#diagnostics
31 posts in Car Diagnostics blog about "diagnostics" — OBD-II diagnostics, DTC codes, repair. ← All posts
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EGR valve and EGR cooler leak: codes P0401, P0402, P0404 and why diesel Passat/A6 "got sooted up"
How to diagnose EGR via OBD-II: difference between P0401 (low flow) and P0402 (high), EGR cooler leak, why "EGR delete" is a short fix with long problems.
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P0455: EVAP large leak — 10 reasons why "just the gas cap" isn't always right
Deep dive into P0455: how OBD-II detects a large leak, difference from P0442/P0456, EVAP smoke test, why gas cap first, what to do with the charcoal canister.
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ATF and the automatic: how OBD-II sees the trans, and when it's really time to change the fluid
What OBD shows about AT: codes P0700, P0741, P0742, adaptation values, ATF temperature, why "sealed for life" is a marketing myth, and how to diagnose slip.
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Battery and alternator via OBD-II: P0562, P0620, diagnose in 5 minutes
What OBD-II says about the battery and alternator: codes P0562 (low voltage), P0563 (high), P0620 (alternator). How to read system voltage and why "overcharge" is more dangerous than "undercharge".
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12 OBD PIDs worth logging every day — early problem detection without Check Engine
The most useful OBD-II PIDs for daily monitoring: LTFT, coolant temp, MAF, boost pressure, misfire counter. How to set up automatic logging in Car Diagnostics and what to do with the trends.
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CAN Bus for car owners: how the "nervous system" of a car works and why it matters for diagnostics
Plain-English CAN Bus: how 60+ ECUs communicate over 2 wires, what ISO 15765-4 and ISO-TP are, high-speed vs low-speed CAN, and why "lost communication with ECU" (U0100+) is critical.
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OBD-II on Chinese cars: Chery, Geely, BYD, Great Wall — diagnostic specifics
What OBD-II can do on Chery Tiggo, Geely Coolray, BYD Tang, Great Wall Haval: SAE J1979 standard, custom PIDs, GB codes, brand tools vs universal ELM327.
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Not all DTCs are equal: 4 severity levels (low, medium, high, critical) and how to prioritize
What DTC severity level means, how to tell "light from dust on the MAF" from "stop the engine now", which codes can wait, which are critical right now.
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Fuel Trim (STFT and LTFT): how the fuel correction tells you about engine health
What Short-Term (STFT) and Long-Term (LTFT) Fuel Trim mean, which values are normal, and how to diagnose a vacuum leak, dirty MAF or clogged injector from Fuel Trim numbers alone.
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HVAC deep-dive: climate control via OBD, blend-door actuators, servo motors — when the cabin heats "from one side"
Dual-zone climate control via OBD: codes B12xx, blend-door actuator state, mode-door, vent output temperature. How to diagnose "warm on the right — cold on the left".
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OBD-II on motorcycles: BMW Motorrad, Harley, Yamaha — where the standard port lives and where it doesn't
What OBD-II can do on a bike: BMW Motorrad K1600 (16-pin OBD), Harley-Davidson (4-pin DDL), Yamaha K-Line, Ducati, KTM. Which adapters and specific codes are available.
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O2 sensor waveform: how to tell a dead lambda from a live one by shape
Visual analysis of upstream and downstream O2 sensor signals: normal oscillations, response signs, aging indicators, how to distinguish a "flat" curve from a real problem.
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Freeze Frame: how OBD "photographs" the moment of a fault and why it matters more than the DTC itself
What OBD-II Freeze Frame data is, which parameters it captures, how to read the ECU snapshot, and why an experienced diagnostician looks at it before the trouble code itself.
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P0300: random / multiple misfire — 12 causes and a step-by-step diagnostic algorithm
Deep dive into P0300: why misfires are "random", how to narrow 12 possible causes to one in 30 minutes, what the AI agent reads from Freeze Frame and Live Monitor.
