These are the trouble codes that set off the check engine light most often, what each one means, and what it typically costs to fix in 2026. Ordered by how commonly drivers encounter them.
Typical repair cost by code
Mid-point of the typical US repair cost for the most common OBD2 codes, 2026. Actual cost depends on the true root cause.
P0420 - Catalyst efficiency (Bank 1)
$900
Often an O2 sensor, sometimes the catalytic converter
P0011 - Camshaft timing over-advanced
$700
VVT solenoid or oil-flow issue
P0401 - EGR flow insufficient
$450
EGR valve or clogged passages
P0300 - Random / multiple misfire
$400
Plugs, coils, or fuel/vacuum issue
P0171 - System too lean (Bank 1)
$300
Vacuum leak, MAF, or fuel delivery
P0128 - Coolant below thermostat temp
$300
Usually a stuck-open thermostat
P0102 - MAF sensor circuit low
$250
MAF sensor or intake leak
P0442 - Small EVAP leak
$150
Often just a loose or bad gas cap
The most important thing to know about these codes: the code names the affected system, not the exact broken part. P0420 does not mean "replace the catalytic converter." It means the catalyst is reading inefficient, which is frequently a failing oxygen sensor that costs a fraction of a converter.
Guessing at a fix based on the code alone is how people spend $1,500 on a converter when a $60 sensor was the problem. Read the code, then diagnose the cause.
Full data table
Rank
Code
Meaning
Common cause
Typical cost
1
P0420
Catalyst efficiency below threshold (Bank 1)
O2 sensor or catalytic converter
$200 - $2,000
2
P0300
Random or multiple cylinder misfire
Spark plugs, coils, fuel or vacuum
$150 - $1,000
3
P0171
System too lean (Bank 1)
Vacuum leak, MAF, fuel delivery
$100 - $600
4
P0128
Coolant temp below regulating temp
Stuck-open thermostat
$150 - $500
5
P0442
EVAP system small leak
Loose or faulty gas cap, EVAP hose
$20 - $300
6
P0401
EGR flow insufficient
EGR valve or clogged passages
$250 - $650
7
P0011
Intake camshaft timing over-advanced
VVT solenoid, oil flow
$300 - $1,200
8
P0102
MAF sensor circuit low input
MAF sensor or intake leak
$100 - $400
📎 Cite or reproduce this data (free, CC BY 4.0)
Citation
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You may reproduce this data with attribution and a link to https://www.ampauto.io/most-common-check-engine-codes-2026. Journalists: nate@amptekgrowth.com for the raw numbers or a custom cut.
FAQ
What is the most common check engine code?
P0420 (catalyst efficiency below threshold) and P0300 (random misfire) are among the most frequently reported. P0420 is often a failing oxygen sensor rather than the catalytic converter itself.
Does a code tell me exactly what to replace?
No. A code identifies the affected system or symptom, not the specific failed part. The same code can have several causes at very different price points, so diagnose the root cause before replacing anything.
Where can I look up my specific code?
Use the free AmpAuto OBD2 code library at /codes, or run a free AI diagnosis that ranks the likely causes for your exact vehicle.
Can I reuse this data?
Yes, under CC BY 4.0 with attribution and a link to this page.
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