Maintenance procedure · Written to ASD-STE100 Issue 9 · Formatted to ANSI Z535

Read and Record a Diagnostic Trouble Code with an OBD-II Scanner

Model year 1996 and later — retrieve the stored codes, the freeze frame and the readiness monitors, write them down, and hand them over without clearing anything
ApplicabilityLight-duty cars and trucks of model year 1996 and later with a standard onboard diagnostic connector
Task time18 min to 44 min editorial estimate, not a measurement
PersonnelOne
Revisionv1 · 2026-07-29
BEFORE YOU BEGIN

Before you begin. This procedure is general reference information, not professional advice. Equipment varies by manufacturer, model, and year. Always follow the documentation supplied with your equipment — where it differs from this page, follow the manufacturer. Verify that you have the tools, the working conditions, and the ability to do this task safely. If anything is unclear, if the equipment is damaged, or if the task exceeds your experience, stop and use a qualified professional. You are responsible for your own safety and for the outcome. This site accepts no liability for any loss, damage, or injury arising from the use of this information.

Specifications marked unverified could not be traced to a manufacturer source. Do not rely on them — obtain the value from your equipment documentation.

Summary

Every car sold in the United States since model year 1996 carries a self-diagnostic system that watches the emission control equipment, stores a trouble code when something fails, and turns on the check engine light to tell you it did. A twenty-dollar reader will pull those codes out through a standard connector, and this page is about doing that properly. It is deliberately not about deciding what the code means — EPA's own inspection rules say that proper diagnosis by a qualified technician is required to positively identify the source of an emission-related problem, and that is the honest boundary for a page like this one.

So the aim here is to end up with a written record: the codes, the freeze frame data captured when the fault was detected, and the status of the readiness monitors. Two things matter more than everything else on this page. A check engine light that flashes is not the same as one that stays on, and a flashing one means stop rather than carry on. And clearing a code does not fix anything — it also wipes the readiness monitors, and a car with unset monitors is failed at an emissions inspection under 40 CFR 85.2222. Read, record, hand over. Do not clear.

Necessary equipment

ItemNote
OBD-II scannerA basic code reader is satisfactory for this procedure.
Flashlight optionalThe light below the instrument panel is low.
Pen and paperWrite each code on paper. Do not keep the code only on the display.
Vehicle documentationThe documentation gives the position of the connector for this vehicle.

Look at the lamp first

Estimated 2 min

NOTEA lamp that flashes and a lamp that stays on are not the same condition.
  1. Look at the instrument panel.
    <1m
  2. Find the check engine light.
    • The regulations call this lamp the malfunction indicator lamp.
    • The symbol on many vehicles is the shape of an engine.
    <1m
WARNING

IF THE CHECK ENGINE LIGHT FLASHES, DO NOT DRIVE AT HIGH SPEED.
Stop the vehicle in a safe place away from traffic. Mazda gives this instruction in its owner documentation. Traffic can kill you.

  1. If the light flashes, stop the vehicle in a safe place.
    • Mazda tells owners not to drive at high speeds in this condition.
    • Speak to a vehicle repair shop before you drive again.
    <1m
  2. If the light is on and steady, continue with this procedure.
    • Make a note of the conditions at the time the light came on.
    • Mazda tells owners that these conditions are important for the repair shop.
    <1m

Attach the scanner

Estimated 6 min to 22 min

Position of the diagnostic connector
Unverified — do not rely on this value
CONSULT THE VEHICLE DOCUMENTATION

No tier-1 source was found for this value. Take it from the documentation supplied with your equipment.

First model year with a standard diagnostic connector
Manufacturer or standard verified
1996 model year

Source: 40 CFR 85.2207 — Onboard diagnostic test standards · tier 1 · accessed 2026-07-29

  1. Get an OBD-II scanner.
    • 40 CFR 85.2231 asks a test system to communicate with the standard connector.
    1m
WARNING

DO NOT USE A SCANNER WHEN YOU DRIVE.
Stop the vehicle before you attach the scanner. Traffic can kill you.

  1. Stop the vehicle in a safe place away from traffic.
    <1m
  2. Set the parking brake.
    <1m
  3. Set the ignition to off.
    <1m
  4. Read the vehicle documentation for the position of the diagnostic connector.
    • The connector is below the instrument panel on the driver side of many vehicles.
    • A panel or a cover hides the connector on some vehicles.
    2m
WARNING

DO NOT DISCONNECT THE CABLES OF THE BATTERY WHEN THE CHECK ENGINE LIGHT IS ON.
Mazda states that the engine can become damaged and can catch fire.

