Auto Service World
Feature   May 1, 2009   by Claire Newman

Taking A First Look At Permanent DTCs

Designed To Make Emissions Testing Easier, Removing Ways To Cheat

Did you know that modern vehicles are really high-tech and automotive technology has changed over the years?

That statement usually makes technicians roll their eyeballs. No one knows better than technicians do just how advanced vehicles have become. And really, it’s not as if the people who spend their days working with vehicles haven’t figured this out yet.

The constant quest for better fuel economy and lower emissions has made engineers come up with incredible ways to make simple things very complicated, and technicians are sometimes left to clean up the aftermath of the engineers’ decisions, often by being as clever and skillful as the engineers themselves.

On that note, there’s another change coming down the pipes that technicians need to be aware of — Permanent DTCs. Starting with certain 2009 vehicles, and mandatory for all 2010 model year vehicles (and there’s already some of these on Canadian roads), this new type of DTC is being phased in to meet new American emission legislation.

Permanent DTCs are designed to stop dishonest folks from cheating on emission tests by just clearing the codes when the vehicle has an emission-related fault, without fixing the problem and allowing the vehicle to leave the shop without a proper fix. So the idea behind the technology is indeed noble.

But there’s one hitch. Permanent DTCs aren’t cleared in the usual way, with a scan tool or in some cases by disconnecting the battery. They have a unique way to clear them, and they’re already giving technicians a few headaches.

Permanent DTCs can only be cleared by proving to the computer that the problem has been fixed and the condition is not returning. And since emission programs and legislation are already causing more than enough headaches for Canadian shop owners and technicians, it’s important to understand how these new DTCs work to prevent problems and misdiagnosis.

Here’s a quick look at DTCs in 2009 and beyond:

Current DTCs

This type of DTC isn’t new, and these are still almost always the best kinds to deal with. What is new is quite often these used to be made up of a letter, followed by four numbers. Now the first digit is still a letter, but quite often letters are mixed-in with the four numbers that follow that prefix. Whichever way the DTC is constructed, the DTC will set when a problem is happening — making it much easier to diagnose and repair the cause of the recorded problem. Oneand two-trip (or more) detection logic still applies, meaning the code may not set the first time the fault happens. But the bottom line about current DTCs is the computer received an input that wasn’t within what its programming says is normal, and it’s made a note of it to help fix the problem — and whatever the problem was, it’s still happening right then. These can still be cleared in the usual way, but they’ll likely return quickly if the problem is not fixed.

History DTCs

These are readings that were out of parameter before, but aren’t out of range now. After a certain number of “clean” trips — trips when a fault doesn’t reoccur — a “current” DTC will become a “history” DTC. These can still be cleared in the usual way, after the data is recorded and you’re sure you won’t need it. The important thing here is not to erase information that could speed up diagnosis.

Pending DTCs

These are DTCs that were out of parameter once, but need to be out of parameter again before the MIL light comes on. Sometimes the problem was just a glitch, sometimes it’s something more serious. The important thing to understand is the cause of the pending DTC needs to be investigated further.

Permanent DTCs

These are the new, emission-related DTCs that can’t be cleared from the ECM or TCM until the computer knows for sure that the problem was fixed. This usually involves performing a Universal Trip Drive Pattern exactly as described, but there are other ways, such as performing a dedicated monitor cycle. Permanent DTCs first appeared on certain 2009 vehicles, and are being legislated on 2010 vehicles. As with any new technology, the “glitches” may need to be worked out in the field, so reading the repair manual and TSBs are important steps before beginning diagnosis.

Universal Trip Drive Pattern

So, how to clear a Permanent DTC? By performing a Universal Trip Drive Pattern that meets the strict criteria in the repair manual, to prove to the computer that the fault has indeed been repaired and no one is trying to cheat on an emissions test. After performing this Drive Pattern, if the code doesn’t clear it’s usually because the strict criteria were not followed exactly. It’s a pain, but that’s the way it goes.

Typically, this Universal Trip Drive Pattern starts off by clearing all the DTCs so that only the Permanent DTC remains (always refer to service information for applicable information and procedures).

Then turn the ignition ON, start the vehicle and let it idle for at least 30 seconds. Then, without cycling the key, drive the vehicle for at least five minutes at more than 40 km/h. Then (again without cycling the key) let the vehicle idle for at least 30 seconds before shutting it OFF. The whole drive cycle must take at least 10 minutes. After performing a Universal Trip Drive Pattern, the computer clears the code, since it’s “seen” that the problem is fixed and not reoccurring. Sounds simple enough, and most problems have apparently come from not performing the Universal Trip Drive Pattern correctly, but time will tell if these new DTCs will be problematic.

Dedicated monitor cycle

If you’d rather not perform a Universal Trip Drive Pattern, performing three dedicated monitor cycles will also clear a permanent DTC. This way takes more time and some research, but it works and it’s always good to know more than one way to get things done.

To clear the Permanent DTC using a dedicated monitor cycle, start by repairing the fault that caused the DTC in the first place. Then clear all the codes except for the permanent DTC (if it were that easy, there’d be no problems), and make sure the Permanent DTC is the only one left.

Then look up the details for that specific monitor drive cycle in the repair manual, and road test the vehicle under those conditions — three separate times. You’ll need to complete three of these cycles (from cold start if applicable) to successfully clear the code. After the monitor runs three times with no problems the code will clear and you’ll be happy (confirm this, though).

Changing technology is nothing new, and Permanent DTC legislation is nothing you can’t handle. A diagnostic trouble code is still, in its simplest form, a sign that an input to the computer wasn’t within the range of what the computer’s been told is acceptable. The basics are still the same, even if the repair strategy changed a bit.

Understanding what’s going on, performing the monitor drive cycle exactly as the repair manual directs, and remembering that the driving force behind this change is a fairer testing process (and ultimately lower pollution levels) will hopefully make this new technology easier to deal with. Because we all know that even more changes will be arriving shortly.


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9 Comments » for Taking A First Look At Permanent DTCs
  1. Ken L says:

    Great description of the codes. Recommend you re-order the descriptions to the sequence they occur in: Pending, current, permenant, historic.

  2. Michael says:

    How do I perform a Trip Cycle ‘A’ procedure and Trip Cycle ‘B’ procedure on a 2012 Nissan Xterra Pro 4X 4.0L automatic?

    • Dario Enrique Altamirano says:

      Great article. I did a dumb thing and tried to start my car with the fuel pump fuse pulled out. Then when I went to start my car, it gave me some permanent DTCs. I was very worried until I came across this article. It’s very well written too. Thanks again.

  3. Jimmy says:

    Definitley a good read and advice. Thank you for posting, as im still learning the new OBD2

  4. John Patrick Saathoff says:

    Thank you! Very well written and easy to understand.

  5. Argentin Filsaime says:

    Thank you so much for this article! It’s very well written, methodical and informative. I was dumb founded when I saw a permanent code on my scanner. This article really help me to understand what it it is. I thought working on helicopters was bad, but the new technology in today’s car is unbeleivable. Thanks again for the education.

  6. R says:

    How do you clear Historic DTC’S? Do you just clear the code with a scan tool?

  7. Jeff says:

    Is there a law that states you cannot clear an emission related code until you fix it ?

  8. Dale eather says:

    I have a 2004 Holden ra rodeo 2wd auto
    And it has repetly logged the code P1626
    Obd2 says theft deterrent fuel enable circuit.
    It’s has been cleared but comes back a couple days later.

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