OBD scan, brakes, tires, leaks – a practical deep-dive for smart buyers
Introduction:
Buying used can be brilliant value, but only if you spot issues before money changes hands. This step-by-step guide shows the first 10 checks every buyer should do, with quick tools, pass/fail cues, and when to walk away.
The First 10 Checks (with pro tips)
1) OBD scan & readiness
Plug in a basic OBD-II/EOBD reader before the test drive. Check for stored and pending codes, readiness monitors, and pull the VIN from Mode 09.
- Pass: No current/pending engine codes. Most readiness monitors show Ready.
- Fail/Walk: Many monitors Not Ready (can mean codes were just cleared). VIN mismatch.
- Tip: Take a short mixed drive, then rescan.
2) Brakes: pads, discs, fluid, handbrake
Look through the caliper for pad thickness. Feel for pulsation under braking and listen for squeal or grind. Test the parking brake on a slope.
- Pass: Pads with healthy material (aim to replace at 3 mm). Smooth, straight stops.
- Fail/Walk: Grinding, blue/heat-spotted discs, soft pedal, fluid below MIN.
- Tip: Budget pairs: pads + discs together on an axle.
3) Tires: tread, age, wear pattern & pressures
Measure tread across the full width and check the DOT date code (week/year). Inspect for cracks, bulges, and even wear.
- Pass: ≥ 3 mm tread for wet-grip headroom; DOT age reasonable.
- Fail/Walk: < 1.6 mm (illegal in EU/UK), cords showing, old/hard rubber, odd wear (alignment/suspension issues).
- Tip: Uneven inside-edge wear often means poor alignment or worn bushings.
4) Leaks under and around the engine bay
Park on clean ground or use a sheet. Check under the car and around the engine, gearbox, diff and power-steering.
- Pass: Dry undersides, no drips.
- Fail/Walk: Oily drips, green/orange/pink coolant puddles, red ATF, fuel smell.
- Tip: Milky “mayo” on oil cap or coolant tank suggests cross-contamination.
5) Suspension & steering
Bounce each corner; look for leaking struts, split CV boots and perished bushings. During a drive, listen for knocks and feel for wander.
- Pass: No clunks over bumps, tracks straight, even tire wear.
- Fail/Walk: Clonks/creaks, steering play, pull under braking, saw-tooth tire wear.
- Tip: Knocks over speed bumps often point to top mounts or drop links.
6) Body, rust & structure
Inspect panel gaps, overspray, and trim clips. Look under sills, wheel arches, subframes, jacking points and the spare-wheel well.
- Pass: Even gaps, factory sealant, surface rust only.
- Fail/Walk: Crumpled rails, fresh underseal hiding rot, bubbling at structural points.
- Tip: A small magnet helps detect thick filler.
7) Paperwork, VIN & recalls
Confirm the VIN on the body matches the registration and the paperwork. Check for open safety recalls and verify inspection history where available.
- Pass: Full-matching VINs, clear history, no outstanding recalls.
- Fail/Walk: Missing logbook, VIN inconsistencies, unresolved recall.
- Tip: Print/list the last servicing and timing-belt interval if applicable.
8) Electricals & battery
Cycle all lights, windows, mirrors, seats, infotainment and HVAC. Watch for warning lights during ignition-on self-check.
- Pass: Everything operates promptly; no warning lights after start.
- Fail/Walk: Dim lights at idle, slow windows, warning icons that return.
- Tip: Many cars show battery voltage in a hidden service menu; otherwise use a multimeter.
9) Test drive: cold start to motorway
Start from cold if possible. Listen for rattles at idle, misfire, turbo whistle or belt squeal. On the road, test straight-line braking and hill pull.
- Pass: Clean idle, smooth shifts, no vibrations, brakes pull straight.
- Fail/Walk: Misfire, slipping clutch, harsh shifts, wheel shake over 100 km/h.
- Tip: Radio off for the first 5 minutes.
10) Fluids: oil, coolant, brake, ATF
Check levels and condition. Oil should be at the mark and not gritty. Coolant should be colored and clean.
- Pass: Fluids at spec, clean and proper color.
- Fail/Walk: Low/black oil with sludge, brown coolant, burned-smell ATF.
