Reading OBD fault codes: how to read out the fault memory yourself before you buy

What OBD and OBD2 actually are

OBD stands for on-board diagnostics — your car's built-in self-diagnosis system. Control units continuously monitor the sensors and actuators of the engine, exhaust system and ancillary systems. If a control unit detects a value outside the permitted range, it stores a fault code and, depending on severity, switches on a warning light. OBD2 is the second, unified generation of this system, which standardises diagnostic access across manufacturers through the connector shape, the signals and a base set of standardised codes.

This very standardisation is the reason you can read the codes yourself at all. In Europe, OBD2 has been mandatory on petrol cars since the 2001 model year and on diesels since 2004 — practically every used car changing hands today has the standardised interface. A cheap reader or a Bluetooth adapter costing a few euros therefore fits almost any vehicle, regardless of brand. It was different before OBD2: back then you needed separate connectors and protocols for every make.

It is important to set the right expectation. OBD is a diagnostic tool, not a seal of approval. The system reports that a control unit has detected a deviation — it doesn't automatically tell you which part is faulty, how expensive the repair will be or whether the finding is safety-critical. A code that has been read out is a starting point for tracing the cause, not a final result. This distinction between “code” and “diagnosis” runs through this entire article and is at the heart of what a buyer needs to understand.

Where the diagnostic port sits

The OBD2 socket is standardised: a 16-pin, trapezoidal connector that your reader or adapter fits into in one direction only. On the vast majority of vehicles you'll find it in the driver's footwell, on the left under the dashboard near the steering column — often within reach above the pedals or behind a small cover flap.

The position isn't entirely uniform, though, and if you don't find it on the first try, keep looking in the usual fallback spots. Some manufacturers hide the socket behind the ashtray, in the centre console, in the glovebox or behind a panel below the steering wheel. Grab a torch and feel along the lower edge of the dashboard — the connector feels like an angular plastic frame. If you still can't find it, a look in the owner's manual or a quick search for the exact position of the specific model helps.

For buying a used car, the location also matters for a practical reason: you want to know where the socket is before the viewing, so that you're not rummaging under the dashboard for minutes at the appointment while the seller waits. Also check whether the socket is accessible and undamaged at all. A taped-over, blocked or visibly tampered-with diagnostic port is a warning sign in its own right — it can be an attempt to make reading the codes harder.

How to read out the fault codes

Reading the codes itself is straightforward and done in a few minutes. You just need a device and the right ignition state — tools or expert knowledge aren't needed for the first step.

You have two routes, which differ in price but hardly in the basic result. The first is a standalone OBD2 handheld device with its own display: plug in, read off, done — rugged and without a smartphone. The second is a small Bluetooth or Wi-Fi adapter that goes into the socket and beams the data to an app on your phone. The adapter route is cheaper and more convenient, but depends on a suitable app and a stable connection. For the sole purpose of seeing fault codes, either one is enough.

The procedure is the same for both routes:

• Plug in the adapter while the ignition is off.
• Switch on the ignition, engine off. Usually the second detent position of the ignition key, or the corresponding start-button mode without starting the engine, is enough — the control units are then active and responsive.
• Connect the device or app and select the vehicle type if the app asks for it.
• Start reading the fault memory. The device queries the control units and lists the codes it finds.
• Note down or photograph the codes — every single one, along with the plain text that many apps supply.

A crucial point for buyers: do not clear the fault memory. Most devices offer a “clear” function, but on a used car belonging to a seller you don't know, that destroys exactly the information you want to assess — and you'd be tampering with the owner's vehicle without their consent. Read yes, clear no. If a seller themselves offers to “quickly make the faults go away”, you should prick up your ears.

How a fault code is structured

An OBD fault code looks cryptic, but follows a clear logic that you can read in a few minutes. It consists of one letter and four digits, for example P0301. The letter names the system affected, the first digit says whether the code is standardised or manufacturer-specific, and the rest narrows down the fault.

The leading letter divides the codes into four large system groups. This immediately tells you which direction a finding points in, even before you interpret the digits.

For buying a used car, the P codes are the most common and the most important, because they cover the engine, exhaust and transmission — exactly the expensive areas. Within the P codes, the first digit decides the origin and readability: a 0 (P0xxx) means a code standardised across manufacturers, whose meaning is the same on every vehicle and which you can look up in any code list. A 1 (P1xxx) marks a manufacturer-specific code that the vehicle maker defined themselves — the same number can mean completely different things on two brands, and simple devices often show only the number without plain text for it.

The further digits narrow the fault down, for instance to a particular cylinder, a sensor circuit or a subsystem. P0301, for example, is standardised for a detected combustion misfire in cylinder 1. That sounds precise, but only appears to be: the code says where the system noticed the misfires, not why — spark plug, ignition coil, injector, compression or a broken wire could all be to blame. This is exactly where reading off a number parts ways with the actual root-cause diagnosis.

Active, stored or intermittent — the important difference

Not every code in the memory means the same thing. A good reader distinguishes between faults that are present right now and those that occurred in the past and are merely logged. This distinction determines how serious a finding is.

An active fault is present at the moment of reading — the control unit detects the deviation right now, usually accompanied by an illuminated warning light. An active engine fault is an immediate signal you must follow up on before any purchase. A stored or passive code, by contrast, documents an event that had occurred but is no longer present. It can point to a solved problem — or to a problem that only comes back intermittently.

