Questions and answers
Car diagnostics: frequently asked questions
The questions that come up most when you read a car's trouble codes, answered in two or three sentences and then in depth. For drivers who want to understand what their car is saying and for shops that explain it every day.
Reviewed by Daniel Morales, founder of Reprorace SL, on
Topics
Diagnostics basics
What OBD is, which protocol your car uses, what you can do yourself and how often it is worth checking.
What is OBD-II and EOBD?
What OBD-II and EOBD are, which model years have them, what you can read with them and how they differ, explained in plain words.
Which diagnostic protocol does my car use: CAN, K-Line or DoIP?
How to tell whether your car is diagnosed over CAN, K-Line or DoIP (Ethernet), what changes with the car's age and why a good reader detects it by itself.
Can I diagnose my car myself?
With an OBD reader you can read trouble codes, live data and freeze frame data. What you can do yourself, where the limits are and when to go to a workshop.
Can an OBD reader damage the car or drain the battery?
Reading codes with an OBD reader does not damage the car. The risks are readers that keep the car awake, writing to the control units and security.
How often should you run diagnostics on your car?
There is no fixed interval: when it is worth reading your car's codes (warning lights, service, inspection, a trip, buying used) and what constant monitoring adds.
What is live data and what is it for?
Which live data you can see through the OBD port (rpm, temperature, mixture, sensors), how to read it and why it helps find faults that left no code.
Can you read the trouble codes of a van or a truck with an OBD scanner?
12 V vans, 24 V trucks, 16-pin or 9-pin connectors and the J1939 protocol: what changes when you read the codes of a commercial vehicle and which tool works.
Shop diagnostics or remote diagnostics: when does each make sense?
An honest comparison: what remote diagnostics solves without moving the car, what still needs a shop visit and how to decide by symptom, with no promises.
Can you diagnose a hybrid or electric car with an OBD scanner?
Whether an OBD scanner works on a hybrid or an EV, which codes they have, why high voltage is for professionals and what you can safely check yourself.
Codes and warning lights
How a car stores a fault, what data comes with it and why the warning light comes on.
What does a pending, stored or permanent code mean?
What it means for a trouble code to be pending, confirmed (stored), permanent or in history, and when the check engine light turns on and off.
What is freeze frame data?
Freeze frame data stores the rpm, temperature, load and mixture at the moment a fault was recorded. What it is for and how to read it.
What are readiness monitors and why do they matter for inspections?
What readiness monitors are, why they reset when codes are cleared or the battery is replaced, how they complete and why the inspection or MOT looks at them.
Why does the check engine light come on after refueling?
A loose gas cap, a leak in the vapor system, poor-quality or wrong fuel: why the check engine light comes on after refueling and what to do.
What do U codes and lost communication (U0100) mean?
U codes are faults in the network between control units. What lost communication such as U0100 means, its most common causes and what to check first.
What does a yellow warning light mean compared with a red one?
What a red, a yellow or amber and a green or blue dashboard light mean, when you have to stop right away and when it is enough to have it checked soon.
Can I pass an emissions inspection with a pending code?
What a pending code is, whether the emissions inspection takes it into account when the check engine light is off and what to do before the test.
How long does the check engine light take to go off after the repair?
Why the check engine light does not go off as soon as the fault is fixed, how many drive cycles it needs and when it makes sense to clear it with a scan tool.
Why does the check engine light come on and then go off by itself?
Why the check engine light comes on for a few days and then goes off by itself, which intermittent faults cause it and why the code is still worth reading.
Why is the warning light on when the scan tool shows no codes?
Why an OBD scanner says no codes or P0000 with the light on: other modules, manufacturer codes, basic readers or a warning light from another system.
Why does the trouble code come back after I clear it?
Clearing a code fixes nothing: why the light and the code return within days, what it means when they come back quickly or slowly and how to find the cause.
I have several trouble codes at once: which one should I fix first?
What to do when the scanner shows several codes at once: where to start, why a weak battery or a network fault multiplies them and which ones are urgent.
Exhaust and emissions
The systems that clean the exhaust gases and cause the most failures at inspection.
What is an oxygen sensor and when does it fail?
What the oxygen sensor (lambda sensor) does, the upstream and downstream sensors, why it fails, its symptoms and its trouble codes.
What is a catalytic converter and why does it fail?
What the catalytic converter does, how the control unit knows it has lost efficiency (P0420, P0430), what damages it and what to check before replacing it.
What is EGR and what happens when it fails?
What the EGR valve does, why it clogs (especially on diesels), the symptoms of an EGR stuck open or closed and its codes, such as P0401.
What is a particulate filter and how long does a regeneration take?
