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Automotive Calculators

OBD2 Diagnostic Calculator

2026-09-27 6 min

Diagnostic severity score and confirmation drive cycles from stored, pending and permanent code counts.

Severity Score
5 pts
Higher = worse
Confirm Drive Cycles
3
Trips to validate repair

Calculator

Stored Codes Help

Confirmed faults

Pending Codes Help

Failed once, unconfirmed

Permanent Codes Help

Emissions history (cannot clear)

Results update live as you type

Live statistics

Severity Score

5 pts

Higher = worse

Confirm Drive Cycles

3

Trips to validate repair

Results

Optimal

Severity Score

5pts

Higher = worse

Confirm Drive Cycles

3

Trips to validate repair

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

Visualizations

Results overview

5Severity Score
3Confirm Drive Cycles

Severity Score vs. Stored Codes

Severity Score

5pts
020

Formula

Weights code types by diagnostic significance: permanent codes prove chronic emissions faults.

Severity = stored x 2 + pending x 1 + permanent x 3

Alternative Form

Confirm cycles = severity/2 (1-8): the road test needed after repair

Variables

SymbolNameUnitDescription
SSeverityptsTriage score

Explanation

Stored codes failed twice (matured); pending failed once; permanent codes are the ECU long memory that blocks inspection passing.

Step-by-Step Calculation

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

Step 1

Record the inputs.

Stored Codes:2
Pending Codes:1
Permanent Codes:0
Result: 3 inputs ready
Step 2

Apply the formula.

Severity = stored x 2 + pending x 1 + permanent x 3
Result: Primary result = 5 pts
Step 3

Read every derived output.

Severity Score:5 pts
Confirm Drive Cycles:3
Result: Severity Score = 5 pts
Final Answer
5

Worked Examples

See how the calculator works with different scenarios.

Scenario

Post-repair verification plan.

Inputs

Stored:2
Pending:1
Permanent:0

Calculation

Severity = 5; cycles = 3

Result

Severity 5, ~3 drive cycles to confirm.

Interpretation

Fix stored first, then run 3 full warm-up cycles and re-scan.

Understanding Your Result

Your Result: Severity Score: 5 pts

What This Means

How sick the ECU thinks the car is, and how much driving proves the fix.

Why It Matters

Prioritizes the repair order and prevents premature inspection attempts.

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

Code counts triage the diagnostic session.

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)

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

Practical Tips

  • •Never clear codes before inspection
  • •Fix stored before pending
  • •Permanent codes need clean miles, not scanners

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.

Stored vs pending codes?
Pending = failed once; stored/confirmed = failed twice across cycles. Permanent = emissions history that only self-heals with clean driving.
What is a drive cycle?
A prescribed warm-up, cruise, decel and idle sequence that runs all monitors — typically 15-30 min of mixed driving.

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Related Guides

Deepen your understanding with these educational resources.

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.

Authoritative References

Trusted sources and standards referenced by this calculator.

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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