Key takeaways
- Clearing codes turns the light off, but it resets counters that any proper OBD-II scan can read.
- Read four things together: readiness monitors, distance since clear, warm-ups since clear and permanent codes.
- A permanent code with zero stored codes is the clearest single sign of a recent clear on newer vehicles.
- Cold weather slows some monitors, especially EVAP, so one incomplete monitor in January is not proof.
- A recent clear after a documented repair is normal. A recent clear with no paperwork should change your offer.
What happens when someone clears check-engine codes?
Clearing codes, with a scan tool’s “erase” command (OBD-II Mode 04) or on many vehicles by disconnecting the battery, does more than turn the light off. It erases stored and pending codes and the freeze-frame snapshot, sets every readiness monitor back to incomplete, and restarts the counters for distance, warm-ups and time since the clear.
What it does not do is fix anything. The fault is still there, and the light comes back once the relevant self-test runs again and fails. That can take a few days or a few hundred km, which is exactly the window a quick clear before an appraisal is meant to exploit.
What are readiness monitors and why do they matter?
Readiness monitors are the self-tests the engine computer runs on emission-related systems. Each one needs particular driving conditions to finish, and after a clear each reads “not ready” until those conditions happen.
| Monitor | What it tests | What it usually needs to finish |
|---|---|---|
| Misfire, fuel system, components | Continuous checks | Normal running; these complete quickly |
| Catalyst | Catalytic converter efficiency | Warm engine and steady cruising |
| Oxygen sensor and heater | Sensor response and heater circuits | Warm-up, steady speed and deceleration |
| EVAP | Fuel vapour system leaks | Often a cold soak, a fuel level between about a quarter and three-quarters, and mild temperatures |
| EGR / VVT | Exhaust gas recirculation or valve timing | Cruise and deceleration |
| Diesel: NOx/SCR, particulate filter, boost | Aftertreatment and turbo systems | Longer highway runs and regeneration |
Exact conditions vary by manufacturer, model and year.
A car that has been driven normally for weeks will show nearly everything ready. A car with half its monitors incomplete has either had its codes cleared recently or has a fault stopping a test from running.
How do distance, warm-ups and time since codes cleared reveal a reset?
Three counters restart at zero when codes are cleared, and a scan reads them directly:
- Distance since codes cleared (PID 31), in km, up to 65,535. A maxed-out reading means the clear was long ago.
- Warm-ups since codes cleared (PID 30), up to 255. A warm-up is a drive where coolant temperature rises at least 22°C from the start and reaches about 71°C.
- Time since codes cleared (PID 4E), in minutes of engine running.
Subtract the distance from the odometer and you get the odometer reading at the last clear. Say a truck shows 187,340 km, 38 km and 3 warm-ups since the clear. The codes were cleared at about 187,300 km, which is roughly the drive from the owner’s driveway to your lot. Not every vehicle reports all three counters, which is one more reason to read them together.
Appraise flags a clear as recent when any of the three is under 200 km, 10 warm-ups or 300 minutes. You can change those thresholds for your dealership.
Why is a permanent code with no stored codes a red flag?
Permanent codes (read with OBD-II Mode 0A) are stored in memory a scan tool can’t erase, and a battery disconnect won’t clear them either. Only the engine computer removes one, after the self-test that set it has run again and passed. Most vehicles from about the 2010 model year on support them.
So if a scan shows zero stored codes but a permanent code is still there, someone cleared the codes and the car hasn’t yet proved the fault is fixed. A permanent P0420 (catalyst efficiency below threshold) on a car with a clean dash is a classic example, and a catalytic converter is not a small recon line.
Can winter or a dead battery make monitors look not ready?
Yes, and this matters on Canadian lots. Many EVAP monitors won’t run when it is cold or the fuel tank is nearly full or nearly empty. Short winter trips with a block heater may never give the catalyst monitor the steady cruising it needs. A new battery or a shop repair also resets everything on many vehicles. Read the pattern, not a single line:
| What the scan shows | Most likely explanation |
|---|---|
| Only EVAP not ready, 1,500 km and 60 warm-ups since the clear, January | Cold weather; normal |
| Most monitors not ready, 30 km and 2 warm-ups since the clear | Codes cleared very recently |
| Zero stored codes, one permanent code, a pending code | Cleared, and the fault is already coming back |
| Recent clear, invoice for a new battery or repair | Legitimate reset; verify the repair matches |
What other scan readings back up the story?
- Pending codes. Faults detected once but not yet confirmed. After a clear, a pending code is often the original problem returning.
- Distance with the light on (PID 21). A long distance means the owner drove with the light on for a while, which says something about maintenance.
- Engine computer VIN (Mode 09). If it doesn’t match the car’s VIN, the computer has been swapped.
- Odometer from the engine computer (PID A6) on newer vehicles. Compare it with the dash.
- Battery voltage. A low reading supports the “battery was disconnected or replaced” explanation and is a recon item on its own.
Why doesn’t anything catch cleared codes in Western Canada?
Where emissions testing reads readiness monitors, a car with too many incomplete monitors is turned away until it has been driven, which makes a quick clear less useful. Alberta has no routine emissions test for passenger vehicles, and BC ended its AirCare program in 2014. Nothing at registration checks readiness, so on most western lots the appraiser is the only check.
What should you do when a scan shows a recent clear?
- Stay neutral. Ask, “Has it been in for any work lately?” Plenty of honest owners just had a repair or a new battery.
- Ask for the invoice and check that the repair matches the code: an oxygen sensor replaced for a sensor heater code makes sense; an oil change does not explain a permanent catalyst code.
- Price the risk if there is no paperwork. Treat the most likely cause of any pending or permanent code as recon, or offer to re-scan after the owner drives it for a few days.
- Write it down neutrally. On a customer offer sheet, “codes were cleared before the appraisal, so some self-tests had not finished” is accurate and doesn’t accuse anyone.
Appraise’s scanner reads all of this in one scan and puts the verdict at the top, for example “Codes cleared 38 km ago”, with each flag listed underneath.
Key facts
- Distance since codes cleared is OBD-II PID 31, reported in km up to 65,535.
- Warm-ups since codes cleared is PID 30 (maximum 255); a warm-up means coolant rising at least 22°C and reaching about 71°C.
- Time since codes cleared is PID 4E, in minutes.
- Permanent codes are read with Mode 0A and can’t be erased by a scan tool or a battery disconnect.
- Appraise flags a recent clear at under 200 km, 10 warm-ups or 300 minutes since the clear; each dealership can change the thresholds.
- Generic OBD-II covers engine and emissions systems only, not ABS, airbag or body modules.
Frequently asked questions
Does disconnecting the battery clear check-engine codes?
On many vehicles, yes. It erases stored codes, resets the readiness monitors and restarts the since-clear counters, much like a scan tool clear. It does not erase permanent codes.
How long does it take readiness monitors to set after codes are cleared?
There is no fixed distance. Most monitors complete after a few days of normal mixed driving. EVAP and catalyst monitors can take longer, especially in cold weather or on a car that only does short trips.
Can a scan tell exactly when the codes were cleared?
Not as a date and time, but it comes close. Time since codes cleared gives minutes of running, and distance since cleared gives the odometer reading at the clear. Appraise shows both as an estimated clear point.
Is it illegal for a seller to clear codes before a trade-in?
Clearing codes is not illegal on its own. You don’t need to prove anyone’s intent anyway: price what the scan shows, ask for paperwork, and note the finding neutrally.
What if the car is too old to support permanent codes?
Rely on the readiness monitors and since-clear counters, which almost every 1996-and-newer vehicle reports in some form, plus pending codes and a careful test drive.