  1. Find the diagnostic connector.
    • A vehicle with no connector fails an emissions inspection under 40 CFR 85.2207.
    1m
  2. Attach the scanner to the diagnostic connector.
    • The connector engages in one direction only. Do not use force.
    <1m
  3. Set the ignition to the on position.
    • Do not start the engine.
    <1m
  4. Wait for the scanner to make a connection to the vehicle.
    • If the scanner makes no connection, set the ignition to off.
    • Then attach the scanner again and set the ignition to the on position.
    <1m

Read the codes

Estimated 5 min to 11 min

Meaning of a diagnostic trouble code
Unverified — do not rely on this value
CONSULT A QUALIFIED TECHNICIAN

No tier-1 source was found for this value. Take it from the documentation supplied with your equipment.

  1. Select the function that reads the stored codes.
    • The scanner can call this function Read Codes or Trouble Codes.
    <1m
  2. Write each code on paper exactly as the display shows it.
    • 40 CFR 86.1806 gives P0456 as an example of such a code.
    • Do not change a character. Do not write down only part of a code.
    2m
  3. Write down the number of codes that the scanner shows.
    <1m
  4. Select the function that shows the freeze frame data.
    • The vehicle keeps the conditions at the time it found the fault.
    • Not all scanners show this data.
    <1m
  5. Write down the freeze frame data.
    2m

Read the readiness monitors

Estimated 3 min to 6 min

Monitors that are not complete allowed at an inspection for a 2001 or later vehicle
Manufacturer or standard verified
1 monitor

Source: 40 CFR 85.2222 — Onboard diagnostic test procedures · tier 1 · accessed 2026-07-29

Monitors that are not complete allowed at an inspection for a 1996 to 2000 vehicle
Manufacturer or standard verified
2 monitor

Source: 40 CFR 85.2222 — Onboard diagnostic test procedures · tier 1 · accessed 2026-07-29

  1. Select the function that shows the readiness monitors.
    • The scanner can call this function Readiness or Monitors.
    <1m
  2. Write down which monitors are complete.
    • 40 CFR 85.2222 calls this step the readiness evaluation.
    1m
  3. Write down which monitors are not complete.
    • An inspection can fail a vehicle of model year 2001 or later with more than one such monitor.
    1m

Disconnect and hand over the record

Estimated 3 min

NOTEDo not use the clear function of the scanner.
NOTICE

DO NOT USE THE CLEAR FUNCTION OF THE SCANNER.
A clear operation removes the codes and the status of the readiness monitors. A vehicle with monitors that are not complete can fail an emissions inspection.

  1. Do not select the function that clears the codes.
    • A clear operation also removes the status of the readiness monitors.
    • The fault stays in the vehicle after a clear operation.
    <1m
  2. Set the ignition to off.
    <1m
  3. Remove the scanner from the diagnostic connector.
    <1m
  4. Install the cover on the connector if the vehicle has one.
    <1m
  5. Record the date and the distance on the odometer with the codes.
    <1m
  6. Give the record to a qualified technician.
    • A code shows the test that failed. A code does not show the part that failed.
    1m

Terminology

This page uses one term for each item and holds it throughout, as ASD-STE100 requires. Equipment sellers and other guides use the names below for the same parts.

diagnostic trouble code
Also called: DTC, fault code, check engine code, engine code, trouble code, P0420, P0171
malfunction indicator lamp
Also called: check engine light, MIL, engine light, service engine soon light, the orange engine symbol
diagnostic connector
Also called: OBD2 port, OBD-II port, data link connector, DLC, the port under the dash, where the code reader plugs in
OBD-II scanner
Also called: code reader, OBD2 reader, scan tool, diagnostic tool, car code scanner
readiness monitor
Also called: I/M readiness, readiness flag, monitors not ready, drive cycle status
freeze frame
Also called: freeze frame data, snapshot data, conditions when the code set

When to use a professional

A code shows the test that the vehicle failed. A code does not show the part that failed. Only a qualified technician can find the cause. EPA gives this same instruction in the report that an inspection station must give a motorist.

Stop if the check engine light flashes. Do not drive at high speed. Speak to a vehicle repair shop immediately.

Do not replace a part because of a code alone. Give the codes, the freeze frame data and the status of the monitors to the technician. Ask the technician to do a test on the part before the replacement.

Do not clear a code before an emissions inspection. A vehicle with monitors that are not complete can fail the inspection.

Common questions

What does the check engine light actually mean?

It means the car's onboard diagnostic system detected a malfunction in something it monitors and stored a code about it. EPA's rule for these systems puts it plainly: OBD systems must detect malfunctions in the emission control system, store trouble codes corresponding to detected malfunctions, and alert operators. It does not tell you how serious the problem is, and it does not tell you what part is at fault. That is what the code and a technician are for.

Where is the OBD2 port in my car?

