Why does my car say "not ready" for an emissions test?

What does "not ready" mean?

Every gasoline car and light truck sold in the US since the 1996 model year has an OBD-II system that keeps testing its own emission controls (CARB). Each of those self-tests is a readiness monitor. California's Bureau of Automotive Repair describes them as "self-tests performed by the vehicle's OBD system to verify emission control functionality" (BAR).

A monitor reads ready (or "complete") once it has run and finished. It reads not ready (or "incomplete") when it hasn't run yet since its results were last wiped. "Not ready" does not mean a part has failed. It means the car can't yet vouch for that system.

The monitors named in BAR's reference include:

• catalyst
• evaporative system (EVAP)
• oxygen sensor and oxygen sensor heater
• EGR (exhaust gas recirculation)
• secondary air system
• misfire
• fuel system
• comprehensive components

Diesels add others, such as the particulate filter and NMHC catalyst. Your car only runs the monitors that apply to its hardware.

An OBD emissions test reads these flags along with the check-engine light and stored codes. If too many monitors are not ready, the station can't judge the car, and it is turned away or failed until they run.

Why are my monitors not ready?

The New York DMV lists four common causes (NY DMV):

1. Codes were cleared with a scan tool after a recent repair.
2. The battery was disconnected or replaced recently.
3. The car's computer needs a software update.
4. A problem is developing that hasn't turned on the check-engine light yet.

BAR adds that monitors must rerun after "disconnecting a vehicle's battery or replacing a defective emissions component" (BAR).

The fourth cause is the one to take seriously. A monitor that won't finish can be the first sign of a fault, and a developing fault may already have logged a pending code before the light comes on. Our guide to pending, stored and permanent codes explains the difference.

Clearing codes to hide a fault before a test doesn't work. It resets the monitors, so the car arrives "not ready". EPA told states that a rising number of not-ready cars "may suggest attempts to clear OBD problem codes by disconnecting and reconnecting the battery without completing appropriate repairs" (EPA, 2001).

How many monitors can be not ready?

This is where the rules differ by place. What we checked:

The EPA numbers come from its April 5, 2001 rule, which allows states to test cars with that many unset monitors rather than reject them. It is a ceiling states may use, not a requirement. California diesels have their own limits: none for 1998–2006, and only the particulate filter and NMHC monitors for 2007 and newer. For gasoline cars, California's test ignores the misfire, fuel system and comprehensive component monitors, which run continuously (BAR).

Readiness allowances don't excuse anything else. Under the same EPA rule, the car still fails if the check-engine light is commanded on. California also fails 2010-and-newer cars that have a permanent trouble code stored. The exception is a car that has done at least 15 warm-up cycles and 322 km (200 miles) since its codes were last cleared (BAR).

In Canada: we checked Ontario and British Columbia, and neither tests passenger cars any more.

• Ontario removed the Drive Clean test requirement for light passenger vehicles on April 1, 2019 (Ontario). The rules on emission control systems still apply.
• BC's AirCare program, which tested light-duty passenger cars and trucks in Metro Vancouver, ended on December 31, 2014 (BC).

Rules change, and many US states don't test at all. Check the current rules where your car is registered.

How do I get my monitors ready?

Start with normal driving. The California Air Resources Board says cars that fail for incomplete readiness should be driven as they normally are "for about a week or so".

If that doesn't do it, follow your car's drive cycle. A drive cycle is the mix of driving a car needs to run all its monitors: usually a cold start, some idling, city driving and steady highway driving, spread over normal days. The exact steps vary by manufacturer and model, so use the one in your owner's manual or service information, or ask a dealer or repair shop to set the monitors for you.

If you fail only for readiness, ask about extra time. New York gives a 10-day extension to drive the car and come back (NY DMV).

What if a monitor never sets?

• Look for a pending or stored code. A fault can stop a monitor from finishing. Fix the fault, and the monitor usually follows. If the code is P0420 or P0171, our guides cover the usual causes.
• Ask a dealer about software updates. The New York DMV lists an out-of-date computer as a cause.
• Some models are known to be hard to set. In 2008 EPA published an I/M OBD Vehicles Readiness Exception List for inspection programs (EPA), and California's BAR keeps its own current list of models with known readiness issues (BAR). If your car won't set a monitor after a week of normal driving and no codes, ask your test program whether your model is a known case.

If the check-engine light is on or flashing, that matters more than readiness. See Check engine light on: can I keep driving?