Pending, stored and permanent trouble codes: what's the difference?

What's the difference between pending, stored and permanent codes?

A pending code is a first detection with the light still off; a stored code is a confirmed fault that turns the light on; and a permanent code is a copy of it that no scan tool can clear. Every OBD-II car and light truck (US model year 1996 and newer) stores diagnostic trouble codes (DTCs). A scan tool asks for each kind of code with a different request, called a mode or service. Any standard scan tool can make these requests, no subscription needed. The rules below come from California's OBD-II regulation, 13 CCR § 1968.2, which EPA adopts, in its 2013 version, for 2017 and newer vehicles in 40 CFR § 86.1806-17.

Misfire, evaporative-system (EVAP) and fuel-system faults have some special timing rules, so treat the "drives" above as the typical case, not a guarantee.

What is a pending code (mode 07)?

When a monitor first detects a problem, the car stores a pending code within ten seconds but doesn't turn on the check engine light. The regulation describes it as a code stored "upon the initial detection of a malfunction... prior to illumination of the MIL," and expects pending codes to give technicians "a quick and accurate indication of a pending failure."

What happens next depends on the next drive in which that monitor runs:

• Fault seen again: the code becomes confirmed and the light comes on.
• Fault not seen: the pending code is erased at the end of that drive.

That's why pending codes come and go. A single pending code after one odd cold morning may mean little. The same pending code appearing drive after drive is worth attention before it matures into a lit light. (Our inference; the regulation only sets when codes are stored and erased.)

Pending codes are required to be readable through the diagnostic port on 2005 and newer vehicles, using mode 07 (the regulation names "Mode/Service $07").

What is a stored code (mode 03)?

A stored or confirmed code is what most people mean by "a check engine code." It's set when the fault is detected a second time, and at that point the check engine light must stay on. The federal emissions-test procedure, 40 CFR § 85.2222, reads these with a mode 03 request whenever the car is commanding the light on, and fails the car.

Stored codes clear themselves in two steps:

1. The light goes out after three consecutive driving cycles in which the monitor runs and the fault isn't found.
2. The code itself stays in memory as history. The car may erase it once the fault hasn't come back for 40 warm-up cycles; on 2019 and newer vehicles it must, no sooner than 40 and no later than 41.

For what the light means for your driving today, see check engine light on: can I keep driving?

What is a permanent code (mode 0A)?

A permanent code is a copy of a confirmed code that is currently turning the light on, written to non-volatile memory (NVRAM) that keeps its contents with the power off. Permanent codes were phased in on 50% of 2010, 75% of 2011 and 100% of 2012 model-year vehicles, per EPA's 2013 OBD readiness memorandum.

The regulation is blunt: permanent codes "may not be erasable by any scan tool command (generic or enhanced) or by disconnecting power to the on-board computer." They're read with a separate request, which EPA identifies as J1979 "Mode/Service $0A," so inspection equipment can tell them apart from regular codes.

Why they exist. EPA says permanent codes were introduced "to make it more difficult for a vehicle owner or technician to clear the fault memory, which can potentially allow the vehicle to pass inspection without being repaired."

How they clear. Only the car's own OBD system can erase one, in one of two ways:

• Codes not cleared: the permanent code is erased when the light goes out, after the fault is absent for three consecutive driving cycles.
• Codes cleared with a scan tool or battery disconnect: the permanent code stays. It's erased at the end of a drive in which the monitor that found the fault runs and passes. For some monitors (misfire, fuel system and circuit checks, among others) the car must also have completed, on one drive (not necessarily the same one), at least 10 minutes (600 seconds) since engine start, at least 5 minutes (300 seconds) at 40 km/h (25 mph) or faster, and at least 30 seconds of continuous idle.

So the only real fix is to repair the fault and drive normally.

What are drive cycles and warm-up cycles?

The regulation defines both:

• Driving cycle: a trip from engine start to engine shutoff.
• Warm-up cycle: a trip long enough for the coolant to rise at least 22.2 °C (40 °F) from start and reach at least 71.1 °C (160 °F). A short hop in winter may not count.

A monitor only "runs" when its own monitoring conditions are met, and the regulation sets those per monitor. That's why a code can take several days of ordinary driving to clear, and why two drives around the block may not do it.

Why doesn't clearing codes before an emissions test work?

Because clearing resets the readiness monitors, and the permanent code stays behind. It's tempting to clear the light and head to the test station. It doesn't work, for two reasons.

1. Clearing resets the readiness monitors. Each monitor reports "complete" or "not complete" since the fault memory was last cleared (13 CCR § 1968.2). Clear the codes and every monitor goes back to not complete. The federal procedure allows a 2001 or newer vehicle to pass with at most one unset monitor (1996–2000 vehicles, two); with more, it's sent away to drive more, and failed if it's still not ready. GM's owner's manual for the 2025 Chevrolet Equinox warns it "might require several days of routine driving" to become ready. See why your car says "not ready".

2. The permanent code is still there. In California, the Bureau of Automotive Repair has failed 2010 and newer vehicles with a permanent code present since July 2019. The code is ignored only if the car has completed at least 15 warm-up cycles and 200 miles (about 320 km) since its OBD information was last cleared (BAR; BAR newsletter). California also allows only the EVAP monitor to be incomplete on 2000 and newer gasoline vehicles.

Rules differ by state. We checked the federal procedure and California's; check your own state's program before you test.

How do you read all three?

A generic OBD-II scan tool or app has to send each request separately: mode 03 for stored codes, 07 for pending and 0A for permanent. Some basic readers only ask for stored codes, so a car can show "no codes" while a pending code is building (inferred from how the requests work). If you're chasing an intermittent problem, a tool that shows all three tells you far more. Common codes worth reading up on: P0420 and P0171.