Expert guides for UK drivers — fault codes, check engine lights, MOT advice and more.
This hub is organised around the decisions drivers usually need to make: whether the car is safe to drive, whether an MOT is at risk, which scanner to buy, and how to challenge a repair quote with better questions.
Start here if you already have a fault code and want an indexable guide with UK repair-cost and MOT context.
Start here if you need to know whether a code, light, or readiness issue is likely to affect your MOT.
Start here if you still need a scanner, adapter, or app to read codes at home.
Start here for quick answers on codes, reports, payments, and how the service works.
Use the articles below for UK-specific explanation and decision support, then follow the strongest destination page for the next step: methodology, proof, scanner choice, or the main fault-code guide.
How we generate and review diagnostics content, what evidence layers we rely on, and where the limits are.
Real user feedback on whether the product is useful, understandable, and aligned with UK driver expectations.
The main canonical page for scanner and app recommendations if you still need a tool to read codes at home.
High-intent guides for drivers deciding whether the car is safe to drive or needs immediate attention.
Your check engine light is flashing — here's exactly what it means, whether it's safe to drive, and what to do next. UK-specific advice.
Check engine light on but your car feels normal? Here's what it likely means for UK drivers, which codes to watch out for, and what to do before your MOT.
Will an engine management light fail MOT in the UK? What counts as a fail, what to check before the test, and when it is safe to drive.
P0299 limp mode UK guide. Why turbo underboost sends the car into limp mode, what is usually safe to drive, and what to check before replacing the turbo.
Pages focused on avoiding preventable MOT failures and understanding emissions-related outcomes.
Nearly 1 in 3 UK cars fail their MOT. Use this 2026 checklist to spot common failures before your test and pass first time — including OBD2 checks.
Which OBD2 fault codes cause the most UK MOT failures in 2026? Data-driven guide covering P0420, P0171, P0300 and more — with UK repair costs and fix advice.
Will an engine management light fail MOT in the UK? What counts as a fail, what to check before the test, and when it is safe to drive.
Will P0171 fail MOT in the UK? Lean mixture faults, warning-light risk, emissions impact, and what to fix before you book the test.
Foundational guides for reading codes, choosing scanners, and understanding OBD2 in the UK.
What is OBD2 and how does it work? Complete UK guide to On-Board Diagnostics — fault codes, scanners, MOT implications and how to use it to save money on repairs.
Don't have an OBD2 scanner? Here are the cheapest ways to read your car's fault codes in the UK, including free apps, cheap dongles and AI diagnosis.
The best OBD2 scanners for UK drivers in 2026. From budget dongles to professional tools, reviewed and ranked with UK-specific advice.
Pages intended to help drivers challenge weak quotes and understand likely repair paths before paying a garage.
Step-by-step DIY guide to fixing fault code P0420 in the UK. Check oxygen sensors, exhaust leaks and catalytic converter efficiency before paying a garage £400+.
How much does a catalytic converter replacement cost in the UK in 2026? Prices by car make, DIY vs garage costs, and how to avoid being overcharged.
P0299 repair cost UK guide. Typical turbo underboost causes, what garages usually miss, and realistic repair costs before you approve a quote.
P0300 repair cost UK guide. What a random misfire usually means, how urgent it is, and the typical UK repair costs before you authorise work.
P0171 symptoms UK guide. Signs of a lean running engine, MOT implications, and typical repair costs before you replace the wrong part.
P0420 symptoms UK guide. What a catalyst efficiency fault usually feels like, when the car still drives normally, and what to check before replacing the cat.
P0420 repair cost UK guide. Realistic cost ranges, the cheapest likely fixes first, and when a catalytic converter quote is probably justified.
Will P0171 fail MOT in the UK? Lean mixture faults, warning-light risk, emissions impact, and what to fix before you book the test.
P0299 limp mode UK guide. Why turbo underboost sends the car into limp mode, what is usually safe to drive, and what to check before replacing the turbo.
These are some of the most commercially valuable and commonly searched fault-code pages on the site. They connect blog readers to indexable diagnostic pages with stronger repair and MOT intent.
One of the highest-intent emissions fault pages, with UK repair costs and catalytic-converter decision guidance.
Strong diagnostic-intent page for rough running, flashing engine lights, and urgent drivability questions.
Common search path for air leaks, fueling imbalance, and MOT-related emissions concerns.
High-value page for modern petrol and diesel turbo faults, limp mode, and quote-challenge research.
Network-diagnostics page for ECM/PCM communication loss, low-voltage events, and multi-warning-light investigations.
Useful when blog readers move from MOT or dashboard-warning content into ABS and safety-system fault diagnosis.
These pages support blog journeys that start with MOT, dashboard-warning, or scanner research and then branch into ABS, airbag, or communication faults rather than engine-only codes.
For readers seeing several warnings at once and needing a first-pass guide to network faults, voltage issues, and module dropouts.
For MOT and brake-warning journeys where the ABS module stops responding and multiple safety systems can be affected.
For ABS and traction-control investigations where a wheel-speed sensor fault is the likeliest starting point.
For body and restraint-system searches where an SRS or airbag warning needs its own diagnostic path.
These are the pages that already have unique content and should absorb the strongest internal-link equity right now, without sending crawlers through redirects or placeholder routes.
Primary repair-intent guide for catalyst quotes, sensor checks, and MOT emissions risk.
Strong page for intake leaks, MAF plausibility, and emissions-related quote checks.
Urgent drivability page for rough running, flashing engine lights, and catalyst-protection decisions.
