Preparing for an inspection: why "clear the codes first" is the worst possible advice

“Clear the codes before the test” has circulated on forums for decades. It worked until OBD-II introduced two things designed specifically against it.

Why clearing does not work

Readiness monitors

Every engine system has a self-test, and a monitor is the flag meaning “that test has run since the last reset”.

Wipe the memory and every flag drops to not ready. They only return after a complete drive cycle: cold start, warm-up, city, highway. That takes anywhere from a few days to a week of driving.

The inspector looks precisely at those. “Zero faults + monitors not ready” reads unambiguously: the memory was just cleared. Most petrol cars are permitted one incomplete monitor, diesels up to two. Three or more is a failure.

Monitor mechanics are in their own guide.

Permanent codes

The second layer of defence. Mode 0A codes cannot be erased by a scan tool at all. They clear themselves only after the module has repeatedly confirmed the fault is gone.

So if the problem is real, clearing does not help: stored codes disappear, permanent ones stay.

How far ahead to prepare

The right answer: two weeks, not one day.

How far ahead	What to do
2 weeks	Read codes, pending and permanent included
2 weeks	Repair whatever turned up
1 week	Drive a complete drive cycle
2–3 days	Verify the monitors have reached ready
Test day	Clear nothing

The week is for the monitors specifically. It cannot be accelerated — the cycle requires cold starts, and you get one a day. The procedure is in the drive cycle guide.

What to check two weeks out

Through diagnostics:

1. Stored codes. The obvious part.
2. Pending codes (Mode 07). What has not confirmed yet but will, precisely when it should not.
3. Permanent codes (Mode 0A). If present, clearing is pointless — repair is required.
4. Readiness monitors. How many are not ready, and which.
5. EVAP readiness — the most stubborn monitor and the most common reason for “everything ready except one”.

By hand — what the inspection lane checks:

• Lighting: every lamp, brake lights and side lights included.
• Brakes: balance, effectiveness, handbrake.
• Tyres: tread depth, matching across an axle.
• Play in the suspension and steering.
• Exhaust: integrity, no unusual noise.
• Glass, wipers, mirrors, seatbelts.
• Corrosion of structural members.

Which monitors stick most often

Monitor	Why it will not complete
EVAP	Wrong fuel level or a leak — see P0455
Catalyst	No sustained constant-speed driving
Oxygen sensors	Engine never reached stable operating temperature
Secondary air	A faulty system — see P0410
EGR	No closed-throttle deceleration

One important detail: fuel level for the EVAP test must be between a quarter and three quarters. A full or nearly empty tank blocks the test, and the monitor will not complete however far you drive.

What to do if there genuinely is a code

The honest sequence:

1. Read the freeze frame — the conditions when it was recorded. It often identifies the cause faster than the code does; covered in its own article.
2. Fix the cause.
3. Clear the code — now it is appropriate.
4. Drive a complete cycle and confirm it has not returned.
5. Wait for the monitors.

The difference from “just clear it” is that here clearing happens after the repair rather than instead of it, and there is a week between it and the test.

What not to do

• Clear codes the day before. Guaranteed failure.
• Fill the tank before the test. EVAP will not complete.
• Disconnect the battery — same effect as clearing.
• Fit oxygen sensor spacers. The catalyst monitor still will not complete correctly, and it shows at the test.

In short

• Clearing before a test resets the monitors — the main cause of failures.
• Permanent codes cannot be erased at all; they exist for this reason.
• Prepare two weeks out: one week for repairs, one for monitors.
• Fuel between a quarter and three quarters, or EVAP will not run.
• Disconnecting the battery beforehand is the same as clearing codes.

Download the app — readiness monitors, pending and permanent codes are read with an ELM327 adapter over Bluetooth SPP on Android.