Accelerator Pedal 1 Position (SPN 91) is one of the most fundamental control parameters in modern diesel engines, monitoring the primary operator input for engine speed and torque requests. This critical parameter is generated by virtually every diesel engine ECM including Cummins ISX, ISM, and QSK series, Detroit Diesel DD13/DD15/DD16, Caterpillar C7/C13/C15/C18/3406E, Volvo D11/D13/D16, PACCAR MX-11/MX-13, John Deere PowerTech series, and Mercedes-Benz OM470/OM471 engines. The parameter represents the calculated ratio of the accelerator pedal’s actual position to its maximum travel, expressed as a percentage from 0% (idle) to 100% (full throttle). This SPN is essential for diagnostics because it directly correlates operator intent with engine response, making it crucial for troubleshooting performance complaints, throttle response issues, drivability problems, and electronic throttle control malfunctions across heavy-duty trucks, construction equipment, agricultural machinery, marine applications, and stationary power generation systems.
Technical Overview
The accelerator pedal position is typically measured using dual potentiometric position sensors or Hall effect sensors mounted within the pedal assembly or connected via mechanical linkage to the throttle control. Most modern implementations utilize redundant sensor configurations for safety—commonly employing two independent analog voltage signals with different voltage ranges (typically 0.5-2.5V and 2.5-4.5V) to enable cross-checking and fault detection. Cummins INSITE, Detroit Diesel DDDL, and Caterpillar ET systems display this as a processed value where the ECM performs rationality checks between sensor inputs, applies calibrated scaling factors, and implements manufacturer-specific algorithms to determine the final percentage. The ECM continuously monitors both sensor circuits for proper voltage levels, signal correlation, and rate-of-change limitations. Normal operating characteristics include smooth voltage transitions during pedal movement, proper return-to-idle functionality (typically 3-8% at rest depending on calibration), and consistent tracking between redundant sensors within manufacturer-specified tolerances (usually ±5-10%). Advanced systems incorporate kick-down switches, idle validation switches, and cruise control integration that can influence the final calculated position value transmitted as SPN 91.
J1939 Network Behavior
SPN 91 is transmitted within Parameter Group Number (PGN) 61443 (0xF003) – Electronic Engine Controller 2 (EEC2), which is broadcast at a standard 10Hz (100ms) transmission rate by the engine ECM using its designated source address (typically 0x00). This high-frequency transmission ensures real-time responsiveness for engine control and allows other network modules to monitor operator intent for coordinated system responses. The transmission control module uses this data for shift scheduling and torque converter lockup strategies, while aftertreatment systems reference pedal position for regeneration inhibit logic and DPF loading calculations. Body control modules monitor this parameter for idle shutdown timers, PTO engagement permissions, and auxiliary system control. The parameter is encoded as a 2-byte value with 0.4% resolution per bit, providing precise granularity for smooth engine response. During network diagnostics, technicians can observe this PGN using tools like the NEXIQ Pro-Link iQ, Noregon JPRO, or OEM-specific software to verify real-time pedal position data, confirm proper ECM broadcasting behavior, and identify communication faults that might cause delayed throttle response or intermittent control issues across the vehicle’s integrated systems.
Diagnostic Importance
Faults affecting accelerator pedal position are classified as critical safety issues because they directly impact vehicle control and can trigger immediate engine protection responses. When SPN 91 faults occur, ECMs typically activate engine derate strategies ranging from 25% power reduction to complete engine shutdown depending on fault severity and manufacturer calibration. Cummins engines may implement “limp-home” mode limiting RPM to 1200-1500, while Detroit Diesel systems often trigger amber or red engine protection lamps with corresponding fault codes in the 3597, 3598, or 3599 families. Caterpillar systems frequently generate event codes in the 157-xxx series with associated engine derates. Critical consequences include loss of throttle response, uncontrolled engine speed variations, inability to achieve rated power, and potential safety hazards in mobile applications. The ECM may default to a predetermined throttle position (typically 20-40%) or rely on cruise control inputs when primary accelerator signals are compromised. Ignoring active fault codes can lead to complete loss of operator speed control, triggering more severe protection modes that may strand equipment or vehicles. Modern emission-compliant engines integrate accelerator position with aftertreatment strategies, so faults can also disrupt DPF regeneration cycles and SCR system efficiency, potentially causing secondary emission-related fault codes and additional system complications requiring comprehensive diagnostic attention.