High-value page for limp mode, boost leaks, actuator faults, and turbo-replacement decisions.
Supporting article for one of the site’s highest-value commercial research journeys.
Supporting article for searchers checking whether a lean fault will fail a UK MOT.
Published 1 Jun 2026
Will an engine management light fail MOT in the UK? What counts as a fail, what to check before the test, and when it is safe to drive.
Published 1 Jun 2026
Will P0171 fail MOT in the UK? Lean mixture faults, warning-light risk, emissions impact, and what to fix before you book the test.
Published 1 Jun 2026
P0171 symptoms UK guide. Signs of a lean running engine, MOT implications, and typical repair costs before you replace the wrong part.
Published 1 Jun 2026
P0299 limp mode UK guide. Why turbo underboost sends the car into limp mode, what is usually safe to drive, and what to check before replacing the turbo.
Published 1 Jun 2026
P0299 repair cost UK guide. Typical turbo underboost causes, what garages usually miss, and realistic repair costs before you approve a quote.
Published 1 Jun 2026
P0300 repair cost UK guide. What a random misfire usually means, how urgent it is, and the typical UK repair costs before you authorise work.
Published 1 Jun 2026
P0420 repair cost UK guide. Realistic cost ranges, the cheapest likely fixes first, and when a catalytic converter quote is probably justified.
Published 1 Jun 2026
P0420 symptoms UK guide. What a catalyst efficiency fault usually feels like, when the car still drives normally, and what to check before replacing the cat.
Published 5 Jun 2026
P0420 on a Vauxhall Astra? UK guide covering common causes, checks worth doing first, and realistic repair costs for 1.4, 1.6, and 1.8 petrol models.
Updated May 2026
What is OBD2 and how does it work? Complete UK guide to On-Board Diagnostics — fault codes, scanners, MOT implications and how to use it to save money on repairs.
Updated May 2026
Which OBD2 fault codes cause the most UK MOT failures in 2026? Data-driven guide covering P0420, P0171, P0300 and more — with UK repair costs and fix advice.
Updated May 2026
How much does a catalytic converter replacement cost in the UK in 2026? Prices by car make, DIY vs garage costs, and how to avoid being overcharged.
Updated May 2026
Step-by-step DIY guide to fixing fault code P0420 in the UK. Check oxygen sensors, exhaust leaks and catalytic converter efficiency before paying a garage £400+.
Updated April 2026
The best OBD2 scanners for UK drivers in 2026. From budget dongles to professional tools, reviewed and ranked with UK-specific advice.
Updated April 2026
Check engine light on but your car feels normal? Here's what it likely means for UK drivers, which codes to watch out for, and what to do before your MOT.
Updated April 2026
Your check engine light is flashing — here's exactly what it means, whether it's safe to drive, and what to do next. UK-specific advice.
Updated April 2026
Nearly 1 in 3 UK cars fail their MOT. Use this 2026 checklist to spot common failures before your test and pass first time — including OBD2 checks.
Updated April 2026
Don't have an OBD2 scanner? Here are the cheapest ways to read your car's fault codes in the UK, including free apps, cheap dongles and AI diagnosis.
Published 9 Jul 2026
Complete UK guide to EVAP emission fault codes P0440, P0442, P0446, P0455, P0456 and P0457. Common causes, repair costs, and MOT risk.
Published 9 Jul 2026
UK guide to EGR fault codes P0400, P0401, P0402, P0403 and P0411. Common causes, repair costs, and MOT risk for diesel and petrol cars.
Published 9 Jul 2026
UK guide to O2 sensor fault codes P0130 through P0161. Oxygen sensor causes, repair costs, and MOT emissions risk.
Published 9 Jul 2026
UK guide to coolant temperature fault codes P0115, P0116, P0117, P0118, P0128, P0217 and P0480. Causes, repair costs, and MOT risk.
Published 9 Jul 2026
UK guide to fuel pressure fault codes P0087, P0088, P0089, P0093, P0190 and P0191. Common causes, repair costs, and MOT risk.
Published 9 Jul 2026
UK guide to turbo and boost fault codes P0234, P0235, P0236 and P0299. Overboost, underboost, and sensor causes with repair costs.
Published 9 Jul 2026
UK guide to timing, camshaft, crankshaft and ignition coil fault codes including P0010-P0016, P0335-P0345 and P0351-P0354.
Published 9 Jul 2026
UK guide to automatic transmission fault codes P0700, P0715, P0720, P0730, P0741, P0750 and P0755. Causes, repair costs, and MOT risk.
Published 9 Jul 2026
UK guide to diesel particulate filter fault codes P2002, P2003 and P2459. Soot loading, ash accumulation, and regeneration causes.
Published 9 Jul 2026
UK guide to diesel AdBlue and NOx fault codes P11A0, P2200, P2201, P20E8 and P203F. SCR system causes, repair costs, and MOT risk.
Published 10 Jul 2026
All 33 Show Me Tell Me questions for the 2026 UK driving test. Free answers, diagrams, and MOT-linked safety checks every driver should know.
Published 10 Jul 2026
Complete guide to Show Me Tell Me engine bay checks for the UK driving test. How to check oil, coolant, brake fluid, and power steering — with step-by-step instructions.
Published 10 Jul 2026
Show Me Tell Me exterior checks guide for the UK driving test. How to check lights, tyres, mirrors, wipers, and windscreen — with MOT-linked tips.
Published 10 Jul 2026
How much does a garage charge to read OBD2 fault codes in the UK in 2026? Compare garage diagnostic costs, cheap alternatives, and how to save money on car diagnostics.