Drive cycles, readiness monitors and emissions tests

A drive cycle is the driving a car needs so its OBD-II monitors can run their self-tests. After trouble codes are cleared, or the battery is disconnected or replaced, the monitors reset to "not complete". An emissions test that reads the OBD-II port checks this readiness, and too many incomplete monitors means the car cannot be tested yet. Normal driving usually completes them within about a week.

What readiness monitors are

The engine computer tests each emission-related system. Some tests run all the time. Others run only when the conditions set by the vehicle maker are met, such as a start after the car has sat for hours, a steady cruise or an idle, so they only run on some trips.

For each monitor, the computer reports one of two states to a scan tool:

• Complete (ready): the test has run since codes were last cleared. Complete does not mean passed. A monitor that found a fault also shows complete, and the fault shows up as a code.
• Not complete (not ready): the test has not finished since codes were last cleared.

Continuous monitors always show complete.

Which monitors a car reports

A car only reports the monitors for systems it has. A car with no secondary air pump shows that monitor as "not supported", which is not a fault.

What resets the monitors

Every monitor goes back to "not complete" whenever fault memory is cleared:

• Clearing codes with a scan tool.
• Disconnecting the battery, or a battery that went flat or was replaced.
• Some repairs, where the shop clears codes afterwards.

Turning the car off normally does not reset them. Permanent codes are the exception to clearing: a scan tool cannot erase them, and they clear themselves only after the computer has re-tested the system and found it fixed.

How many "not ready" monitors an emissions test allows

Under the US federal rule for OBD inspections (40 CFR 85.2222):

• 1996 to 2000 model years: up to two monitors may be not ready.
• 2001 and newer: up to one monitor may be not ready.
• More than that, and the driver is told to come back after driving the car. If monitors are still not ready at the retest, the car fails.
• The car fails if the check engine light is commanded on with a stored code, even with every monitor ready.
• The car fails if the check engine light does not come on at all with the key on and the engine off (a burned-out bulb).

States run their own inspection programs within this rule, so check your state's program for its exact process. California's Smog Check, for example, uses the OBD inspection as the main test for 2000 and newer vehicles.

How to get monitors to complete

1. Fix the fault first. If the fault is still there, its code comes back and fails the test anyway.
2. Do not clear codes just before a test. That resets every monitor.
3. Drive the car normally for several days, with a mix of city and highway driving. CARB says about a week of normal driving usually completes them.
4. Check readiness with a scan tool before you go to the test station.

Some monitors are allowed to stay off in extreme conditions. CARB's rule lets a maker disable a monitor below 20°F (−7°C) or above 8,000 feet (2,438 m), and lets it disable tests such as misfire detection when the fuel tank is 15% full or less. So in very cold weather, at high altitude or with a nearly empty tank, some monitors may not run.

Checking readiness without a scan tool

On some vehicles the check engine light shows readiness. With the key on and the engine off, the light stays on steadily for 15 to 20 seconds if every monitor is complete. If one or more is not complete, it then blinks once per second for 5 to 10 seconds. This is optional, so not every car does it. A scan tool reads the same status directly.

Ford's published drive cycle

Ford publishes a short readiness drive cycle in the "Readiness for Inspection/Maintenance (I/M) testing" section of its owner guides. Two versions appear in Ford Motor Company owner guides.

2006 Mercury Mountaineer owner guide. After the powertrain or battery has been serviced, at least 30 minutes of city and highway driving is needed:

1. Let the vehicle sit for at least eight hours without starting the engine.
2. Start the engine and let it warm up to normal operating temperature. Do not turn it off until the cycle is complete.
3. Drive for at least 10 minutes on an expressway or highway.
4. Then drive for at least 20 minutes in stop-and-go, city-type traffic, with at least four idle periods.

2010 Ford Fusion owner guide. Ford notes that a complete check may take several days of normal driving. If the vehicle is still not ready, this cycle may be performed:

1. Let the vehicle sit for at least eight hours without starting the engine.
2. Start the engine and let it warm up to normal operating temperature. Do not turn it off until the cycle is complete.
3. Drive for 15 minutes steadily on an expressway or highway.
4. Then drive for 20 minutes in stop-and-go traffic, with at least four idle periods of 30 seconds each.
5. If the vehicle is still not ready, repeat the cycle.

The 2010 guide also gives a readiness check with no tool: turn the key on for 15 seconds without cranking. If the "Service engine soon" light blinks eight times, the vehicle is not ready for I/M testing. If it stays on solid, it is ready.

Use the owner guide for your own model year, because the wording changed over time. Ford's procedures for completing a single monitor after a repair are in Ford's workshop service information, not in the owner guide. Ford vehicles can also show code P1000 while monitors are not complete; see what Ford's P1000 means.

Other makers publish their own drive cycles in owner manuals or service information. There is no single cycle that completes every monitor on every car. Always drive within traffic laws; no drive cycle requires unsafe driving.

Codes tied to the monitors

• The evaporative monitor shows complete only when both its purge valve check and its leak check have run. Codes such as P0442, P0455 and P0456 are evaporative system leaks.
• The catalyst monitor reports P0420 when the converter's efficiency is below threshold.
• Misfire (P0300) and fuel system (P0171) monitors are continuous. They always show complete, so a fault there shows up as a code and a lit check engine light rather than a "not ready" monitor.

For how codes are stored and cleared, see how to read an OBD-II code. For the terms used here, see the OBD-II glossary.

OBD DashX reads and clears OBD-II trouble codes on Android with a Bluetooth ELM327 adapter.