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P0430: catalyst bank 2 — why the "other side" of the engine fails separately and how to diagnose
Difference between P0420 and P0430, when both appear together vs separately, how OBD-II sees bank 2, why V-engines fail asymmetrically, and repair cost.
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UDS Mode 22: what manufacturers hide beyond the OBD-II standard
What UDS (Unified Diagnostic Services) ISO 14229 is, difference vs OBD-II, how Mode 22 reads DPF soot mass, VGT position, TPMS voltage — and why 90% of the "interesting stuff" lives here.
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UDS SecurityAccess: how seed-key challenge protects the ECU and why "hacking" isn't that simple
What UDS SecurityAccess ($27) is, how seed-key challenge works, why some ops (read PID) are open but others (write, flash) require auth, and what's legally permitted.
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Engine Stalls While Driving: 10 Causes and Urgent Diagnosis
Engine suddenly dies while driving — under braking, at lights, on the motorway: how to find the cause in 15 minutes and whether you can keep driving.
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Common Ford Faults: Focus, Fiesta, Kuga, Mondeo — DTC Codes and Repair
Catalogue of common Ford Focus, Fiesta, Kuga, Mondeo, Escape faults: EcoBoost turbo engines, PowerShift DCT, Duratec, TDCi diesels.
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Common Mercedes-Benz Faults: W204, W205, W211, W212 — DTC Codes
Catalogue of common Mercedes C-Class, E-Class, GLK, GLC faults: OM651 diesel, M271/M276 petrol, 7G-Tronic, AirMatic.
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Common BMW Faults: E90, F30, G20 — DTC Codes and Repair
Catalogue of common faults on BMW 3-Series (E90, F30, G20) and 5-Series (E60, F10, G30): VANOS, timing chain, HPFP, DPF, injectors, electronics.
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Car Will Not Start: 12 Causes and a Quick Diagnostic Algorithm
Engine cranks but no fire — or dead silence: how to tell in 5 minutes whether it is the starter, the battery, fuel, plugs, immobiliser, or something deeper.
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Diesel Engine Power Loss: 10 Causes and Step-by-Step Diagnosis
Why a diesel car does not pull, how to quickly distinguish turbo, EGR, DPF or injector issues, and which DTC codes to read first.
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Fuel Consumption Went Up: 12 Causes and How to Diagnose via OBD-II
Sudden fuel-use increase: which DTC codes and live parameters to watch, how to distinguish worn engine from a clogged filter.
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Oil Leaking Under the Engine: 10 Leak Points and How to Pinpoint
Found an oil stain under the car: how to identify in 20 minutes which gasket or seal is leaking, and how much repair costs.
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Common Renault Faults: Megane, Duster, Logan — DTC Codes and Repair
Catalogue of common Renault Megane, Duster, Logan, Sandero, Captur faults: EGR, DPF, 1.5 dCi injectors, TCe turbo, UCH electronics.
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Common Skoda Faults: Octavia, Fabia, Yeti — DTC Codes and Repair
Catalogue of common Skoda Octavia A5/A7, Fabia, Yeti, Superb faults: TSI engines, DPF, DSG, EPC, VAG platform shared with Volkswagen.
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Steering Wheel Vibration: 10 Causes and How to Pinpoint the Speed
Steering wheel vibration at 60, 90, 120 km/h: how to tell in 5 minutes whether it is wheels, brakes, CV joint, wheel bearing, or the engine.
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Common Toyota Faults: Camry, Corolla, RAV4 — DTC Codes and Repair
Catalogue of common Toyota Camry, Corolla, RAV4, Prius faults: oxygen sensors, catalyst, DPF, VVT-i, hybrid system, EGR.
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Common Volkswagen Faults: Top 20 DTC Codes and Typical Problems
Catalogue of typical VW Golf, Passat, Polo, Tiguan faults: EPC, ABS, DPF, EGR, ignition coils, TSI engines. With DTC codes, symptoms and repair.
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Check Engine Light On: What to Do — Step-by-Step Guide
A driver's step-by-step guide: what the Check Engine light means, how to read DTC codes without a scanner, when it is safe to keep driving, and when to call a tow truck.