Seeing a check engine light that won’t turn off is frustrating. You might have tried clearing it, only to have it come right back. This is often due to a Permanent Diagnostic Trouble Code, or PDTC. This guide will explain exactly How To Clear Permanent Diagnostic Trouble Code and get your vehicle’s system back to normal.
Unlike regular codes, a PDTC is a special type of DTC that your car’s computer stores even after the fault is fixed. It’s part of modern On-Board Diagnostics II (OBD2) systems. Its job is to help verify that a repair was successful. You can’t simply erase it with a basic scanner. You need to complete a specific drive cycle to clear it.
How To Clear Permanent Diagnostic Trouble Code
Clearing a PDTC isn’t about deleting it with a tool. It’s about satisfying your car’s computer that the problem is truly gone. The process involves fixing the root issue first, then performing a drive cycle. This allows the vehicle to run its own tests and automatically clear the code. Here’s the overall strategy.
What is a Permanent DTC (PDTC)?
Since 2010, OBD2 regulations required automakers to use PDTCs. Think of a regular DTC as a “current alarm.” It turns on the check engine light immediately. A PDTC is more like a “confirmed fault log.” Even after you fix the problem and the light goes off, the PDTC remains in memory.
The car keeps it to monitor the repair. It wants to see that everything is working correctly over time and under various conditions. Until it does, the PDTC stays. This prevents someone from just clearing a serious code without fixing it.
Permanent vs. Pending vs. Active DTCs
It’s key to understand the difference:
- Active DTC: This is a current, confirmed problem. It illuminates the check engine light (MIL). It needs immediate attention.
- Pending DTC: This is a potential fault the computer has seen once or twice, but not enough times to confirm it. It does not turn on the check engine light.
- Permanent DTC: This is a confirmed fault that has been stored permanently until a successful drive cycle is completed after the repair. The check engine light may be off, but this code remains.
Tools You Will Need
You don’t need expensive equipment, but you do need the right tools.
- An OBD2 Scanner: Not all scanners are equal. You need one that can read both “Active” and “Permanent” DTCs. Many basic Bluetooth dongles and apps can do this.
- Your Vehicle’s Repair Manual: The drive cycle specifics vary by make and model. The manual provides the exact parameters.
- Basic Hand Tools: To perform the actual repair that caused the code in the first place.
Choosing the Right OBD2 Scanner
For PDTCs, ensure your scanner advertises “Readiness Monitor” status and can access “Stored” or “Permanent” codes. Mid-range scanners from brands like Autel, Launch, or even a good app like Torque Pro paired with a solid Bluetooth adapter will work. Cheap code readers that only clear active codes won’t help you see the PDTC.
Step-by-Step Guide to Clearing a PDTC
Follow these steps in order. Skipping step one is the most common mistake.
Step 1: Diagnose and Repair the Underlying Problem
This is the most critical step. You must identify and fix the issue that caused the code. Use your scanner to read the specific PDTC (e.g., P0420 – Catalyst System Efficiency Below Threshold).
Research what that code means for your vehicle. It could be a bad oxygen sensor, a leaking exhaust manifold, or a failing catalytic converter. Perform the necessary repair. Do not try to clear the PDTC before the repair is complete—it will not work.
Step 2: Clear All Active DTCs (If Any)
After the repair, connect your scanner. If the check engine light is still on, there will be an active DTC. Use your scanner’s “Clear Codes” function to erase the active codes. This will turn off the check engine light. The PDTC will remain, but that’s normal.
This step resets the monitors and prepares the system for its tests. Your scanner will likely show “Monitors Not Ready” or similar.
Step 3: Perform a Complete Drive Cycle
This is how you clear the PDTC. A drive cycle is a specific set of driving conditions that allows the car’s computer to run tests on all its systems. It is not just a long drive.
A generic OBD2 drive cycle often includes:
- Cold Start: The engine must be cold (coolant temp below 122°F and close to air temp).
- Idle: Start the engine and let it idle for 2-3 minutes with all accessories (A/C, lights, radio) off.
- City Driving: Accelerate gently to around 25-35 mph. Drive at steady speed for 1-2 miles. Include several stops with idle time.
- Highway Driving: Accelerate onto a highway. Drive at a steady 55-65 mph for 5-10 miles. This tests things like catalytic converter and fuel trim at load.