- Tip: Brown or sweet-smelling puddles are red flags.
Quick Tools to Bring
- OBD-II/EOBD scanner (reads codes + readiness)
- Tyre tread gauge & pressure gauge
- Flashlight, paper towels, small magnet
- Notepad/phone checklist and VIN list
Summary Checklist Table (print-friendly)
OBD scan
- What “Good” Looks Like: No current/pending codes; monitors Ready; VIN matches
- Quick Tool: OBD-II reader
- Red Flags: Many monitors Not Ready; VIN mismatch
Brakes
- What “Good” Looks Like: No noise/vibration; pads healthy (~3 mm replace); firm pedal
- Quick Tool: Flashlight
- Red Flags: Grinding/squeal; soft pedal; blue discs
Tires
- What “Good” Looks Like: ≥3 mm tread; even wear; DOT date reasonable
- Quick Tool: Tread gauge
- Red Flags: <1.6 mm; cracks/bulges; inside-edge bald
Leaks
- What “Good” Looks Like: Dry undersides
- Quick Tool: Clean sheet/torch
- Red Flags: Oil, coolant (green/pink), red ATF, fuel smell
Suspension/steer
- What “Good” Looks Like: Straight tracking; no knocks
- Quick Tool: Test drive
- Red Flags: Clunks; pull; scalloped wear
Body/rust
- What “Good” Looks Like: Even gaps; factory sealant
- Quick Tool: Magnet/torch
- Red Flags: Rot at sills/subframes; fresh underseal
VIN/recalls
- What “Good” Looks Like: All VINs match; no open recalls
- Quick Tool: VIN list/online
- Red Flags: Missing docs; unresolved safety recall
Electricals
- What “Good” Looks Like: All functions work; no warnings
- Quick Tool: Ignition self-check
- Red Flags: Persistent warnings; slow motors
Test drive
- What “Good” Looks Like: Smooth power and shifts
- Quick Tool: Your senses
- Red Flags: Vibrations; slipping clutch
Fluids
- What “Good” Looks Like: Clean, correct levels
- Quick Tool: Rag
- Red Flags: Milky oil; brown coolant; burnt ATF
Conclusion:
These ten checks catch the big risks fast: hidden engine faults, unsafe brakes, illegal or worn tyres, leaks and structural problems. If anything fails hard, walk away. If it passes, book an independent pre-purchase inspection for peace of mind and use any minor findings to negotiate.
Glossary (Acronyms & Jargon)
- ATF (Automatic Transmission Fluid) – Oil used inside an automatic or dual-clutch gearbox for lubrication, cooling and hydraulic control. Burnt-smell ATF can signal overheating or internal wear.
- CV boot – A rubber sleeve that covers a constant-velocity (CV) joint on a driveshaft, keeping grease in and dirt/water out. Split boots can quickly lead to joint failure.
- Differential (“diff”) – A gear unit that splits power between left/right wheels (or front/rear axles) while allowing them to rotate at different speeds, especially in corners.
- DOT date code – Four-digit code moulded into a tyre sidewall (week + year of manufacture). Example:
1622= week 16 of 2022. Used to judge tyre age. - HVAC – Heating, Ventilation and Air Conditioning system; covers cabin temperature control, airflow and demisting/defogging.
- Mode 09 – An OBD/OBD-II diagnostic mode that lets a scan tool read vehicle information such as the VIN and calibration IDs stored in the ECU.
- OBD / OBD-II / EOBD – On-Board Diagnostics systems that let you read fault codes and live data from a car’s ECUs. OBD-II is the modern standard; EOBD is the harmonised European version.
- Pre-purchase inspection – A detailed check of a used car by an independent mechanic or specialist, typically including a lift inspection, test drive and written report before you buy.
- Readiness monitors – Self-tests that the engine control unit runs on emissions-related systems (catalyst, oxygen sensors, EVAP, etc.). “Ready” means tests have completed; many “Not Ready” flags can suggest codes were recently cleared.
- VIN (Vehicle Identification Number) – A unique 17-character code identifying each vehicle. It should match across the car’s body stampings, data plates, and registration / title documents.
I’m not inventing a new wheel ; here’s the tool I used:
ChatGPT (Plus), used with my custom CarAIBlog.com blogging prompt.