Common Failure Patterns
The most frequent failure patterns involve accelerator pedal position sensor degradation due to environmental contamination, particularly in construction and agricultural applications where sensors are exposed to moisture, dirt, and temperature extremes. Potentiometric sensors commonly develop worn resistance tracks causing erratic voltage signals, dead spots during pedal travel, or complete signal loss. Wiring harness issues represent another major failure category, including connector corrosion at the pedal assembly (especially in high-moisture environments), chafing damage from cable routing near moving components, and intermittent connections causing signal dropouts during vehicle operation. Hall effect sensors typically fail due to magnetic field contamination from metal debris or internal electronic component degradation. Calibration drift occurs over time as sensor characteristics change, causing incorrect position reporting where 100% pedal position may only register as 85-90% in the ECM, resulting in reduced power availability. Mechanical linkage problems in systems using remote-mounted sensors include binding, wear, or misalignment affecting position accuracy. Ground circuit integrity issues frequently cause voltage reference problems leading to incorrect position calculations. Multi-sensor systems may develop correlation faults where one sensor provides accurate data while the secondary sensor fails, triggering rationality fault codes even when actual pedal position tracking remains functional, requiring careful diagnosis to identify the specific failed circuit.
Diagnostic Approach
Effective diagnosis requires a systematic approach beginning with real-time parameter monitoring using professional diagnostic tools like INSITE, DDDL, ET, VCADS, or ServiceLink to observe SPN 91 behavior during controlled pedal movement. Initial verification involves confirming proper idle position reading (typically 3-8%), smooth progression through the operating range without dead zones or erratic jumps, and achievement of 100% at full pedal depression. Voltage testing at the ECM harness connector using a digital multimeter with the pedal disconnected should verify proper reference voltage supply (usually 5V) and clean ground circuits with less than 0.1V resistance. Individual sensor circuit testing requires back-probing connector pins while cycling the pedal to observe smooth voltage transitions within specified ranges—primary sensors typically operate 0.5-4.5V while secondary sensors may use inverse scaling or different voltage windows. Advanced diagnostics include monitoring both sensor circuits simultaneously to verify proper correlation and identifying which sensor has failed in dual-sensor configurations. Resistance testing of potentiometric sensors with the system de-energized can reveal worn tracks or internal opens, while oscilloscope analysis may be necessary to identify intermittent signal dropouts or noise issues. When basic electrical checks pass but faults persist, calibration procedures using OEM software may be required to reset adaptive parameters or re-establish sensor scaling values, particularly after component replacement or mechanical adjustments to pedal assemblies.