- Deceleration: Let off the gas and coast down slowly (in gear) from highway speed. This creates high vacuum for EVAP and other tests.
- Repeat: Some monitors may need a second warm-up cycle. A total drive of 30-50 minutes is typical.
Step 4: Verify the PDTC is Cleared
After completing the drive cycle, park and turn off the ignition. Wait a few minutes, then reconnect your scanner. Check the code menu. The PDTC should no longer be listed in the “Permanent” or “Stored” section.
Also, check the “I/M Readiness Monitors.” All monitors should show “Ready” or “Complete.” This confirms the vehicle has performed all its self-checks. If the PDTC is gone and monitors are ready, you have successfully cleared it.
What If the PDTC Won’t Clear?
Sometimes, the code comes back or stubbornly remains. Here are common reasons and solutions.
The Repair Was Incomplete or Incorrect
This is the number one cause. The fault might be intermittent, or you may have fixed one part but not the root cause. For a P0442 (small EVAP leak), you might have replaced the gas cap, but the leak could be in a hose. Double-check your repair work.
The Drive Cycle Was Not Properly Executed
Each vehicle has subtle differences. Your car might require a specific coolant temperature or a longer highway drive. Consult your owner’s manual or a service database for the exact drive cycle procedure for your model year. Missing one parameter can prevent a monitor from running.
There is an Additional Related Fault
Some codes have prerequisites. For example, the oxygen sensor heater circuit must work before the catalyst monitor can run. If there’s a separate, unrelated pending code, it might inhibit the monitor you need. Scan for any other codes, even pending ones, and address them.
Preventive Tips to Avoid Future PDTCs
Good maintenance reduces the chance of check engine lights and the hassle of PDTCs.
- Use a quality scanner regularly to check for pending codes before they become active.
- Address minor issues promptly. A faulty oxygen sensor can lead to a failed catalytic converter.
- Always tighten your fuel cap until it clicks three times. A loose cap is a common cause of EVAP codes.
- Follow your manufacturer’s recommended maintenance schedule for tune-ups, fluid changes, and filter replacements.
When to Seek Professional Help
If you’ve gone through multiple drive cycles and the PDTC persists, it’s time to consult a mechanic. They have advanced scan tools that can look at live data and see exactly why a specific monitor is not completing. They can also access manufacturer-specific drive cycle patterns that are not publicly available. This can save you hours of frustration.
Frequently Asked Questions (FAQ)
Can I clear a permanent DTC with a scanner?
No, you cannot directly clear a PDTC by using the “clear codes” function on any standard OBD2 scanner. The vehicle’s computer must clear it automatically after a successful drive cycle following a repair.
How long does it take to clear a permanent DTC?
It depends on the vehicle and the specific monitor that needs to run. A single, perfectly executed drive cycle can take 30 to 50 minutes of driving. Sometimes it may take a few days of normal driving for all conditions to be met.
Will disconnecting the battery clear a permanent DTC?
Disconnecting the battery will clear active codes and reset the computer, but it will not typically clear a PDTC. The PDTC is designed to survive a battery disconnect. It may also reset your readiness monitors, making you start the drive cycle process from scratch.
What is the difference between clearing a code and erasing a PDTC?
Clearing a regular (active) code simply deletes it from the short-term memory. Erasing a PDTC refers to the process of the vehicle’s computer automatically removing it after it verifies the problem is solved through a drive cycle. You initiate the first, the car does the second.
My check engine light is off, but I have a permanent code. Is this okay?
Yes, this is normal after a repair. The PDTC will remain stored as a historical record until the drive cycle is complete. As long as the check engine light is off and you have fixed the problem, you can proceed with the drive cycle to clear the PDTC.
Can a permanent DTC cause a smog check failure?
Yes, absolutely. In most areas, if any DTC (including a Permanent DTC) is present in memory, or if the readiness monitors are not all set to “Ready,” your vehicle will fail the OBD2 portion of the emissions test. Clearing the PDTC via a drive cycle is essential for passing.
Successfully dealing with a Permanent Diagnostic Trouble Code requires patience and methodical approach. Remember the sequence: fix the problem, reset the computer, then perform the precise drive cycle. With the right tools and this guide, you can turn that persistent check engine light off for good and ensure your vehicle is running properly. Always refer to your vehicle-specific information for the best results, as each model can have its own quirks.