On the great majority of cars it is under the dash on the driver's side, within reach of the driver's seat, sometimes behind a small hinged cover. It is a trapezoidal socket with 16 pins. This page will not promise you an exact location because manufacturers place it differently and some hide it behind trim, and your owner's manual is the source that knows. What is guaranteed is that a 1996-or-later car has one and that it is not allowed to be missing — 40 CFR 85.2207 fails a vehicle at an emissions inspection if the connector is missing, tampered with or otherwise inoperable.

My light is flashing rather than staying on. Is that different?

Yes, and it is the one distinction on this page that should change what you do in the next minute rather than the next week. Mazda's owner's manual tells owners that if the check engine light remains on or flashes continuously, they should not drive at high speeds and should consult a dealer as soon as possible. Treat a flashing light as an instruction to get off the road safely and stop, not as a slightly more urgent version of a steady one. Do not run a scan while you are moving.

What does P0420 mean?

This page will not tell you, and that is deliberate rather than lazy. A generic code identifies which self-test the vehicle failed, not which component caused it, and the same code is set by several genuinely different faults. EPA requires inspection stations to hand motorists a statement saying exactly this: that faults may be an indication of a malfunction, that multiple or seemingly unrelated faults can point at something that has already passed, and that proper diagnosis by a qualified technician is required to positively identify the source. Write the code down, hand it over, and let someone test before anyone buys a part.

Can I just clear the code and carry on?

You can, and it is usually a bad idea. Clearing does not repair anything — if the fault is still there the code comes back, and in the meantime you have deleted the freeze frame data that would have helped diagnose it. Worse, clearing also resets the readiness monitors, and 40 CFR 85.2222 fails a vehicle at an emissions inspection when those monitors are not complete: a 2001-or-later car is allowed no more than one unset monitor, and a 1996 to 2000 car no more than two. Clearing codes the day before a test is the classic way to fail it.

The light went off by itself. Is it fixed?

Maybe, and maybe the condition simply stopped occurring. The system turns the lamp off again after the fault has not recurred over a number of drives, but the code often stays in memory as a stored or pending record for a while afterwards. That is exactly why it is worth scanning even after the light goes out. A code with no lamp is also not a fail at an emissions inspection: 40 CFR 85.2222 says a vehicle passes if the lamp is not commanded on, even when codes are present.

Do I need an expensive scanner?

No. For this job — read the stored codes, read the freeze frame, read the readiness monitors — a basic reader does everything the page asks for. The functions themselves are standardised, which is why a cheap tool works on any 1996-or-later car: EPA's inspection rules describe requesting the diagnostic status and the stored codes through defined standard services, and every compliant vehicle answers the same way. Spend the money on the diagnosis instead.

Sources

Every specification on this page is traced to one of these sources. Tier 1 is a manufacturer document, a published standard, or a government safety authority. See Standards for the full tiering rule.

  1. Tier 140 CFR 85.2207 — Onboard diagnostic test standardsOffice of the Federal Register (eCFR) · accessed 2026-07-29 · re-verification due 2027-01-25
  2. Tier 140 CFR 85.2222 — Onboard diagnostic test proceduresOffice of the Federal Register (eCFR) · accessed 2026-07-29 · re-verification due 2027-01-25
  3. Tier 140 CFR 85.2223 — Onboard diagnostic test reportOffice of the Federal Register (eCFR) · accessed 2026-07-29 · re-verification due 2027-01-25
  4. Tier 140 CFR 85.2231 — Onboard diagnostic test equipment requirementsOffice of the Federal Register (eCFR) · accessed 2026-07-29 · re-verification due 2027-01-25
  5. Tier 140 CFR 86.1806-17 — Onboard diagnosticsOffice of the Federal Register (eCFR) · accessed 2026-07-29 · re-verification due 2027-01-25
  6. Tier 1Check Engine Indication/Light — 2022 Mazda CX-30 Owner's ManualMazda North American Operations · accessed 2026-07-29 · re-verification due 2027-01-25

Revision history

Current version 1 · updated 2026-07-29 · next review due 2027-07-29

  1. v1 · 2026-07-29First draft. Built from 40 CFR 85.2207, 40 CFR 85.2222, 40 CFR 85.2223 and 40 CFR 86.1806-17, with Mazda's owner's manual for the behaviour of the check engine light. The page retrieves and records and never interprets: the professional boundary rests on the customer alert statement EPA requires an inspection station to give a motorist. The readiness-monitor allowances are quoted per model-year band from 85.2222(c)(2) rather than reconciled into one number, and no code is decoded anywhere on the page.

Corrections to this page are listed in the public corrections log. Reports that are credible and not yet verified are listed in the public errata register, and an open one renders above the step it concerns.

Comments and corrections

Found an error? A correction with a manufacturer source is applied and logged publicly. You can also write to corrections@fixchecklist.com.

Every step above carries its own report control, which tags the report with that step so a correction reaches the right place. For anything about the procedure as a whole — the order of the sections, a step that is missing, something that did not match your equipment — use this: report something about this whole procedure.

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