If you’ve ever had a check engine light appear on your dashboard, you’ve already brushed up against a world of diagnostic fault codes. These codes are the car’s way of telling you that something isn’t quite right—but not all fault codes are created equal. Some need immediate attention, while others are more of a historical record. So how do you tell the difference?
Let’s break down the main types of fault codes you might encounter and help you understand when it’s time to take action—or take a breath.
What Are Fault Codes?
Modern vehicles are equipped with onboard diagnostic systems (OBD-II for cars made since 1996) that constantly monitor performance. When the system detects a problem—whether it’s with emissions, the engine, transmission, or other critical systems—it stores a diagnostic trouble code (DTC) and may trigger the check engine light.
These codes can be read with a scanner or a mobile car inspection service, helping pinpoint what’s going wrong under the hood.
The 4 Main Types of Fault Codes
1. Stored (Active or Pending) Fault Codes
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What they are: These are current issues your car is experiencing. They’re the most important because they reflect problems happening right now.
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Examples: Misfires, sensor failures, or transmission errors.
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Should you worry? Yes. These codes mean there’s an active problem. If the check engine light is on and you see stored codes, get it checked right away.
2. Pending Codes
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What they are: These occur when the computer has noticed something odd, but not consistently enough to log it as a full-blown fault.
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Examples: An intermittent sensor glitch or temporary emissions hiccup.
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Should you worry? Not necessarily. But they’re worth watching. If they return on your next drive or get upgraded to a stored code, it’s time for action.
3. Permanent Codes
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What they are: These codes remain even after you clear stored codes, and they’re designed to prevent dishonest code clearing (like before selling a used car).
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Examples: Emissions system failures or catalytic converter inefficiencies.
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Should you worry? Yes, but context matters. Permanent codes only clear when the system verifies the problem is genuinely fixed. If one remains, it usually means a real repair still needs to happen.
4. Historic (or History) Codes
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What they are: These are logs of past problems that have since resolved or no longer meet the criteria for stored status.
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Examples: A misfire that happened last week due to bad gas.
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Should you worry? Usually not. If the problem hasn’t reoccurred, it’s likely a fluke. But repeated history codes for the same issue can point to an underlying trend.
How to Read These Codes
You can use an OBD-II scanner, or better yet, call a mobile car inspector who can bring professional-grade tools to your driveway and interpret the results for you. They’ll also check if a seller recently cleared codes to mask an issue—a common scam.
When Should You Be Concerned?
| Fault Code Type | When to Worry | What to Do |
|---|---|---|
| Stored/Active | Immediately | Inspect ASAP |
| Pending | If it persists | Monitor or recheck soon |
| Permanent | If it doesn’t clear | Fix the root cause |
| Historic | If repeated | Keep an eye out or ask an expert |
Final Tip: Always Inspect Before You Buy
If you’re buying a used car and fault codes show up—especially permanent or recently cleared ones—it’s a big red flag. That’s where a mobile pre-purchase inspection can save you thousands. These inspections often reveal not just what the car’s computer is saying, but also what it’s hiding.
Want to know if your next used car has hidden problems?
Book a mobile inspection with iNeedaPPi and get the real story—before you commit.
