“pdtc's, Permanent Headache? Not If You Read This First”
- Nic Wright
- Nov 17
- 9 min read
Updated: Nov 17
Permanent Diagnostic Trouble Codes (PDTCs) and Your California Smog Check
What Every Driver Should Know – from Midnight Smog in San Marcos, CA
If you’ve had a check-engine light recently, you might hear a shop mention “Permanent Diagnostic Trouble Codes” or PDTCs. They sound serious (and they can be) – but they’re also one of the ways California keeps the Smog Check program fair and honest.
At Midnight Smog in San Marcos, we talk to a lot of drivers who are confused or stressed about PDTCs, especially right before a Smog Check.
This post breaks things down in plain English:
Why PDTCs were added to the California Smog Check program
How PDTCs work inside your car’s computer
What they mean for your smog inspection result
How to approach your Smog Check at Midnight Smog if PDTCs are involved
For more background, the California Bureau of Automotive Repair (BAR) has several official write-ups on PDTCs, including their Spring 2019 and Fall 2019 newsletter articles and technical references:
1. What is a PDTC?
Modern vehicles use On-Board Diagnostics (OBD II) to monitor emissions systems and turn on the check-engine light when something isn’t working correctly. When a fault is detected, the car stores a Diagnostic Trouble Code (DTC).
A Permanent Diagnostic Trouble Code (PDTC) is basically the same type of fault code, but with one huge difference:
A regular DTC can be cleared with a scan tool or by disconnecting the battery.
A PDTC cannot be cleared that way – it stays stored until the vehicle proves, through its own self-tests, that the problem has actually been fixed.
BAR explains it this way in their newsletter and OBD reference: PDTCs use the same P-codes as regular DTCs, but cannot be reset by disconnecting the vehicle’s battery or cleared using an OBD scan tool. They can only be cleared by fixing the underlying problem and allowing the vehicle enough drive time to rerun the monitor so the car can verify the fault is gone.
In simple terms, a PDTC is your car’s way of saying:
“At some point, this emissions system failed. I’m not forgetting that until I’m sure everything has run correctly again.”
→ BAR Spring 2019 PDTC overview Bureau of Automotive Repair→ BAR On-Board Diagnostic Test Reference – PDTC section Bureau of Automotive Repair
2. Why did California add PDTCs to the Smog Check program?
On July 1, 2019, the California Bureau of Automotive Repair (BAR) officially incorporated PDTCs as part of the Smog Check failure criteria for 2010 and newer vehicles.
BAR’s Fall 2018 and Fall 2019 newsletters and their PDTC implementation documents explain the reasons for this change:
→ BAR Fall 2019 PDTC article Bureau of Automotive Repair→ BAR OBD Test Reference – PDTC standards table Bureau of Automotive Repair
a) Stopping the “clear the codes and hope” trick
Before PDTCs were part of the program, someone could:
Clear the codes with a scanner or by disconnecting the battery
Drive just enough to get some monitors “ready”
Rush into a Smog Check before the car had a chance to rediscover the fault
BAR notes that PDTCs were specifically designed to prevent “DTC clearing to obtain a passing test” and make sure the fault is genuinely repaired, not just temporarily hidden.
b) Making sure tricky systems don’t get ignored
Systems like the evaporative emissions system (EVAP) and certain diesel after-treatment components can be slow or picky about when their self-tests run. BAR’s analysis found cases where:
An EVAP or after-treatment monitor was incomplete, yet
There was a historic fault (PDTC) for that same system,
And the vehicle still passed under the old rules.
The PDTC criteria were added to close that loophole, especially around EVAP leaks and diesel after-treatment issues.→
See EVAP and diesel analysis slides in the 2018 PDTC workshop PDF Bureau of Automotive Repair
c) Protecting air quality and honest repairs
BAR’s PDTC articles and implementation documents all point back to the same goals:
Improve the effectiveness of the OBD-based Smog Check
Reduce the number of vehicles that pass with unresolved emissions problems
Keep the program fair for both motorists and shops who do proper repairs
3. How PDTCs actually work inside your car
Here’s the basic life-cycle of a PDTC, consistent with BAR and EPA guidance:
A fault occurs
An emissions-related problem happens (for example, an EVAP leak or catalyst issue).
The car sets a regular DTC, may turn on the check-engine light (MIL), and sets a PDTC for that same fault.
The issue is diagnosed and repaired
A technician repairs the underlying problem.
They may clear the codes to turn the light off and reset monitor status.
The car has to prove the fix
After repairs, the car runs a series of self-diagnostic drive cycles during normal driving.
When the OBD system finishes its tests and sees that everything is working correctly, it will erase the PDTC on its own.
BAR summarizes it as: PDTCs can only be cleared by the vehicle’s OBD system after it determines the problem is no longer present and the related monitor has run to completion.
→ BAR Spring 2019 PDTC FAQ Bureau of Automotive Repair→ EPA best-practices document on Permanent-DTC behavior (PDF) EPA
The “warm-up cycles” and miles rule
To avoid over-penalizing motorists when monitors are slow to run, BAR created a specific safety net:
PDTCs will be ignored during Smog Check once the vehicle has completed at least 15 warm-up cycles and been driven at least 200 miles since its OBD information was last cleared.
This “15 warm-ups / 200 miles” standard is documented in BAR’s newsletters and implementation updates:
BAR also clarifies what a warm-up cycle is: basically a trip where the engine is started cold, the coolant temperature rises by at least 40°F, and reaches at least 160°F before shutting off. Most cars will naturally accumulate enough warm-ups and miles through a few days of normal driving.
