Readiness monitors, in plain English

Few results confuse drivers more than a rejected test on a car that seems fine. The word sounds like a fail, and it is not one. This guide explains what the computer is doing, why a recent repair or battery change sets it back, and how the Nevada DMV tells you to get through it.

At a glance

• Rejected (not ready) is not a failure, according to the Nevada DMV.
• The usual causes are a recently disconnected battery or recently cleared codes.
• The DMV's fix is to drive up to one week under varying conditions, city and highway, then retest.
• The Reno emission lab can check readiness at no cost, no appointment, if you arrive before 4:30 p.m.

What the computer is doing

Every light-duty vehicle built in 1996 or later carries the on-board diagnostics system known as OBD II. The DMV describes it as a standardized computer system that continuously monitors the engine's performance and the emissions control systems. When a sensor detects a malfunction, the computer stores a diagnostic trouble code and turns on the check engine light.

Monitoring is not instant, though. The computer has to see the engine run through real driving before it can finish its checks, and the DMV's wording is that the system must complete all its checks under varying conditions. Those checks are what people mean by readiness monitors.

Why a reset sets the clock back

The result of the checks lives in the computer's memory. Disconnect the battery, replace it, or erase codes with a scan tool and that memory is cleared. The computer starts over, and until the checks are done again the car reports that it is not ready to be tested.

The DMV lists the same two triggers every time it explains this: the battery was recently disconnected, or codes were cleared. It adds a related warning to not test immediately after a battery change, because a dead, disconnected or newly replaced battery clears the computer's memory.

A rejection is not a fail

On the DMV's pages the two words are kept apart on purpose. A failed test means the car is emitting too much, or the check engine light is lit. A rejected test only means the vehicle is not ready to be tested. The rejection costs you a return trip, not a repair bill, and it does not count as the failed test that starts the waiver process.

If you were rejected right after a repair, the repair may be fine. The car simply has not been driven enough since it was done.

How to get a car ready again

The DMV's instruction has two parts. First, drive the vehicle for up to one week under varying conditions, both city and highway. Second, return for a retest. Think of it as a week of ordinary errands rather than one long trip: cold starts, stop and go traffic, steady speeds on the freeway.

The DMV also offers a shortcut. After a rejected result, your local emission lab can check whether your vehicle is ready to be tested at no cost. No appointment is needed, but arrive before 4:30 p.m. The Reno lab is at 9155 Double Diamond Pkwy and answers at (775) 684-3580, and our emissions lab guide covers when a trip there makes sense.

What not to do

Do not clear the codes to make a light disappear before the test. The DMV is blunt that using a scanner to erase trouble codes right before a test produces an automatic rejection, since clearing does not fix the problem and only erases the memory the inspector needs to see.

Also avoid scheduling the test for the day after a repair or battery replacement. If the registration deadline is close, ask about the timing, and read about the movement permit as a backstop.

Where it fits in a normal year

For most cars nothing here ever comes up. The computer finishes its checks during normal driving and the test runs in the five minutes or less the DMV quotes for OBD tests. Readiness only becomes a topic after something interrupts the computer, which is why recent work on the car belongs in the first thing you say at the counter. See the not ready page for the short version.