replacement when sensor calibration parameters are mismatched.
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FMI 31: Condition exists
SPN 723 FMI 31 indicates an issue with the second engine speed sensor, often surfacing after ECM replacements or sensor malfunctions. This fault code is crucial in applications with multiple engine speed sensors, as it helps differentiate between primary and secondary readings. In practice, technici