Recently Cleared Codes: How Readiness Monitors Expose It

A used car scans with zero stored fault codes. The dashboard is dark and the seller points at the clean read on your phone as proof. Then you take it out, and the engine hesitates off the line and hangs at a rough idle that feels nothing like a fault-free car should. The scan and the seat-of-the-pants do not agree.

They do not agree because the scan is showing you a snapshot taken minutes after someone wiped the ECU. The codes the car had this morning are gone. What the seller could not delete is the record of that reset, and it sits one screen away from the codes you just read.

Why a clean scan proves almost nothing

Most buyers stop at the code read. They plug in, see no stored codes, and treat that as a pass. A seller who knows the car has a problem also knows this, which is why the easiest way to make a faulty car present well for an hour is to clear the codes right before you arrive.

Clearing codes does three things at once. It erases stored and pending fault codes, it wipes the freeze frame data attached to each one, and it resets every readiness monitor to not-ready. The first two leave you with a blank screen. The third leaves a trail. The monitors are the part the seller cannot fake, because there is no button that marks them complete. Only the car earns that, by driving.

That is the whole game. A code scan tells you what the ECU is complaining about right now. The readiness monitors tell you whether the ECU has had enough time since the last reset to complain about anything at all. Read only the first and you are trusting a screen that was deliberately emptied.

What does "readiness monitors not ready" mean?

Readiness monitors are the ECU's self-test report card. The on-board diagnostics system runs a series of checks on the emissions-related systems, and each check has a status: complete, meaning the test ran and finished, or not-ready, meaning it has not run yet since the last reset.

Not-ready does not automatically mean a part is broken. It means the computer has not yet had the driving conditions it needs to test that system. A car fresh off a battery change shows not-ready monitors and runs perfectly. The status is about test completion, not about pass or fail.

There are typically eight monitors on a petrol car: catalyst, heated catalyst, evaporative system (EVAP), secondary air, oxygen sensor, oxygen sensor heater, EGR, and AC refrigerant. Diesels swap a few of these for the diesel particulate filter and NOx monitors. A handful are continuous and set almost immediately. The rest are non-continuous and need specific conditions, a cold start, a steady cruise, a deceleration, before they run.

When you scan a car and most or all of these read not-ready, one of three things happened recently: the codes were cleared, the battery was disconnected, or the ECU itself was reset. On a car a seller is presenting as fault-free, the most likely of the three is the one that hides a fault.

How long do readiness monitors take to set after clearing codes?

Long enough that a seller cannot reset the codes in the morning and have the monitors back to complete by your afternoon viewing. That gap is exactly what makes the check work.

On most cars, a few days of normal mixed driving, cold starts, city stop-and-go, and a stretch of highway, sets all or nearly all of the monitors. But the spread is wide. The continuous monitors set within minutes. The catalyst and EVAP monitors are the laggards, and they often need anywhere from 50 to 300 km of varied driving with the right temperature and fuel-level conditions before they run. The EVAP monitor in particular is fussy, sometimes refusing to set until the fuel tank sits in a specific range.

So the math at the viewing is simple. If the seller cleared codes that morning and the car has done 30 km of errands since, almost every monitor will still read not-ready when you scan it. A genuinely fault-free car that has been driven normally all week shows the opposite: most monitors complete, maybe one slow one still pending.

A car with 220,000 km on the clock and seven of eight monitors reading not-ready was reset within the last day or two. The dashboard is clean because it was emptied, not because the car is healthy.

How do you tell if codes were cleared on a used car?

Run a three-part read instead of a one-part read. The code scan is step one of three, not the whole job. The full sequence belongs inside a proper pre-purchase inspection, but for the cleared-codes question specifically, here is what to pull.

First, read the readiness monitors. This is the core tell. Count how many read not-ready. On a car that has supposedly been driven and maintained normally, a wall of not-ready monitors is your answer.

Second, read the permanent codes. Permanent diagnostic trouble codes (PDTCs) became standard around 2010 and they behave differently from ordinary codes. A scan tool cannot delete them. A battery disconnect cannot delete them. The ECU only removes a permanent code after it re-runs the relevant monitor and confirms the fault is actually gone. A permanent code present in the scan, especially alongside not-ready monitors, is the strongest single signal you can get: someone cleared the regular codes, but the computer kept a copy it will not let go of until the car proves the problem is fixed.

Third, read freeze frame on anything that did survive. If a pending or permanent code is present, open its freeze frame and check the mileage stamp inside it. A stamp that contradicts the odometer is a separate and serious problem, which the odometer fraud detection guide covers in full, but it travels in the same company as cleared codes more often than not.