4. How PDTCs affect your California Smog Check at Midnight Smog
For 2010 and newer vehicles that support PDTCs:
If a PDTC is stored and you have not yet met the 15 warm-ups / 200 miles conditions→ The BAR-OIS system can fail the vehicle on the PDTC check, even if the check-engine light is currently off.
If a PDTC is stored but you have:
Driven at least 200 miles, and
Completed at least 15 warm-up cycles,
Since the last time the codes were cleared
The system will ignore the PDTC and move on to evaluating the rest of the OBD data (monitors, MIL status, etc.).
So your Smog Check result comes down to a few key questions:
Is the check-engine light off now?
Are the required readiness monitors set to “ready”?
If there’s a PDTC, have you given the car enough time and driving (15 warm-ups / 200 miles) since codes were cleared?
If all of those are in good shape (and you pass the visual/functional parts of the inspection), you can still pass your Smog Check at Midnight Smog even if a PDTC is still in the car’s memory.
BAR’s test reference and PDTC implementation update spell this out as the standard:
5. What should you do if you have (or suspect) PDTCs?
If you recently had a check-engine light and you’re worried about PDTCs before coming in for your Smog Check, here’s a simple game plan from Midnight Smog.
Step 1 – Make sure the problem is actually fixed
PDTCs exist to nudge people toward real repairs, not shortcuts.
Have a qualified technician diagnose and repair the root cause of the original DTC/PDTC.
Avoid just clearing codes and hoping for the best.
If the check-engine light comes back on, or the same PDTC returns during your post-repair driving, that’s a sign something still isn’t resolved and further repair may be needed.
(For tech-level detail on how PDTCs clear after repairs, BAR references SAE J1979 behavior in its workshop and implementation docs.)
Step 2 – Don’t keep clearing codes
Every time codes are cleared:
The 200-mile and 15 warm-up cycle counters reset to zero.
Your readiness monitors can go back to “not ready.”
That pushes your successful Smog Check further away. The best move is usually to leave the codes alone after a proper repair and let the car prove itself.
Step 3 – Drive normally, but enough
After repairs, you want to:
Drive a mix of city and highway
Over several days, if possible
Until you’ve comfortably exceeded 200 miles and 15 warm-up cycles
BAR’s data shows most vehicles complete their self-diagnostic tests well before the 15/200 maximum, but those limits are there as a reasonable upper bound to protect motorists.
→ BAR Fall 2019 PDTC article – most vehicles complete tests before 15/200 Bureau of Automotive Repair
Once you’ve done that, your car either:
Erases the PDTC on its own, or
Leaves it in memory but allows the Smog machine to ignore it thanks to the 15/200 rule.
Step 4 – Then visit Midnight Smog
Once you’ve:
Driven at least 200 miles after the last code clear
Accumulated 15+ warm-up cycles, and
Confirmed the check-engine light has stayed off
Then you’re in the best position to pass your Smog Check with PDTCs in the picture.
At Midnight Smog in San Marcos, we’re always happy to talk through where you’re at with miles, warm-up cycles, and recent repairs before we run the test – especially if you’re unsure whether your car is ready.
6. Common PDTC Questions & Myths (Answered by Midnight Smog)
“Can Midnight Smog clear my PDTC so I can pass?”
No. PDTCs cannot be cleared by a smog station, scan tool, or by disconnecting the battery. BAR emphasizes that PDTCs:
Use the same codes as regular DTCs, but cannot be cleared by code clearing or battery disconnect, and can only be cleared by the OBD system itself after the issue is resolved.
They only:
Clear themselves once the car’s self-tests confirm the issue is fixed, or are ignored by the Smog system after you’ve met the 15 warm-up / 200-mile requirement.
→ BAR Spring 2019 PDTC article Bureau of Automotive Repair→ PDTC Implementation Plan Bureau of Automotive Repair
“Does a PDTC mean my car is broken right now?”
Not necessarily. A PDTC means:
There was an emissions-related fault at some point, and the vehicle hasn’t yet completed enough successful self-tests to fully forget about it.
BAR’s own examples show vehicles can have PDTCs stored while monitors finish running; the key is whether the fault is actually fixed and enough warm-ups/miles have occurred.
If the check-engine light is off, your monitors are ready, and you’ve driven enough since the last repair, you may still pass the Smog Check at Midnight Smog even if a PDTC is still stored.
“Why is California so strict about this?”
Because without PDTCs, it’s too easy to game the system by clearing codes right before inspection. BAR directly states that the purpose of PDTCs is to prevent DTC clearing to obtain a passing test and to improve the effectiveness of the Smog Check program.
→ BAR PDTC workshop – stated purpose of PDTCs Bureau of Automotive Repair→ BAR Fall 2019 PDTC article – implementation and impact Bureau of Automotive Repair
7. Final Thoughts – and How Midnight Smog Can Help
Permanent Diagnostic Trouble Codes sound intimidating, but they don’t have to be a nightmare.
For drivers in San Marcos, Vista, Carlsbad, Oceanside, and the greater North County area, here’s the bottom line from Midnight Smog:
PDTCs protect you from incomplete or low-quality repairs.
They protect the environment by keeping clearly broken cars from sliding through.
If you fix the problem, avoid clearing codes repeatedly, and drive enough, PDTCs won’t stop you from passing your Smog Check.
If you’re unsure whether your car is ready for a Smog Check because of a past check-engine light or recent repairs:
👉 Give Midnight Smog a call or stop by our San Marcos location. We’ll walk you through what PDTCs mean for your specific situation and help you decide the best time to test, so you’re not wasting time, money, or stress.
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