# Iron Fault > Free plain-English explainer for J1939 SPN/FMI diesel and heavy-equipment fault codes, with links to official (legitimate) manual sources. Explanations are original and general. Informational only. Iron Fault explanations are written in our own words and are general — they are not OEM service procedures. Always verify with the manufacturer's service information and follow safety procedures. You are responsible for any work you perform. ## Key pages - [Fault code index](https://ironfault.com/codes/): all SPN and SPN+FMI pages - [FMI 0-31](https://ironfault.com/fmi/): what each failure mode means - [Manual finder](https://ironfault.com/manuals/): official manual sources by brand (we never host manuals) - [Disclaimer](https://ironfault.com/disclaimer/) ## SPNs - [SPN 27: EGR Valve Position](https://ironfault.com/spn/27/): Tracks how far open the exhaust gas recirculation (EGR) valve is compared with where the engine computer asked it to be. - [SPN 84: Wheel-Based Vehicle Speed](https://ironfault.com/spn/84/): Vehicle road speed calculated from a wheel or output-shaft speed signal. - [SPN 91: Accelerator Pedal Position 1](https://ironfault.com/spn/91/): How far the driver is pressing the throttle pedal, sent as a percentage to the engine computer. - [SPN 94: Fuel Delivery Pressure](https://ironfault.com/spn/94/): Low-pressure fuel supply pressure feeding the high-pressure pump, after the lift pump and filters. - [SPN 97: Water in Fuel Indicator](https://ironfault.com/spn/97/): Signals that the fuel/water separator has collected enough water to trip its sensor. - [SPN 100: Engine Oil Pressure](https://ironfault.com/spn/100/): Engine oil pressure measured in the main oil gallery. - [SPN 102: Intake Manifold #1 Pressure (Boost)](https://ironfault.com/spn/102/): Boost pressure in the intake manifold after the turbocharger and charge air cooler. - [SPN 105: Intake Manifold #1 Temperature](https://ironfault.com/spn/105/): Temperature of the air entering the cylinders after the charge air cooler. - [SPN 108: Barometric Pressure](https://ironfault.com/spn/108/): Outside air pressure, used to correct fueling and boost for altitude. - [SPN 110: Engine Coolant Temperature](https://ironfault.com/spn/110/): Temperature of the engine coolant, usually measured near the thermostat housing. - [SPN 111: Coolant Level](https://ironfault.com/spn/111/): Coolant level in the expansion tank or radiator, from a level probe or switch. - [SPN 157: Injector Metering Rail 1 Pressure (Fuel Rail)](https://ironfault.com/spn/157/): High-pressure fuel rail pressure on common-rail engines. - [SPN 168: Battery Potential / Power Input 1](https://ironfault.com/spn/168/): Voltage the engine computer sees on its main power supply. - [SPN 171: Ambient Air Temperature](https://ironfault.com/spn/171/): Outside air temperature, used for cold-start strategies and engine protection. - [SPN 175: Engine Oil Temperature 1](https://ironfault.com/spn/175/): Temperature of the engine oil. - [SPN 190: Engine Speed](https://ironfault.com/spn/190/): Engine RPM, usually calculated from the crankshaft and/or camshaft position sensors. - [SPN 412: EGR Temperature](https://ironfault.com/spn/412/): Temperature of recirculated exhaust gas after the EGR cooler. - [SPN 520: Actual Retarder – Percent Torque](https://ironfault.com/spn/520/): How much braking torque the retarder or engine brake is actually producing, as a percentage. - [SPN 627: Power Supply](https://ironfault.com/spn/627/): General power supply to the controller is outside the expected range or was interrupted. - [SPN 629: Controller #1](https://ironfault.com/spn/629/): The engine controller itself has detected an internal problem. - [SPN 636: Engine Position Sensor (Camshaft)](https://ironfault.com/spn/636/): Camshaft position signal used for injection timing and cylinder identification. - [SPN 637: Engine Timing Sensor (Crankshaft)](https://ironfault.com/spn/637/): Crankshaft position/speed signal, the main reference for engine speed and timing. - [SPN 639: J1939 Network #1 (Primary Data