Running this read by hand means scanning codes, switching screens to monitor status, then checking permanent codes and freeze frame separately and remembering what each means. The Skanyx app does the full 8-step Pre-Purchase Inspection (initial code scan, idle 90s, cruise 60s, acceleration 45s, final scan, fraud detection including recently cleared codes and incomplete readiness monitors, multi-specialist analysis) and returns a Buy / Negotiate / Caution / Walk Away verdict with a PDF and a negotiation script. Run it on the car before you pay

What does the OBD2 scan actually show, and what does it not?

This matters because the cleared-codes check is one of the few used-car red flags that lives entirely inside generic OBD2, with no caveats. It is worth being precise about where the line sits.

What Skanyx and any generic ELM327 adapter give you here: stored, pending, and permanent fault codes; the freeze frame data attached to each stored code; and the readiness monitor status for every monitor the vehicle exposes. All of this rides on standard OBD2 modes that every compliant car since 1996 (petrol, EU 2001) and 2004 (EU diesel) supports. There is no manufacturer-specific protocol involved. The monitor read works the same on a Golf, a 3 Series, or a Sprinter. You also get the live data stream, so you can watch fuel trims and sensor values on the test drive and see whether the car is misbehaving even with a blank code screen.

What the generic scan does not tell you: it cannot tell you when, to the minute, the codes were cleared. There is no timestamp PID. What it gives you instead is the inference, and a strong one: not-ready monitors plus high mileage plus a clean dash equals a recent reset. It also cannot read manufacturer-specific extended data, lifetime regeneration counts on a diesel, brand-specific actuator histories, or anything that lives on Mode $22. For the cleared-codes question you do not need any of that. The standard monitor read is the whole answer.

The other thing the scan cannot do is fix the seller's incentive. A clean dashboard on a high-mileage import is not reassuring on its own. It is a prompt to check the monitors, which is the single most underused screen in the entire used-car inspection.

Which faults do sellers usually hide behind a code clear?

The codes that get cleared most often are the ones with a recurring, drive-it-and-it-comes-back nature. A P0420 catalyst efficiency code clears easily and stays gone for a tank or two before the catalyst monitor runs again and re-flags it. A P0300 random misfire clears in seconds and returns the moment the misfire repeats, which on a genuine fault is usually within a few cold starts. A P0171 lean condition behaves the same way.

None of these is hard to clear and none stays cleared if the underlying fault is real. That is the seller's gamble and the buyer's opportunity. If you scan clean, see not-ready monitors, then drive the car for 30 to 50 km and re-scan, a real fault often re-announces itself before you finish the test drive. A code that returns under your own scan, on your own drive, is evidence the seller cannot argue with.

The monitors and the returning codes work together. Not-ready monitors tell you a reset happened. A code coming back tells you why the reset was done.

A practical buyer workflow for cleared codes

Treat the suspicion as a procedure, not a hunch. The steps are quick and they fit inside a normal test drive.

Scan before you drive. Pull codes, then immediately switch to readiness monitor status and count the not-ready entries. Note whether any permanent code is present. Do this while the car is cold, on the seller's driveway, before anything has had a chance to set.

Drive the car properly. Not a lap of the block. Get it to operating temperature, include some steady cruising and at least one firm acceleration and one long deceleration, and cover enough distance to give the continuous monitors a chance to start setting. Twenty to forty kilometres is a reasonable test-drive ask and is exactly the kind of mixed driving monitors need.

Re-scan at the end. If a code has returned, you have your answer and your negotiating evidence. If the monitors are still all not-ready and the seller cannot explain a recent battery change or repair, you are looking at a car that was reset specifically for your visit.

Ask the direct question. "When were the fault codes last cleared, and why?" A seller with a legitimate answer, recent battery replacement, a repair last week, gives it without hesitation. A seller who clears codes to flip a problem car does not have a clean answer ready, and the hesitation tells you as much as the data. This same logic extends to the wider German used-car import checklist for cars bought sight-unseen across borders, where you cannot run a long test drive before committing.

Make the monitor screen part of every scan

A code scan that returns nothing is not a clean bill of health. It is a snapshot, and on a car you are about to pay for, snapshots are easy to stage. Read the readiness monitors and the permanent codes in the same sitting, and the staging falls apart: a wall of not-ready monitors on a high-mileage car is the seller telling you, without meaning to, that the dashboard was emptied minutes before you arrived. Drive it, re-scan, and let the codes that come back finish the conversation.