The first OBD scan after buying a used car: a 20-minute routine

Checking before you buy is the well-known advice — used car pre-purchase OBD check covers that. This is the scan that matters almost as much and that nobody does: the one after the money has changed hands.

Its purpose is different. You are no longer deciding whether to buy. You are establishing a baseline, so that in six months you can tell whether something has changed.

Step 1 — read every code, then leave them alone

Read stored codes, pending codes and permanent codes. Photograph or export all of them along with their freeze frames.

Resist clearing anything for now. Stored codes are the last honest record of how the car behaved before you owned it. Once cleared, that history is gone forever.

Pending codes deserve particular attention: they are faults on their first strike, invisible on the dashboard, and they tell you what is about to become a problem. See pending vs stored codes.

Step 2 — check the readiness monitors

This is the step that catches a specific kind of seller.

If all the monitors read “not ready”, the codes were cleared very recently — most likely just before you viewed the car. That does not prove dishonesty, but it does mean the clean scan you saw was meaningless, and it tells you to be extra attentive over the next few weeks.

Detail: OBD readiness monitors explained and P1000 readiness not complete.

Drive normally for a week, then check again. Monitors that complete and stay clean are genuine reassurance. A code that appears at that point was always there.

Step 3 — record the baseline numbers

This is the real value of the exercise. Write these down with the date and the mileage.

Value	Healthy range	Why record it
Long term fuel trim, idle	−5% to +5%	Drift over months reveals leaks and ageing sensors
Long term fuel trim, 2500 rpm	−5% to +5%	The idle/load comparison localises faults
Coolant temperature, warm	85–95 °C	Baseline for thermostat behaviour
Battery voltage, engine running	13.8–14.6 V	Charging system starting point
O2 sensor crossings per second	1–5	Sensor ageing shows here first
DPF soot load, if diesel	Under ~45%	Regeneration health
Distance between regenerations	300–600 miles	Rising frequency means trouble

In six months, the comparison against these numbers is worth more than any single reading in isolation. A long term fuel trim that has moved from +2% to +14% is a developing vacuum leak, visible long before a code appears.

Which values are worth watching generally: best OBD PIDs to log on a daily driver.

Step 4 — verify the VIN and mileage

Read the VIN electronically through Mode 09 and compare it against the documents, the windscreen etching and the plate.

The Mode 09 calibration ID and verification number also tell you whether the engine software matches the factory version — useful for spotting a remap that was not mentioned. See ECU remap detection and odometer fraud through OBD.

Paperwork side: vehicle history check before buying and VIN decoder.

Step 5 — a proper drive with logging

Twenty minutes covering a cold start, town driving, and a steady motorway run. Log the values above throughout.

Watch specifically for:

• Coolant temperature climbing smoothly and holding, not stuck low — P0128.
• Fuel trims settling rather than wandering.
• Battery voltage steady under load with lights and blower on.
• On a diesel, whether a regeneration starts, and whether it completes.

That last one matters on any diesel bought without service history. A car that starts a regeneration within the first 100 miles has been doing short trips — DPF light on, what to do and short trips and engine damage.

Step 6 — now decide about clearing

Once everything is recorded, clearing is reasonable. It gives you a clean slate, so anything that appears from here is yours to investigate rather than inherited ambiguity.

Do not clear immediately before an emissions test, for the readiness reason above. See clearing codes, when and why.

What this does not cover

Generic OBD-II reaches the powertrain reliably and very little else. Airbags, ABS module internals, body electronics and comfort systems mostly need manufacturer-level access — scan tool limits.

So an illuminated airbag light will not necessarily show up here, and it is a roadworthiness issue: airbag light on causes.

A scan is also not a mechanical inspection. It says nothing about rust, suspension bushes or brake pad thickness. Pair it with high mileage car, what to check.

Repeat it quarterly

The routine takes twenty minutes the first time and five thereafter. Comparing today’s fuel trims against the day you bought the car is the cheapest early-warning system available to an owner.

Try it on the free tier of the app — pair an ELM327 adapter and save the baseline on day one. Every future reading is only meaningful because you took that first one.