What the particulate filter (DPF) is, how it cleans itself, how long a regeneration usually takes, what interrupts it and when to go to a workshop.
What is AdBlue (DEF) and what happens if it runs out?
What AdBlue (diesel exhaust fluid, DEF) is, why diesels with SCR use it, how much they use, what happens if it runs out or freezes and how to refill it.
Engine
Sensors and parts the control unit watches to keep the engine running well.
What is a mass airflow sensor and what symptoms does it cause when it fails?
What the mass airflow (MAF) sensor measures, why it gets dirty, the symptoms of a failing MAF (stumbling, black smoke, loss of power) and its codes.
What are engine misfires (P0300)?
What a misfire is, how the control unit detects it, what P0300 and P0301 to P0306 mean, their usual causes and when you have to stop.
What are fuel trims and how do you read them?
What short-term and long-term fuel trims are, what a positive or negative value means and how they help diagnose P0171 and P0172.
What does the engine thermostat do and what does P0128 mean?
What the thermostat does, what happens if it sticks open (P0128, weak heat, higher fuel use) or closed (overheating) and how to check it.
What do the crankshaft and camshaft sensors do and what happens when they fail?
What the crankshaft and camshaft position sensors do, why a failure can stop the engine from starting or make it stall, and what P0335, P0340 and P0016 mean.
What does the throttle body do and why does a fault trigger limp mode?
How the electronic throttle body and pedal work, why their faults trigger limp mode and what P2135, P2106 or P2110 mean.
How does a turbocharger work and what does P0299 mean?
How a turbocharger works, what underboost and overboost codes (P0299, P0234) mean, their usual causes and how to look after the turbo.
Why does my car go into limp mode when it is cold?
Why a car goes into limp mode on a cold start and then runs fine: turbo, glow plugs, sensors, throttle body or battery, and what is worth checking.
Battery and electrics
The 12 V battery and what depends on it, such as stop-start.
How can you tell if the 12 V battery is failing?
Signs your car battery is failing, how to check its voltage at rest and running, the types (AGM, EFB) and why some cars need the battery registered.
Why is stop-start not working?
Stop-start will not shut the engine off if the battery is not charged enough, the engine is cold, the climate control needs it or the particulate filter is regenerating.
What happens if I disconnect the battery: are the codes erased?
What happens to trouble codes, readiness monitors and the car's memories when you disconnect the battery, and why it is not a good way to turn off a warning light.
Buying and selling a used car
What to check with a scan tool before you buy, what gives away cleared codes, how to spot an odometer that does not add up and what to check before you sell.
What should you check before buying a used car with an OBD scanner?
Trouble codes, readiness monitors, freeze frame, battery and live data: what to look at with an OBD scanner before buying a used car, and what it will not tell you.
Can you hide faults by clearing the codes before selling a car?
Clearing codes turns the light off but leaves traces: incomplete readiness monitors, counters since the clear and permanent codes. What a buyer can see.
How can you tell if the odometer has been tampered with?
Signs of odometer tampering: mileage that does not match between control units, invoices and inspections, and wear. What diagnostics can and cannot tell you.
What should you check before selling your car?
Before you sell, read the codes, fix or explain what you find, let the monitors complete, gather invoices and inspections and offer a report: you will sell with more trust.
What should you do if a warning light comes on soon after buying a used car?
You bought a used car and a few days later a warning light comes on: how to read it, what freeze frame and monitors show, what to keep and who to turn to.
More guides for drivers
Car systems: how they work and what fails
Every car system explained one by one: engine, cooling, ignition, fuel, exhaust, turbo, transmission, brakes, steering, battery, CAN network and airbags.
Car symptoms: what they mean and what to do
Warning lights, jerking, smoke, noises, smells or a car that won't start: what each symptom means, whether you can keep driving and what to check.
OBD fault codes: what each one means
Glossary of OBD-II/EOBD fault codes (P, C, B and U) in plain English: what each code means, common causes, severity and whether you can keep driving.
Vehicle diagnostics glossary
Nearly 50 vehicle diagnostics terms explained in a sentence or two: OBD, UDS, DTC, ISO-TP, DoIP, CAN FD, K-Line, J1939, DM1, J2534, D-PDU and more.
Remote OBD diagnostics: what it is, how it works and what it can't do
What remote OBD diagnostics is, how it works with ECO-LITE, ECO-LITE TOOL and the platform, what can and can't be done remotely, and how the driver approves it.
For repair shops and professionals: frequently asked questions
Remote diagnostics for repair shops, distributors and fleets: what you need, what it costs to start, consent and GDPR, remote programming, networks under your brand.
See what your car is saying
The ECO-LITE stays in the OBD port and shows the trouble codes and the car's data in your account; your workshop can review them remotely, always with your permission.
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