Link)](https://ironfault.com/spn/639/): The main J1939 CAN data link that modules use to talk to each other. - [SPN 641: VGT Actuator](https://ironfault.com/spn/641/): The electronic actuator that moves the vanes in a variable-geometry turbocharger. - [SPN 651: Injector Cylinder #01](https://ironfault.com/spn/651/): Circuit or performance fault for the cylinder 1 fuel injector. - [SPN 1569: Engine Protection Torque Derate](https://ironfault.com/spn/1569/): The engine has reduced its power on purpose to protect itself. - [SPN 1761: Aftertreatment 1 DEF Tank Volume](https://ironfault.com/spn/1761/): How full the diesel exhaust fluid (DEF) tank is. - [SPN 2791: EGR Valve Control](https://ironfault.com/spn/2791/): The output circuit the engine computer uses to drive the EGR valve. - [SPN 3216: Aftertreatment 1 Intake NOx](https://ironfault.com/spn/3216/): NOx sensor reading in the exhaust before the SCR catalyst (engine-out NOx). - [SPN 3226: Aftertreatment 1 Outlet NOx](https://ironfault.com/spn/3226/): NOx sensor reading in the exhaust after the SCR catalyst (tailpipe NOx). - [SPN 3242: Aftertreatment 1 DPF Intake Gas Temperature](https://ironfault.com/spn/3242/): Exhaust temperature entering the diesel particulate filter. - [SPN 3251: Aftertreatment 1 DPF Differential Pressure](https://ironfault.com/spn/3251/): Pressure difference across the diesel particulate filter — how hard the exhaust is working to push through it. - [SPN 3364: Aftertreatment 1 DEF Quality](https://ironfault.com/spn/3364/): Whether the fluid in the DEF tank has the right urea concentration. - [SPN 3719: Aftertreatment 1 DPF Soot Load Percent](https://ironfault.com/spn/3719/): Estimated soot load in the diesel particulate filter as a percentage of full. - [SPN 4364: Aftertreatment 1 SCR Conversion Efficiency](https://ironfault.com/spn/4364/): How well the SCR system is reducing NOx compared with what it should be doing. - [SPN 5246: Aftertreatment SCR Operator Inducement Severity](https://ironfault.com/spn/5246/): The emissions system is forcing the operator to fix an SCR/DEF problem by stepping up warnings and derates. ## FMIs - [FMI 0](https://ironfault.com/fmi/0/): Value too high (most severe) - [FMI 1](https://ironfault.com/fmi/1/): Value too low (most severe) - [FMI 2](https://ironfault.com/fmi/2/): Data erratic, intermittent, or incorrect - [FMI 3](https://ironfault.com/fmi/3/): Voltage above normal or shorted high - [FMI 4](https://ironfault.com/fmi/4/): Voltage below normal or shorted low - [FMI 5](https://ironfault.com/fmi/5/): Current below normal or open circuit - [FMI 6](https://ironfault.com/fmi/6/): Current above normal or grounded circuit - [FMI 7](https://ironfault.com/fmi/7/): Mechanical system not responding or out of adjustment - [FMI 8](https://ironfault.com/fmi/8/): Abnormal frequency, pulse width, or period - [FMI 9](https://ironfault.com/fmi/9/): Abnormal update rate - [FMI 10](https://ironfault.com/fmi/10/): Abnormal rate of change - [FMI 11](https://ironfault.com/fmi/11/): Root cause not known - [FMI 12](https://ironfault.com/fmi/12/): Bad intelligent device or component - [FMI 13](https://ironfault.com/fmi/13/): Out of calibration - [FMI 14](https://ironfault.com/fmi/14/): Special instructions - [FMI 15](https://ironfault.com/fmi/15/): Value too high (least severe) - [FMI 16](https://ironfault.com/fmi/16/): Value too high (moderately severe) - [FMI 17](https://ironfault.com/fmi/17/): Value too low (least severe) - [FMI 18](https://ironfault.com/fmi/18/): Value too low (moderately severe) - [FMI 19](https://ironfault.com/fmi/19/): Received network data in error - [FMI 20](https://ironfault.com/fmi/20/): Data drifted high - [FMI 21](https://ironfault.com/fmi/21/): Data drifted low - [FMI 31](https://ironfault.com/fmi/31/): Condition exists / not available ## Manuals by brand - [Cummins](https://ironfault.com/manuals/cummins/) - [Caterpillar (Cat)](https://ironfault.com/manuals/caterpillar/) - [John Deere](https://ironfault.com/manuals/john-deere/) - [Detroit Diesel (DTNA)](https://ironfault.com/manuals/detroit/)