Fault codes, warning lights, symptoms, OBD2 adapters and garage budget: clear guides to understand what your car is telling you before you pay for a repair.
Symptoms
Check engine, ABS, airbag, oil, battery: which lights an OBD2 adapter can read, which go through another control unit, and which cannot be read at all.
OBD2 guides
An OBD2 adapter reads the engine, but the ABS, airbag or A/C? What the standard requires, what depends on the make and the adapter, and how to find out.
OBD2 guides
Is your car OBD2? The year it became mandatory in each country, the five protocols, the tricky models and how to check before you buy an OBD2 adapter.
Fault codes
Code U0100: a control unit can no longer hear the engine on the CAN network. Battery, earth, connector, added alarm or real failure: the order of the checks.
OBD2 guides
Check engine light back on after clearing? The freeze frame tells you under what conditions the fault appeared. Two cases explained in detail: P0299 and P0128.
Fault codes
Codes P0130, P0135, P0141: dead O2 sensor, failed heater or wiring? How to tell using the live sensor voltage, before you pay for a brand-new O2 sensor.
OBD2 guides
Disconnecting the battery often does not clear the check engine light, and when it goes off, the fault remains. Permanent codes, checks reset, settings lost.
Symptoms
DPF light on on a diesel: when a drive on the open road is enough, when you need a garage, and what an OBD2 adapter can or cannot read about the filter itself.
Fault codes
Codes P0380, P0670 to P0678 on a diesel: glow plug, glow plug control unit or wiring? The tests to do before replacing all four glow plugs, and the costs.
Symptoms
Airbag light on: clock spring, connector under a seat, weak battery. What the B code says, what can be cleared, and why you should never touch the module.
OBD2 guides
v1.5, v2.1, PIC18F25K80, dual board: what these labels are worth on an ELM327, what the ATZ command returns, and the test to run as soon as it arrives.
Fault codes
Code P0128 often blames the thermostat, sometimes the sensor. A simple test: follow the coolant temperature during 10 minutes of driving, then compare it.
Fault codes
P0335 (crankshaft sensor) or P0340 (camshaft sensor): why the engine stalls or will not start, the rev counter test and the real causes to check, in order.
OBD2 guides
Light off, no codes: is the car healthy, or was its memory just wiped? Three signs you can read at the OBD2 port, and what the port cannot tell you about it.
OBD2 guides
You clear the code and it stays or comes back? Pending, confirmed, stored, permanent: what each status means and how to make the code go away for good.
Symptoms
Check engine light on but your OBD2 reader finds no code? Pending or permanent codes, manufacturer codes, another control unit: here is where to look first.
Fault codes
Code P0299: the turbo pressure is too low, but the turbo itself is rarely the first culprit. The checks to make in order, before paying for a brand-new turbo.
Symptoms
ABS or ESP light on but “no codes” on your OBD2 reader? Standard OBD2 only reads the engine. Where the C codes are, the likely causes and how serious it is.
OBD2 guides
Clearing codes the day before the roadworthiness test leaves three traces in the car: emissions checks not ready, distance since clearing, permanent codes.
OBD2 guides
Your ELM327 connects to the phone but not to the car? Ignition, protocol, clone, BLE, Android 12 permission: 10 causes to check, one after the other, in order.
Garage and budget
Fuel cap, air filter, mass airflow sensor, spark plugs, ignition coil, throttle body, glow plugs: 7 simple jobs, no heavy tools, for the most common codes.
Garage and budget
Arriving at the garage with a fault code, freeze frame data and sensor readings changes the conversation. What to say, what to avoid, how to get a clear quote.
Garage and budget
Code reading, full diagnosis, dealer scan tool: typical prices in France, what you get at each price, and where an app like DiagAuto AI is all you need.
Fault codes
P0087 (rail pressure too low) and P0191 (implausible rail pressure sensor) on common rail diesels: fuel filter, HP pump, regulator, injectors. Checks and costs.
OBD2 guides
In 10 minutes with an OBD2 adapter, spot fault codes, recently cleared codes or unfinished emissions checks. The pre-purchase scan checklist for a used car.
Fault codes
P2002 (DPF efficiency below threshold) and P2463 (soot build-up) on a diesel: short trips, pressure sensor, additive. How to avoid a €1,500 replacement.
OBD2 guides
Engine light on means a re-test. 10 minutes with an OBD2 adapter before your appointment can avoid it. What the inspector checks, and the mistakes to avoid.
Symptoms
On most diesels, a glow plug light flashing while driving means an injection or engine management fault, not necessarily the glow plugs. Causes and next steps.
Fault codes
P0455 (large leak) and P0442 (small leak) in the EVAP system: the fuel cap is the number one cause. What to check before you take the car to the garage.
OBD2 guides
Temperature, fuel trims, air flow, intake pressure, O2 sensors… the 10 live readings that tell you more than a fault code, with their normal values explained.
Symptoms
Smoke colour is a diagnosis in itself. White: water or coolant. Blue: oil. Black: too much fuel. Causes, how serious it is, and what to check on your car.
Fault codes
P0101 (MAF signal out of range) and P0102 (signal too low): air filter, intake leak, dirty sensor or dead MAF? How to check it, and what it costs.
OBD2 guides
Clearing an OBD2 fault code fixes nothing, but it is a valuable diagnostic tool if done at the right time. What clearing does, what it loses, and when not to.
Symptoms
The car tops out at 2,500 rpm, won't accelerate uphill or on the motorway, then is fine again after a stop. That is limp mode. Common diesel and turbo causes.
Fault codes
P0401 (insufficient EGR flow) mostly affects diesels and petrol cars used in town. Symptoms, causes, cleaning, and the cost of replacing the EGR valve.
OBD2 guides
The OBD2 port is always in the cabin, within a metre of the steering wheel, often behind a cover. Common spots on Peugeot, Renault, VW, BMW, Toyota, Ford…
Fault codes
P0300 = random misfires, P0301 to P0306 = misfire on cylinder 1 to 6. Spark plugs, coils, injector or compression: the simple method to isolate the cause.
Symptoms
Unstable idle, shaking engine, yet the OBD2 scanner finds nothing. The causes that never set a code, and how to track them down with live sensor data.
Fault codes
P0171 and P0174 signal a lean mixture. Air leak, MAF sensor, injectors, fuel pump: here is the order to search in and how to use the fuel trims to find it.
OBD2 guides
Bluetooth, BLE or Wi‑Fi? An €8 clone or a €40 model? A practical guide to choosing an OBD2 adapter for Android and DiagAuto AI, without bad surprises.
Symptoms
A flashing check engine light is not a simple warning: it signals misfires that can destroy the catalytic converter in a few kilometres. What to do and avoid.
Fault codes
Code P0420 does not always mean a dead catalytic converter. The real causes, the checks to do before you pay, typical costs and practical advice for drivers.
Symptoms
The check engine light is on but everything feels normal. What it monitors, when to worry, and how to find out in 2 minutes what it means with an OBD2 reader.