Fault Codes for SPN 91
FMI 0: Data valid but above normal operational range (most severe)
SPN 91 FMI 0 indicates that the accelerator pedal position is reporting a value above the normal operational range, signaling a severe condition. This fault often appears in practice when an ECM recalibration is forced or after a throttle position sensor replacement. Technicians usually encounter th
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FMI 1: Data valid but below normal operational range (most severe)
SPN 91 FMI 1 indicates the primary accelerator pedal position sensor reports a signal below the calibrated minimum operational threshold. This fault triggers when the ECM detects pedal position values consistently under 0.5% for more than 200ms. Technicians commonly encounter this code after moistur
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FMI 2: Data erratic, intermittent or incorrect
SPN 91 FMI 2 signifies an erratic, intermittent, or incorrect reading from the accelerator pedal position sensor. This issue often emerges following ECM updates or when the pedal assembly is replaced without proper calibration. Technicians frequently encounter this fault after performing repairs rel
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FMI 3: Voltage above normal or shorted high
SPN 91 FMI 3 indicates the ECM detected voltage above normal or a short to high on the accelerator pedal position sensor 1 circuit. This fault commonly appears after a forced DPF regeneration when heat damages wiring or after replacing the ECM without recalibrating the pedal. The ECM uses a 5V refer
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FMI 4: Voltage below normal or shorted low
SPN 91 FMI 4 is a fault in the Accelerator Pedal Position indicating a voltage level below normal. This issue frequently arises after a forced ECM replacement or when the wiring harness is disturbed during maintenance. Commonly, this code appears when technicians replace an electronic engine control
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FMI 5: Current below normal or open circuit
The SPN 91 FMI 5 fault code relates to the accelerator pedal position, indicating an open circuit or below-normal current in the input device. This commonly occurs when technicians replace the ECM and forget to properly calibrate or connect the pedal sensor. The code is often triggered under low-loa
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FMI 6: Current above normal or grounded circuit
SPN 91 FMI 6 indicates excessive current flow or grounded circuit in the primary accelerator pedal position sensor. This fault commonly appears after water ingress during pressure washing or when harnesses chafe against engine components during heavy vibration. The ECM detects current exceeding manu
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FMI 7: Mechanical system not responding properly
SPN 91 FMI 7 indicates the ECM detected the accelerator pedal 1 position signal is not responding properly to mechanical input. This often occurs after a forced DPF regeneration when soot debris lodges in the pedal hinge, or when a worn return spring prevents full release. The ECM expects a smooth v
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FMI 8: Abnormal frequency, pulse width or period
SPN 91 FMI 8 indicates the ECM detected an abnormal frequency, pulse width, or period on the accelerator pedal position signal. This fault often appears after a recent ECM replacement or harness repair where the pedal signal frequency deviates from the expected 200–500 Hz range. Technicians may find
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FMI 9: Abnormal update rate
SPN 91 FMI 9 indicates an abnormal update rate in the accelerator pedal position sensor. This fault often appears in practice after an ECM software update when the recalibration of the throttle sensor is overlooked. Technicians might encounter inconsistent engine response, particularly during rapid
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FMI 11: Root cause not known
SPN 91 FMI 11 indicates the ECM detects an accelerator pedal position sensor fault but cannot determine the specific root cause. This code frequently appears after component replacements or intermittent electrical issues. Technicians commonly encounter this fault during cold weather operations when
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FMI 12: Bad intelligent device or component
SPN 91 FMI 12 indicates the ECM has detected a failure within the accelerator pedal’s intelligent circuitry, such as a corrupted internal memory or communication fault in the pedal sensor module. This code commonly appears after a forced DPF regeneration when excessive heat damages the pedal’s inter
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FMI 13: Out of calibration
SPN 91 FMI 13 is an out-of-calibration fault for the accelerator pedal position. This fault can occur when the pedal’s position sensor doesn’t correctly report its position relative to its maximum range. Commonly, this issue arises after maintenance involving the throttle assembly or ECM replacement
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FMI 14: Special instructions
SPN 91 FMI 14 indicates the ECM requires special instructions for accelerator pedal position calibration or configuration. This fault commonly appears after ECM replacement in Caterpillar C15 engines or following accelerator pedal sensor replacement in Mercedes-Benz OM471 applications. The fault sig
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FMI 18: Data valid but below normal operating range (moderately severe)
This fault occurs when the ECM detects the accelerator pedal position signal is valid but consistently below the expected minimum threshold. In practice, this often appears after a forced DPF regeneration when the pedal is not fully released, causing the ECM to interpret a low idle request as a faul
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FMI 31: Condition exists
SPN 91 FMI 31 is associated with the accelerator pedal position sensor, indicating a condition in the calculated position ratio. This fault often arises in situations where the ECM fails to receive a consistent signal from the pedal, commonly after maintenance activities like sensor replacement or E