The SAE J1939 Suspect Parameter Number (SPN) 2631, labeled “Engine Charge Air Cooler 1 Outlet Pressure,” is a critical measurement parameter that monitors the absolute air pressure at the outlet of the first or only charge air cooler (CAC) in a turbocharged diesel or natural gas engine. This pressure reading is used by the Engine Control Module (ECM) to calculate charge air density, boost pressure drop across the cooler, and overall intake manifold conditions. It is most commonly found on heavy-duty on-highway trucks from manufacturers such as PACCAR (MX-13, MX-11), Volvo (D13, D16), and Cummins (X15, ISX12 G), as well as off-highway equipment from Caterpillar (C9.3, C13, C18) and John Deere (PowerTech 9.0L, 13.5L). In real-world diagnostics, SPN 2631 is frequently monitored during high-load events such as hill climbing or heavy hauling, where charge air cooler efficiency directly impacts engine performance and emissions compliance.
Technical Overview
From an engineering perspective, SPN 2631 is derived from a dedicated absolute pressure sensor, typically a silicon-based piezoresistive transducer with an integrated signal conditioning circuit. The sensor is physically installed in the outlet duct of the charge air cooler, downstream of the turbocharger compressor and upstream of the intake throttle or intake manifold. The sensor output is an analog voltage signal that varies linearly with absolute pressure, typically ranging from 0.5 V at near-vacuum conditions to 4.5 V at the maximum rated boost pressure (commonly 300–400 kPa absolute for modern engines). The ECM provides a regulated 5 V reference voltage and a ground circuit to the sensor. On the ECM side, the analog voltage is sampled by a 12-bit analog-to-digital converter (ADC) and converted to an engineering value in kilopascals (kPa) using a calibration table stored in the ECM firmware. The normal operating range for SPN 2631 varies by engine model and operating condition; for example, a Cummins X15 at full load and rated speed may see values between 250 and 350 kPa absolute, while a PACCAR MX-13 at idle may read near atmospheric pressure (approximately 100 kPa). Some modern engines, particularly those from Detroit Diesel (DD15, DD13) and Mercedes-Benz (OM 471, OM 473), use a digital sensor communicating via a dedicated SAE J1939 message, though the parameter remains mapped to SPN 2631 in the network layer. The ECM continuously compares this pressure value against a model-based expected pressure derived from engine speed, load, ambient pressure, and turbocharger speed to detect deviations that may indicate a restriction or leak in the charge air system.
J1939 Network Behavior
On the Controller Area Network (CAN) bus, SPN 2631 is transmitted as part of the Engine Fluid Level/Pressure 4 Parameter Group (PGN 65263, or 0xFEEF in hexadecimal). This PGN is broadcast periodically by the engine ECU (source address 0x00) at a rate of once per 100 milliseconds (10 Hz) under normal operating conditions. The PGN contains 8 data bytes, with SPN 2631 occupying bytes 4 and 5 (16-bit unsigned integer, resolution of 0.1 kPa per bit, with a data range of 0 to 6425.5 kPa). The scaling factor and offset are defined in the J1939/71 standard, and the parameter is transmitted as absolute pressure. Other ECUs on the network—such as the transmission controller, aftertreatment control module, or vehicle control unit—may use this data for coordinated torque limiting, exhaust temperature management, or diesel particulate filter regeneration strategies. For instance, a Volvo I-Shift transmission may use SPN 2631 data to adjust shift timing under high boost conditions, while a PACCAR aftertreatment system may reference this pressure to calculate exhaust mass flow for DEF dosing. The parameter is also broadcast in the diagnostic message (DM1) if a fault is active, where the SPN is combined with the corresponding Failure Mode Identifier (FMI) to indicate the specific nature of the malfunction (e.g., FMI 1 for low voltage, FMI 3 for high voltage, FMI 4 for signal out of range, or FMI 18 for data rate deviation).
