Stored, pending and permanent codes and freeze frame
A scanner can show three kinds of trouble codes plus a freeze frame snapshot. Knowing the difference tells you whether a fault is confirmed, still being checked or already fixed but not yet verified.
How the check engine light behaves
- When you switch the ignition on, the light comes on briefly as a bulb check, then goes off after the engine starts if no fault is stored.
- It stays on when the OBD system has detected and confirmed an emission related malfunction.
- It blinks when a misfire is happening that could damage the catalytic converter. Reduce driving and have the car checked as soon as possible.
- It can turn off on its own if the fault does not return over three consecutive drive cycles. The code history can still be read with a scanner.
Stored, pending and permanent codes
| Type | What it means | Turns on the light? | Can you clear it with a scanner? |
|---|---|---|---|
| Pending | A fault seen once. If it is seen again on the next drive cycle it becomes a confirmed code. If it does not return, it is eventually erased. | No | Yes |
| Stored (confirmed) | A fault the system has confirmed, usually on two drive cycles. | Yes | Yes, but it returns if the fault is still there |
| Permanent | A copy of a confirmed code kept in memory that survives clearing. | Not by itself | No. It clears itself after the system verifies the repair |
Pending codes are useful for checking a repair: after fixing something and clearing codes, a pending code after one drive shows the fault is still there. Permanent codes cannot be erased by clearing codes or disconnecting the battery. The OBD system removes them itself once the light is commanded off and the related monitor has run without finding the fault.
What is freeze frame data?
Freeze frame is a snapshot of engine conditions saved at the moment an emission related code is stored. It typically includes:
- Engine speed (RPM) and vehicle speed
- Engine coolant temperature
- Calculated engine load
- Fuel system status (open or closed loop) and fuel trim values
Usually only one freeze frame is kept. A misfire or fuel system code takes priority and replaces an earlier freeze frame from another fault.
How to use freeze frame
- Read and save the freeze frame before clearing codes. Clearing the codes can erase it.
- Note the coolant temperature and speed: they show whether the fault happened on a cold start, at idle or on the highway.
- Compare with live data under the same conditions to try to reproduce the fault.
- If several codes are stored, note which code the freeze frame belongs to.
Reading all of this with OBD Master Pro
OBD Master Pro reads stored, pending and permanent codes and shows freeze frame data, so you can save everything before you clear a code. After a repair, check for pending codes and the emission readiness check before an inspection.
Try it on your own car
OBD Master Pro turns your Android phone and a generic ELM327 adapter into a code reader: read and clear codes, view freeze frame and live data, and check emissions readiness.
Keep reading
Related guides: Check engine light on: what to do · How to read an OBD2 code · OBD2 readiness monitors explained · ELM327 adapter not connecting · How to clear the check engine light · What is fuel trim · OBD2 live data and PIDs · OBD2 port pinout · What is a U code? · OBD2 glossary
Common codes: P0171 System Too Lean · P0420 Catalyst Efficiency · P0300 Random Misfire · P0455 EVAP Large Leak
All OBD2 guides · OBD2 by car brand · OBD2 trouble code guides
Sources
- U.S. Code of Federal Regulations, 40 CFR 86.1806-05, On-board diagnostics (govinfo.gov)
- U.S. EPA: Performing Onboard Diagnostic System Checks as Part of a Vehicle Inspection and Maintenance Program (EPA420-R-01-015)
- Ford service information: On Board Diagnostics II PCED, Quick Test and Continuous Memory (pending codes)
- Ford service information: On Board Diagnostics II PCED, Freeze Frame Data
- California Bureau of Automotive Repair: On-Board Diagnostic Test Reference
- California Bureau of Automotive Repair: Permanent Diagnostic Trouble Codes, workshop presentation (2018)
- California Air Resources Board (CARB): On-Board Diagnostic II (OBD II) Systems fact sheet
- Clemson University Vehicular Electronics Laboratory: On-Board Diagnostic Systems
- OBD Master Pro on Google Play (app features, supported connections and protocols)
Published . Updated . General information only. Details generally vary by model, year and market, so always follow the owner's manual and service information for your vehicle.