Your truck speaks in fault codes, and Fault Mode Indicator (FMI) codes are one of its clearest languages.
Whether it’s a wiring issue, a faulty sensor, or a communication delay between components, FMI codes help you identify the root cause. In this article, we’ll break down the most common FMI codes, what they mean, and how you can use them to take the right next step fast.
What are FMI codes?
FMI codes work alongside SPN (Suspect Parameter Number) codes to describe what type of fault occurred. While SPN tells you where the issue is, FMI tells you what’s wrong. This is crucial for determining whether you’re looking at a simple wiring issue or a deeper component failure.
Let’s look at the most common FMI categories:
Component failures & wiring issues:
These faults typically deal with physical parts like sensors, actuators, or modules and their connections.
FMI 5 – Open Circuit or Low Current
Often, it means a wire is broken or disconnected. Think of it as the truck saying, “I’m not getting power where I should be.”
FMI 7 – Not Responding or Out of Adjustment
Indicates a part is there, but it’s acting out of spec or failing to respond.
FMI 12 – Bad Component or Module
Straightforward and serious, this means a part has likely failed and needs replacement.
Tip: Component failures are often easier to diagnose because the DTC usually points directly to the failing part, reducing the guesswork.
Data communication errors (CAN System)
Your truck’s brain relies on internal communication across the Controller Area Network (CAN). When that communication lags or breaks down, you’ll see these:
FMI 9 – Abnormal Update Rate
A key system (like the VGT actuator or EGR valve) isn’t sending updates fast enough. The engine control module starts asking, “Where’s the data?”
FMI 10 – Abnormal Rate of Change
Signals data is coming in, but it’s erratic or slower than expected. For example, SPN 3216 + FMI 10 could mean a slow or stuck NOx sensor.
These issues don’t always mean the part is broken; it could be a wiring glitch, a sensor lag, or a module beginning to fail.
Condition-based failures:
Not all faults are electrical. Some FMI codes indicate that operating conditions have drifted out of range:
FMI 0 or 1 – Above or Below Expected Range
Often triggered in aftertreatment systems. For instance, SPN 3364 + FMI 1 means the DEF quality is below the acceptable level.
FMI 31 – Condition Met
A catch-all code that shows a predefined condition has occurred, like low DEF or derate mode. It’s not always a fault in the traditional sense, but it helps explain why your truck is acting the way it is.
Diagnose Smarter with OTR Diagnostics
When you pair this code knowledge with a diagnostic tool like OTR Diagnostics, you gain the power to interpret and act immediately without waiting on a dealer.
The OTR Diagnostics app doesn’t just read fault codes; it helps you understand their meaning, perform resets, and monitor active data. It’s your shortcut from “What’s going on?” to “Problem solved.”
FMI codes are more than just numbers, they’re messages your truck sends to help you stay on the road. By understanding what these codes mean and how they relate to wiring, components, or system conditions, you can make faster, smarter decisions.
When paired with a tool like OTR Diagnostics, this knowledge becomes actionable power. Instead of wasting time guessing or sitting at the dealer, you’ll know what needs attention and when to take action.
Learn more about how OTR Diagnostics can save you time and money.
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