Diagnostic Importance
Faults associated with SPN 2631 are considered high-priority because they directly impact the ECM’s ability to manage air-fuel ratio, boost pressure control, and exhaust gas recirculation (EGR) flow. If the ECM detects an implausible or missing charge air cooler outlet pressure signal, it typically activates an engine protection strategy that may include derating power output, limiting engine speed, or initiating a forced regeneration of the diesel particulate filter. For example, on a Cummins X15 engine, a persistent SPN 2631 fault with FMI 2 (data erratic) will cause the ECM to substitute a default value based on ambient pressure and engine speed, leading to reduced torque and increased fuel consumption. Ignoring an active fault code for this parameter can result in cascading failures: an undetected charge air cooler leak may cause excessive turbocharger speed (overspeed condition), elevated exhaust temperatures that damage downstream aftertreatment components, or increased soot loading in the EGR cooler. In severe cases, as documented in Caterpillar service literature for C13 engines, a failed sensor causing a false low-pressure reading has been known to trigger repeated regeneration cycles, leading to oil dilution and eventual engine bearing failure. Therefore, any diagnostic trouble code (DTC) involving SPN 2631 must be addressed promptly to avoid costly repairs and downtime.
Common Failure Patterns
Technicians encounter several recurring failure patterns with SPN 2631. The most frequent is a wiring or connector issue—specifically, corrosion or fretting at the 2-pin or 3-pin Deutsch connector leading to the pressure sensor, which causes intermittent signal loss or voltage drift. This is especially common on equipment operating in wet or salty environments, such as refuse trucks or agricultural tractors from John Deere. The second most common pattern is sensor degradation due to thermal cycling and vibration; the internal diaphragm of the sensor can develop cracks or fatigue, causing the output to drift high or low over time. Contamination is another significant failure mode: oil mist from the turbocharger compressor seal or moisture from the charge air cooler can migrate into the sensor port, clogging the reference vent or filling the internal cavity with sludge. This is frequently observed on PACCAR MX-13 engines operating with excessive crankcase ventilation oil carryover. Calibration drift, while less common, occurs when the sensor’s internal reference voltage shifts due to aging components, leading to a fixed offset error that may only be detectable by comparing the reading against a known reference pressure during a key-on, engine-off test. Mechanical failures, such as a collapsed charge air cooler hose or a failed CAC core causing a restriction, will also manifest as a low or erratic SPN 2631 reading, though these are typically secondary to a sensor or wiring fault in terms of diagnostic frequency. On Detroit Diesel DD15 engines, a specific failure pattern involves the sensor sharing a 5 V reference line with other sensors (e.g., intake manifold pressure, ambient pressure); a short in any one sensor can pull down the entire reference voltage, causing multiple SPN faults simultaneously.
Diagnostic Approach
A systematic diagnostic approach for any fault code involving SPN 2631 begins with a thorough visual inspection of the sensor connector and wiring harness for signs of corrosion, chafing, or loose terminals. The recommended tools include a digital multimeter (DMM) capable of measuring voltage, resistance, and frequency, along with a breakout box (e.g., Cummins INLINE 6 adapter or Volvo VTT) for accessing ECM pins without damaging the connector. The first electrical test is to verify the 5 V reference supply at the sensor connector with the ignition on and engine off—this should be within ±0.1 V of 5.0 V. Next, check the sensor ground circuit for continuity to chassis ground (less than 1 ohm). The signal wire should be probed while the sensor is connected; at key-on, engine-off, the voltage should correspond to the local barometric pressure (e.g., approximately 1.0 V at 100 kPa). Using a hand vacuum pump or pressure calibrator, apply known pressures (e.g., 0, 100, 200, 300 kPa) and verify the sensor output follows the manufacturer’s specified transfer function. For example, a Bosch sensor commonly used on MAN D26 engines outputs 0.5 V at 0 kPa and 4.5 V at 400 kPa. If the sensor output is erratic or fails to track pressure, replace the sensor. If the sensor checks good, the next step is to inspect the charge air system for leaks or restrictions: use a smoke machine to pressurize the intake system to 20–30 psi and look for smoke escaping from hoses, CAC core, or EGR cooler. Reference values for pressure drop across the CAC should be obtained from OEM service data—typically 10–30 kPa at full load for a healthy system. If all electrical and mechanical checks pass,
Fault Codes for SPN 2631
FMI 0: Data valid but above normal operational range (most severe)
The ECM detected that the charge air cooler 1 outlet pressure signal exceeded the maximum calibrated threshold, indicating a boost overpressure condition. This code commonly appears after a forced DPF regeneration or during high-load operation on a dyno, when the wastegate or VGT actuator fails to o
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FMI 1: Data valid but below normal operational range (most severe)
