How to Diagnose & Fix Common Check Engine Codes
You ever turn the key and see that dreaded light glow and think
“Is my engine about to explode”
“Can I fix this or do I need to remortgage the house”
“Do I even want to know”
Been there
Felt that panic
But here’s the truth — most check engine codes are minor
If you know what to look for
Here’s my straight–talk guide to how to diagnose & fix common check engine codes
No nonsense
No overpriced repairs you don’t need
I’m Myriam
Makeup artist
Car worrier turned car fixer
And here’s how you handle it like a pro
Why Does The Check Engine Light Come On?
Because your car’s computer is a bit dramatic
Even a loose petrol cap can trigger it
✅ Sometimes serious — like misfires
✅ Sometimes silly — like a dirty sensor
✅ Always worth checking — because guessing costs more
How to Diagnose & Fix Common Check Engine Codes Step by Step
Here’s exactly what I do — every single time
Step 1: Stay Calm
✅ If the light is steady — not flashing — it’s usually safe to drive home or to a shop
✅ If it’s flashing — stop driving — that means a serious misfire that can wreck your catalytic converter
Step 2: Get A Code Reader
✅ You can buy one online for £15–£30
✅ Or many car parts shops will plug theirs in for free
✅ Plug it into the port under your dashboard (usually by your knees)
✅ Read the code — it’ll look something like P0420 or P0171
Step 3: Look Up The Code
✅ Use the little booklet that comes with the reader
✅ Or just Google it — plenty of simple explanations online
✅ Don’t just guess based on symptoms — that’s how you waste money
Common Check Engine Codes & What To Do
Here are the ones I see most often
P0420 – Catalyst System Efficiency Below Threshold
✅ Usually means your catalytic converter isn’t happy
✅ Sometimes just an oxygen sensor going bad
What to do:
✔ Check for exhaust leaks
✔ Replace O2 sensor if cheap
✔ If the converter itself is bad — shop job, not DIY
P0171 – System Too Lean
✅ Engine is getting too much air or not enough fuel
What to do:
✔ Check for vacuum leaks — cracked hoses
✔ Clean or replace mass airflow sensor
✔ Replace dirty fuel filter
P0300 – Random Misfire Detected
✅ Engine is misfiring — could be spark plugs, coils or fuel
What to do:
✔ Check spark plugs and ignition coils
✔ Make sure fuel injectors are clean
✔ If you hear knocking or it runs rough — get help fast
P0455 – Large Evaporative Emissions Leak
✅ Big leak in the fuel system — often just the petrol cap
What to do:
✔ Tighten or replace the fuel cap
✔ Check hoses around the fuel tank for cracks
My Own Check Engine Light Story
Once my light came on on the way to a wedding
Panicked
Thought it was the end of the world
Reader said P0455 — fuel cap loose
Twisted it until it clicked
Light went off next day
Cost me nothing but 2 minutes of my life
Quick Tips To Handle It Like A Pro
☑ Don’t ignore it — but don’t freak out
☑ Get a cheap reader — they pay for themselves the first time you use it
☑ If it’s flashing — stop driving
☑ Fix the easy stuff first — fuel cap, spark plugs, sensors
☑ If it’s something big — find a trusted shop and take your code with you
FAQs About How to Diagnose & Fix Common Check Engine Codes
Can I just reset the light?
Yes — but it’ll come back if the problem isn’t fixed.
Is it safe to drive with the light on?
If steady — usually yes for a short time. If flashing — no.
Do I need a mechanic for every code?
No — many fixes are cheap and easy at home.
Why does it come on after filling petrol?
Loose cap or vapour system leak — tighten the cap properly.
Does it always mean something serious?
No — half the time it’s just sensors or minor issues.
Final Word On How to Diagnose & Fix Common Check Engine Codes
If you follow this guide on how to diagnose & fix common check engine codes
You’ll save money
You’ll feel in control
And you’ll stop fearing that little light
Check it
Fix what you can
And don’t get talked into unnecessary repairs
