AUTOMOTIVE DIAGNOSTIC KNOWLEDGE

What problem are you having with your car?

Describe what you see, hear, or feel. IAR turns symptoms into a clear diagnostic path—so you can test the cause before buying parts.

TRY:Cranks but won't start P0300 Battery warning light Voltage-drop test

44DETAILED
DIAGNOSTIC GUIDES

43GENERIC OBD-II
CODE PAGES

35MANUFACTURERS
STRUCTURED

✓NO MADE-UP SPECS
Exact values require a cited source.

01 / START HERE

Select your vehicle

Enter the exact configuration you know. Optional engine, trim, transmission, and drivetrain details help narrow searches without IAR inventing fitment.

Model years 1940–1995 can be entered manually; NHTSA vPIC model-name suggestions begin with 1996. Saving still records what you chose; IAR does not infer engine, trim, transmission, drivetrain, or repair applicability from a model listing. Published technical facts still display their own applicability and source status.

DIAGNOSTIC WORKSPACE · GUIDED + AI

Turn symptoms into a test plan.

Start with deterministic IAR decision trees and structured guides, then use Gemini when you need help organizing a more specific symptom pattern. Your selected vehicle carries into both workflows.

Open Diagnose workspace →Go directly to IAR AI →AI key stays server-side · static IAR guidance works without it

02 / DIAGNOSE BY SYSTEM

Where is the problem?

Explore all car systems →

Engine

Starting, ignition, fuel, air and mechanical health

Misfire · Rough idle · Loss of power↗04

Electrical & networks

Power distribution, wiring, modules, sensors and communication

Intermittent faults · No communication · Blown fuse↗06

Brakes

Hydraulics, friction, ABS and pedal concerns

Vibration · Grinding · Soft pedal↗08

Steering

Driver input, assist, geometry and wheel direction

Pulling · Vibration · Heavy steering↗09

Cooling

Temperature control, circulation, airflow and sealing

Overheating · Leaks · No cabin heat↗11

HVAC

Cabin heating, ventilation, air conditioning and air distribution

A/C warm · Weak airflow · Odor↗

03 / START WITH WHAT YOU NOTICE

Not sure which system? Start with a symptom.

You do not need to know the part name. Tell us what changed and when it happens.

Browse every symptom →

01Won’t start→02Check engine light→03Engine shaking→04Overheating→05Battery dying→06Grinding noise→07Steering vibration→08Oil leak→09Transmission slipping→10A/C not cold→11Rough idle→12Loss of power→

04 / OBD-II CODE LOOKUP

A code is a clue.
Not a diagnosis.

Look up a generic trouble code, understand what the monitor detected, and follow a test-first path.

FEATURED DIAGNOSTIC GUIDE12 MIN READ

INTERMEDIATE✓ GENERAL KNOWLEDGE

Engine cranks but won’t start

The starter turns the engine, but combustion never begins. Separate fuel, ignition, compression, timing, and security concerns in a logical order.

6 diagnostic steps4 cause groups

Example only: this ignition-coil range applies if testing confirms an ignition fault.

Planning range · per coil · not a quoteRounded USD conversion · not live rates

USD $100–$500CAD CA$140–CA$690EUR €90–€460GBP £80–£395AUD A$155–A$765

Open diagnostic guide →

THE IAR METHOD

Good diagnosis is a process—not a parts cannon.

Observe

Capture the exact symptom and the conditions that trigger it.

Prioritize

Rank plausible causes using evidence and system logic.

Test

Use the safest, least invasive checks before disassembly.

Verify

Confirm the fault and confirm the repair changed the result.

RECENTLY ADDED

Go deeper with connected knowledge

Each page connects symptoms, components, codes, testing, and repair so you can follow the evidence naturally.

COMPONENT · GENERAL KNOWLEDGE

Ignition coil

Function, symptoms, failure modes, and a practical testing overview.

Explore component →REPAIR GUIDE · BEGINNER–INTERMEDIATE

Replace an ignition coil

Preparation, removal, installation, verification, and specification boundaries.

Open repair guide →OBD-II · VERIFIED GENERIC

P0300 misfire

Severity, source-backed context, cause groups, tests, and common mistakes.

Understand the code →VEHICLE HUB · COVERAGE EXPANDING

2015 Lexus IS 250

A transparent vehicle hub that separates sourced facts from general guidance.

Open vehicle hub →

BROWSE THE PLATFORM

Knowledge for diagnosis, repair, and ownership

Start from whichever fact you have: a symptom, a code, a vehicle, a part name, or a maintenance question.

BUILT TO SCALE

Browse manufacturers

Vehicle coverage is expanding. Unsourced specifications are never filled in to look complete.

✓INFORMATION
STANDARD

WHY TRUST IAR

Clear about what is known—and what must be verified.

Verification statusEvery guide identifies whether it is general knowledge, vehicle-specific, or needs confirmation.

Source-ready architectureVehicle claims can carry source, publication date, retrieval date, and verification status.

No invented specificationsTorque, capacities, pressures, intervals, and programming steps require exact service information.

Work safely

Automotive repair can involve hot, pressurized, moving, electrical, fuel, airbag, and high-voltage hazards. Information varies by vehicle configuration. Consult the correct manufacturer service information and use a qualified professional whenever a procedure exceeds your training or equipment. Read the hybrid and EV high-voltage safety guide →

What problem are you having with your car?

Select your vehicle

Turn symptoms into a test plan.

Where is the problem?

Engine

Electrical & networks

Brakes

Steering

Cooling

HVAC

Not sure which system? Start with a symptom.

A code is a clue. Not a diagnosis.

Engine cranks but won’t start

Good diagnosis is a process—not a parts cannon.

Observe

Prioritize

Test

Verify

Go deeper with connected knowledge

Ignition coil

Replace an ignition coil

P0300 misfire

2015 Lexus IS 250

Knowledge for diagnosis, repair, and ownership

Browse manufacturers

Clear about what is known—and what must be verified.

What problem are you having with your car?

Select your vehicle

Turn symptoms into a test plan.

Where is the problem?

Engine

Electrical & networks

Brakes

Steering

Cooling

HVAC

Not sure which system? Start with a symptom.

A code is a clue. Not a diagnosis.

Engine cranks but won’t start

Good diagnosis is a process—not a parts cannon.

Observe

Prioritize

Test

Verify

Go deeper with connected knowledge

Ignition coil

Replace an ignition coil

P0300 misfire

2015 Lexus IS 250

Knowledge for diagnosis, repair, and ownership

Browse manufacturers

Clear about what is known—and what must be verified.

A code is a clue.
Not a diagnosis.

A code is a clue.
Not a diagnosis.