SPN 2631 FMI 1 indicates a significantly low pressure at the outlet of the Engine Charge Air Cooler 1. This often occurs after a forced DPF regeneration when excess soot and debris have been cleared, leading to a temporary drop in pressure. In practice, technicians may encounter this fault following
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FMI 2: Data erratic, intermittent or incorrect
SPN 2631 FMI 2 indicates erratic or incorrect data from the engine charge air cooler outlet pressure sensor. This issue often arises after an engine component replacement or when the sensor connections are compromised. Technicians frequently encounter this code following the installation of a new ch
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FMI 3: Voltage above normal or shorted high
SPN 2631 FMI 3 indicates the charge air cooler outlet pressure sensor voltage exceeds normal operating parameters, typically above 4.5V reference. This fault commonly appears after turbocharger replacement when technicians inadvertently damage sensor wiring during installation. The ECM interprets th
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FMI 4: Voltage below normal or shorted low
This fault indicates the ECM has detected a voltage below the normal operating range on the charge air cooler outlet pressure sensor signal circuit. In practice, this code commonly appears after a forced DPF regeneration when excessive heat damages the sensor wiring harness near the turbocharger out
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FMI 5: Current below normal or open circuit
SPN 2631 FMI 5 indicates a potential open circuit or below-normal current in the charge air cooler outlet pressure sensor. This fault often arises after technicians replace sensors without fully verifying connector integrity. In practice, this code can appear after a vehicle undergoes a routine main
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FMI 6: Current above normal or grounded circuit
SPN 2631 FMI 6 indicates excessive current or grounded circuit in the charge air cooler outlet pressure sensor, critical for turbocharger system monitoring. This fault commonly appears during post-DPF regeneration diagnostics when elevated exhaust backpressure affects intake system readings. The ECM
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FMI 7: Mechanical system not responding properly
SPN 2631 FMI 7 indicates the Engine Charge Air Cooler 1 Outlet Pressure sensor signal is mechanically unresponsive, often due to a stuck boost pressure actuator or a collapsed charge air hose. Technicians commonly see this after a forced DPF regeneration when excessive heat warps the actuator linkag
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FMI 9: Abnormal update rate
SPN 2631 FMI 9 indicates an abnormal update rate for the Engine Charge Air Cooler 1 Outlet Pressure sensor, measured in kPa. This fault often arises after ECM replacement or when recalibrating the ECM following a forced DPF regeneration. The issue could lead to inaccurate pressure readings, affectin
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FMI 11: Root cause not known
SPN 2631 FMI 11 represents an unknown root cause fault in the engine charge air cooler outlet pressure monitoring system. This fault commonly appears in heavy-duty applications after aggressive driving cycles or following turbocharger replacement procedures. Technicians frequently encounter this cod
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FMI 12: Bad intelligent device or component
This fault indicates the ECM has detected an internal failure within the charge air cooler 1 outlet pressure sensor or its signal processing circuit. The sensor reports values outside a physically plausible range, often due to corrupted internal logic or a shorted ASIC. Technicians frequently encoun
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FMI 13: Out of calibration
The SPN 2631 FMI 13 code indicates that the engine charge air cooler 1 outlet pressure is out of calibration. This fault is critical as it affects engine efficiency and emissions. Typically, this code surfaces after mechanical interventions such as turbocharger replacements or after an ECM update. D
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FMI 14: Special instructions
Engine Charge Air Cooler 1 Outlet Pressure sensor requires special manufacturer-specific calibration or initialization procedures according to FMI 14 protocol. This fault commonly appears after ECM replacement in Cummins ISX engines or following turbocharger service on Detroit DD15 units, where the
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FMI 18: Data valid but below normal operating range (moderately severe)
SPN 2631 FMI 18 indicates the charge air cooler 1 outlet pressure is below the normal operating range but data is valid. This fault often appears after a forced DPF regeneration when the charge air system has a leak or restriction. Technicians may see it after replacing the ECM, requiring a calibrat
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FMI 31: Condition exists
SPN 2631 FMI 31 indicates a condition with the pressure at the outlet of the first charge air cooler. This issue often arises after ECM replacements or repairs, highlighting irregularities in the pressure sensor readings. Technicians frequently encounter this fault when the charge air cooler system