Particularly tricky are intermittent faults that come and go: a loose contact, a sensor that only plays up at a certain temperature, a fault that only occurs under load. On a short viewing lap something like that often stays invisible, but it leaves traces in the memory. Intermittent codes in particular are valuable, because they point to problems that a normal test drive doesn't trigger — provided nobody cleared the memory beforehand. How such under-load symptoms manifest specifically on a diesel, for instance at the EGR valve and diesel particulate filter, is explored in more depth in this cluster's own article on diesel symptoms; here it's about the method, not the individual part.

A simple device does show you whether a code is active or stored, but it doesn't tell you how often an intermittent fault occurred, under what conditions or how critical it is. What's missing is the context that turns a number into an assessment.

The empty fault memory — why “nothing” can be suspicious

Many buyers breathe a sigh of relief when the device reports “no faults found”. On a visibly high-mileage, older vehicle, however, a spotlessly clean memory isn't automatically a good sign — it can mean that someone tidied up shortly before the sale.

Clearing the fault memory takes seconds and is possible with the same cheap device you use to read it. A seller who wants to get rid of an illuminated engine warning light can simply acknowledge the codes — the light goes out, the memory is empty, the car looks healthy. The underlying defect isn't fixed by this, just made invisible. After clearing, depending on the fault it takes a while and a few driving cycles until the system detects the fault again and stores it once more — a window that can reach exactly until after the sale.

There is a telltale detail that a professional read-out reveals but a simple device usually doesn't: the so-called readiness codes, or readiness monitors. These are internal self-tests that the vehicle only completes after several full driving cycles. Right after a memory reset, these monitors show “not ready”. So if an inspector encounters a completely empty fault memory in which the readiness monitors are simultaneously reset, that is strong evidence that clearing happened only recently — even though the car has long been driven. Recognising this combination is part of a professional read-out and is one of the points where a simple handheld can no longer help.

In practice this means: an empty memory is only reassuring when it fits the vehicle. On a young car with low mileage, it's normal. On an older high-mileage car whose seller dodges the question about faults, it's a reason to look more closely.

The limits of a simple reader

A cheap OBD2 device is a sensible first step — but it has clearly defined limits you need to know before you base a buying decision on it. It shows you codes. It doesn't show you what the code means, how urgent it is and what the repair costs.

The first limit is the missing root-cause diagnosis. As seen with the misfire example, a code names the symptom, not the cause. Between the code displayed and the part that is actually faulty lies a fault-finding process that demands measurements, experience and often a targeted process of elimination. A simple device gives you the number and leaves you alone with the interpretation.

The second limit is the live data. Meaningful diagnosis lives not only on stored codes, but on current readings during operation — for instance the differential pressure across a particulate filter, a regeneration counter, lambda readings, ignition or injection parameters. Cheap devices show little to none of this, and even when they output a value, the layperson lacks the benchmark to tell “normal” from “conspicuous”. Yet it's precisely this live data that often makes the difference between whether a part is really at the end of its life or just grumbled once.

The third limit is the prioritisation by urgency. A fault memory can contain several codes at once — a harmless sensor defect alongside a safety-relevant finding. A simple device lists them on an equal footing. It doesn't tell you which code you must take seriously immediately, which can wait and which is merely the consequence of another. This prioritisation is what turns a list of numbers into a dependable buying decision — and it doesn't come from the device, but from the experience of whoever operates it.

What a professional read-out adds

It's exactly at the three limits of the simple device that the on-site check picks up. A car appraiser doesn't just read the codes, they evaluate live data, spot a freshly cleared memory from the readiness monitors and rate every finding by urgency and rough cost range.

The difference lies in three things you don't get with an aftermarket handheld. First, the depth: professional diagnostic equipment reaches more control units and reads manufacturer-specific codes in plain text where a simple device shows only an unintelligible number. Second, the live view: the inspector observes the vehicle in operation and judges readings that reveal whether a part is genuinely at the end of its life. Third, the assessment: they separate the cosmetic sensor fault from the expensive powertrain finding and put the result in the context of what they saw and heard on the bodywork, the underbody and the test drive. For them, reading the OBD is one building block among many, not an isolated result.

checkdenwagen comes straight to the vehicle, checks over 100 points and delivers the report within 24 hours; the on-site appointment takes approx. 1.5 hours. The report documents every finding with its condition and assessment — including the codes read out along with their rating — and is thereby also your negotiating document. The Standard Check costs from €289 incl. VAT and travel, the Premium Check from €339 incl. VAT and travel with an additional repair-cost calculation. This is how a fault code that a cheap device merely displays becomes a statement you can genuinely base your buying decision on.

When a professional read-out pays off

For a first impression, your own device is enough. But as soon as money or uncertainty is at stake, the professional diagnosis pays off:

• Your device shows one or more P codes in the powertrain or exhaust area whose meaning you can't reliably place.
• The fault memory is suspiciously empty even though the vehicle has visibly high mileage — and you want to know whether it was cleared recently.
• The seller dodges questions about the fault memory or offers to “quickly make the faults go away”.
• It's about a diesel suspected of EGR or particulate-filter problems, where the live data in particular is decisive.
• The purchase price is so high that the inspection amounts to only a fraction of a possible bad buy.

In all these cases, checkdenwagen inspects on-site, reads out professionally and delivers a report you can decide on with confidence.