Litres consumed per 100 kilometres; lower is better. Standard metric efficiency unit.

Energy lost deforming tires as they roll. Low-pressure or wide tires raise it.

Automotive Calculators

OBD2 Code Lookup

2026-09-27 6 min

Decode a diagnostic trouble code: system letter, subsystem group and cylinder from code type and number.

Subsystem Group: 3; 0-2 fuel/air, 3 ignition, 4 emissions, 5 idle, 6 computer, 7+ trans
Cylinder / Sub-code: 1; Last two digits (misfire = cylinder)

Calculator

Code Letter Help

0=P powertrain, 1=C chassis, 2=B body, 3=U network

Code Number Help

Digits after the letter (e.g. 301)

Results update live as you type

Live statistics

Subsystem Group

0-2 fuel/air, 3 ignition, 4 emissions, 5 idle, 6 computer, 7+ trans

Cylinder / Sub-code

Last two digits (misfire = cylinder)

Results

Optimal

Subsystem Group

0-2 fuel/air, 3 ignition, 4 emissions, 5 idle, 6 computer, 7+ trans

Cylinder / Sub-code

Last two digits (misfire = cylinder)

Planning estimate from standard automotive formulas. Validate with dyno, timeslip or datalog data when precision matters.

Visualizations

Results overview

3Subsystem Group

1Cylinder / Sub-code

Subsystem Group

010

Formula

Parses the standardized DTC structure: letter selects the controller family, hundreds select the subsystem.

Subsystem = hundreds digit; detail = last two digits

Alternative Form

P03xx = ignition/misfire (last digits = cylinder); P04xx = aux emissions; P07xx+ = transmission

Variables

Symbol	Name	Unit	Description
L	Letter		P/C/B/U controller family
N	Number		Subsystem + specific fault

Explanation

OBD-II codes are addresses, not diagnoses: P0301 means cylinder 1 misfire detected — plugs, coils, injectors and compression are all still suspects.

Step-by-Step Calculation

Follow each step to understand how the result is calculated from your inputs.

Step 1

Record the inputs.

Code Letter:0

Code Number:301

Result: 2 inputs ready

Step 2

Apply the formula.

Subsystem = hundreds digit; detail = last two digits

Result: Primary result = 3

Step 3

Read every derived output.

Subsystem Group:3

Cylinder / Sub-code:1

Result: Subsystem Group = 3

Final Answer

Worked Examples

See how the calculator works with different scenarios.

Scenario

Check-engine light, rough idle.

Inputs

Letter:P (0)

Number:301

Calculation

Subsystem = 3 (ignition/misfire); detail = 01 (cylinder 1)

Result

Group 3, cylinder 1.

Interpretation

Misfire on cylinder 1 — inspect plug, coil, injector, compression in that cost order.

Understanding Your Result

Your Result: Subsystem Group: 3

What This Means

Which system and cylinder a fault code points at.

Why It Matters

Turns a cryptic code into an inspection shortlist.

Professional Interpretation

Treat the primary result as a planning estimate, then validate on a dyno, at the track, or with logged sensor data. If measured numbers disagree by more than ~10%, re-check inputs (units are the usual culprit) before changing parts.

Recommendations

•Double-check units on every input before trusting the result
•Compare against the worked example below to sanity-check magnitude
•Validate with measured data (dyno, timeslip, datalog) when it matters

Learn — fuel economy, deeply

Trouble codes are structured addresses into the fault library.

1 · Where the gallon goes (energy path)

Engine heat

60–70% lost

Coolant + exhaust carry most energy away. Hybrids recover some via Atkinson cycle + regen.

Driveline

5–15% lost

Transmission, differential, accessories. CVTs and 8–10 speed autos lose less.

Aero drag

∝ v²

Doubles from 55→78 mph. Roof boxes hurt more than trunks.

Rolling + brakes

10–20%

Tires, weight, stops. Regen braking (hybrid/EV) reclaims city braking energy.

4 · Measure like a pro (the 3-tank protocol)

Fill to first click at the same pump, record the odometer (Tank 0).
Drive normally for 2–3 tanks; always refill to first click at the same pump.
MPG = total miles ÷ total gallons across all tanks — averaging kills pump-click noise.
Cross-check with the trip computer; a persistent >10% gap means the computer needs calibration — or the car needs service.
Log with an OBD-II dongle for per-trip MPG if you want to isolate commute vs. highway behavior.

Practical Tips

•Read freeze-frame with the code
•Fix the first/oldest code first
•Clear and road-test to confirm

Common Mistakes

Mixing units (inches with mm, psi with kPa)

Convert every input to the unit shown on its field before calculating.

Trusting a single output blindly

Cross-check with the worked example and, when it matters, a dyno or datalog.

Assumptions

This calculator makes the following assumptions to provide estimates:

Steady-state model: Formulas assume stabilized conditions; transients, heat soak and traction limits are excluded.

Standard factors: Empirical constants (BSFC, VE, drivetrain loss) use typical values — your combination may differ.

Limitations

Be aware of these limitations when using this calculator:

Estimate, not measurement: Predictions carry real-world scatter of roughly ±5–15% depending on the model.

No transient effects: Shift time, wheelspin, spool lag and heat soak are not modeled here.

Frequently Asked Questions

Answers to common questions about this calculator.

What does P0301 mean?

Cylinder 1 misfire detected. Check spark plug, coil pack, injector and compression — plus the freeze-frame conditions.

Do codes tell me which part to replace?

No — they identify the failing test or circuit. Diagnose the listed system; do not fire the parts cannon.

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Related Guides

Deepen your understanding with these educational resources.

Automotive Guides

Build planning, measurement protocol and validation how-tos

Glossary

Technical terms used in mortgage calculations explained in plain language.

MPG: Miles per gallon. Distance travelled per gallon of fuel; higher is better. US gallon = 3.785 L.
L/100 km: Litres consumed per 100 kilometres; lower is better. Standard metric efficiency unit.
MPGe: Miles per gallon-equivalent. EV metric: miles travelled on 33.7 kWh (energy of 1 gal of gasoline).
CAFE: Corporate Average Fuel Economy — US fleet-average efficiency standard enforced by NHTSA.
Rolling resistance: Energy lost deforming tires as they roll. Low-pressure or wide tires raise it.
Aerodynamic drag: Air resistance growing with the square of speed — why highway MPG falls fast above 60 mph.
Hypermiling: Driving techniques (coasting, steady speed, anticipation) that maximise real-world MPG.
OBD-II: On-board diagnostics port. Scanners and dongles read live MPG, fuel trim and fault codes.
DTC: Diagnostic trouble code — the standardized P/C/B/U fault identifier.

Authoritative References

Trusted sources and standards referenced by this calculator.

EPA — FuelEconomy.gov DOE — Fuel-Efficient Driving Tips NHTSA — Driving Safety & Efficiency SAE International — Fuel Economy Standards Consumer Reports — MPG Truth IRS — Standard Mileage Rates

Applicable Standards

EPA Fuel Economy Testing (40 CFR Part 600)CAFE — Corporate Average Fuel Economy Standards (NHTSA)WLTP — Worldwide Harmonised Light Vehicle Test ProcedureSAE J1321 / J1711 — Fuel consumption test proceduresISO 2575 — Road vehicles symbols & efficiency markings

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Last Updated:

2026-09-27

Version:

2.0.0

References:

License:

Free to use for educational and professional purposes.