How the OBD smog test works

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1. VIN scan

   The emissions technician scans the vehicle identification number with a bar code reader, for accurate records and identification.

2. OBD connection

   A cable from the analyzer plugs into the vehicle's OBD port, usually under the dashboard. No other connection or probe is needed.

3. Quick analysis

   The analyzer talks to the car's computer and reads its emission data and diagnostic information.

4. Result

   The system shows pass or fail and helps the technician pinpoint a malfunction. The result goes to the DMV in real time.

OBD II, short for On-Board Diagnostics, second generation, is a standardized computer system built into every 1996 and newer light-duty vehicle sold in the U.S. It constantly watches the engine and the emission controls. Nevada's smog test simply asks it what it has seen. Here is the test step by step, as the Nevada DMV describes it.

At a glance

• For 1996 and newer light-duty cars and trucks (Nevada DMV).
• No probe in the tailpipe: one cable to the diagnostic port.
• Most tests take five minutes or less, according to the DMV.
• Results go to the DMV in real time, so you can usually renew right after.

What the analyzer reads

The car's computer stores two things the test cares about. First, whether any emission-related fault is active, the kind that turns on the check engine light. Second, whether its readiness monitors have finished, the self-checks it runs on systems like the catalytic converter and oxygen sensors. A lit check engine light means a fail. Unfinished monitors mean the car is not ready and the test is rejected, which is not a fail.

Why Nevada uses it

The DMV lists the advantages: speed, with most tests done in five minutes or less; accuracy, a precise digital report of the engine's health; cost, because it is less expensive than the dynamometer tests used in some other states; and real-time data, with results sent instantly to the DMV.

It also catches problems early. The DMV points out that OBD detects failing emission components before you notice a change in how the car drives.

Before you pull in

Unplug anything in the diagnostic port. Insurance trackers and other driving monitors live there, and the DMV says technicians are required to fail any vehicle that has a device plugged into the OBD port during the inspection.

If the battery was recently replaced or disconnected, give the car up to a week of normal driving first, so the computer can finish its self-checks.

Which vehicles get it

1996 and newer light-duty vehicles: gasoline cars, SUVs, vans and pickups up to 8,500 pounds. Vehicles from 1995 and earlier and all heavy-duty vehicles keep the traditional two-speed tailpipe test, and light-duty diesels have their own test with a dynamometer and an opacity reading.

After the test

Pass: renew right away, at a station that offers renewal, online through MyDMV, or at a DMV Now kiosk minutes later. The pass is valid 90 days. Fail: you get a report detailing why, and the vehicle needs repairs and a retest. Rejected: drive a few days and come back.

What it means for your visit

Because the test is a computer reading, the car does not need to be warmed up on a machine or run at speed in the bay. Pull in, hand over the keys, and the technician handles the VIN scan and the connection. If you want to renew in the same visit, bring your renewal notice. If you also want an oil change, ask; both locations are inside an Xpress Oil & Lube Center.