What "in and out" emissions testing means

In and out emissions testing is a two-part inspection that checks your vehicle's emissions both when the engine is cold (the "in" test) and after it has warmed up (the "out" test). Most states that require emissions testing use this method because it catches problems that only show up at different engine temperatures.

The test happens in a single visit to an inspection station. You drive in, the technician connects a probe to your vehicle's tailpipe, and the machine measures what comes out. Then your car idles or runs at a set speed while warmed up, and the machine measures again. Both readings go into a report that either passes or fails your vehicle.

The reason for two measurements is practical: a cold engine burns fuel differently than a warm one. A vehicle that pollutes heavily when first started but runs clean after warming up would pass a single test but still contribute to air quality problems during rush hour. The two-part test catches that.

Key Takeaways

  • In and out testing measures emissions twice — once when your engine is cold and once when it is warm — to catch pollution problems that only happen at certain temperatures.
  • The entire test takes place during one visit to an inspection station and usually takes 10 to 15 minutes.
  • Your vehicle must pass both the cold and warm readings to pass the overall test; failing either one means the vehicle fails.
  • States that use in and out testing typically require it every one or two years, depending on your vehicle's age and your state's rules.

How the cold engine test works

When you arrive at the inspection station, the technician will ask you not to warm up your engine before the test. The cold test measures what your vehicle emits within the first few minutes of starting, when the engine is still below normal operating temperature.

The technician inserts a probe into your tailpipe and records the levels of hydrocarbons, carbon monoxide, and carbon dioxide. These numbers are compared against your state's limits for your vehicle's model year and engine type. A vehicle with a fouled spark plug, a clogged air filter, or a failing oxygen sensor often fails the cold test because the engine cannot burn fuel efficiently when it is not yet warmed up.

If your vehicle fails the cold test, you will not proceed to the warm test. You will be told the results and given information about repair shops or resources in your area.

How the warm engine test works

If your vehicle passes the cold test, the technician will ask you to idle or drive the vehicle at a steady speed (usually around 2,500 RPM) for a set time while the engine reaches normal operating temperature. This is the "out" test, because it measures emissions from a fully warmed engine running under load.

The probe remains in the tailpipe, and the machine records the same pollutants: hydrocarbons, carbon monoxide, and carbon dioxide. A warm engine should burn fuel more completely, so these numbers are typically lower than the cold test. However, if your vehicle has a failing catalytic converter, a vacuum leak, or a malfunctioning fuel injector, it may produce high emissions even when warm.

Your vehicle must pass both the cold and warm measurements. Passing one and failing the other means the overall result is a failure.

What happens if you fail

If your vehicle fails either the cold or warm test, the inspection station will provide a report showing which pollutants exceeded the limit and by how much. This report is useful to a mechanic because it narrows down what might be wrong — for example, high hydrocarbons often point to ignition problems, while high carbon monoxide often points to fuel mixture problems.

You will have a set amount of time (usually 30 to 60 days, depending on your state) to have the vehicle repaired and then return for a retest. Some states allow one free retest; others charge a fee for each retest. Keep your repair receipts, because some states require proof that you attempted repairs before allowing a retest.

If you repair the vehicle and it still fails the retest, you may be able to request a waiver in some states, which allows you to register the vehicle anyway if you have spent a certain amount on repairs. The threshold and process vary by state.

Common reasons vehicles fail in and out testing

Vehicles most often fail because of problems that affect how fuel burns. A faulty oxygen sensor, which tells the engine how much fuel to inject, is one of the most common causes. A clogged air filter restricts oxygen flow and causes the engine to run rich (too much fuel, not enough air). A failing catalytic converter cannot clean up emissions the way it should.

Spark plug problems — worn plugs, wrong heat range, or incorrect gap — prevent fuel from igniting completely, especially in a cold engine. A vacuum leak allows unmetered air into the engine, throwing off the fuel mixture. A malfunctioning fuel injector can spray fuel unevenly or at the wrong time.

Less commonly, a vehicle fails because the engine computer itself is not adjusting fuel mixture correctly, or because the vehicle is straightforward very old and its emission control systems have degraded. In those cases, repair costs can be higher.

Preparing your vehicle for in and out testing

Before you schedule an inspection, make sure your vehicle has been driven recently so the engine computer has had a chance to run its self-diagnostic cycles. Vehicles that sit unused for weeks sometimes fail emissions tests even though they are mechanically sound, because the computer has not completed its checks.

Check that your gas cap seals tightly — a loose or missing cap can trigger a check engine light and cause a test failure. Make sure your oil level is correct and your air filter is not visibly clogged. If your vehicle has a check engine light on, get it scanned before the test; the code will often point to the problem.

Do not warm up your engine before arriving at the inspection station. The technician needs to measure cold emissions, and a pre-warmed engine will not give an accurate reading.

How in and out testing differs from other emissions tests

Some states use a loaded mode test, where the vehicle is placed on a dynamometer (a treadmill for cars) and driven at various speeds while emissions are measured. This test is more thorough but also more expensive to administer, so fewer states use it.

Other states use a two-speed idle test, which measures emissions at two different idle speeds but does not include a cold start. This test is simpler and faster but may miss problems that only show up during acceleration or under load.

In and out testing sits in the middle: it is more thorough than a straightforward idle test because it captures both cold-start and warm-running behavior, but it is less expensive and faster than a full dynamometer test. This is why many states have adopted it as their standard.

Frequently Asked Questions

Can I fail the warm test if I passed the cold test?

Yes. Passing the cold test means your vehicle burns fuel efficiently when first started, but a warm engine under load is a different condition. A failing catalytic converter or a fuel system problem might not show up until the engine is fully warmed and running at higher RPM. You must pass both tests to pass overall.

How long does an in and out emissions test take?

The actual testing usually takes 10 to 15 minutes. Add time for check-in, paperwork, and waiting if the station is busy. Plan for 30 minutes total to be safe. If you fail, you will get your results when ready and can leave to arrange repairs.

What if my vehicle has a check engine light on?

Most inspection stations will fail your vehicle if the check engine light is illuminated, because the light indicates a problem the engine computer has detected. Have the code read at a repair shop or auto parts store before the test. The code will tell you what to fix.

Do I need to do anything special to prepare my vehicle for the test?

Drive your vehicle regularly in the days before the test so the engine computer completes its diagnostic cycles. Check that your gas cap is tight and your oil level is correct. Do not warm up the engine before arriving at the station — the technician needs a cold start for the first measurement.

What if I fail and cannot afford repairs?

Some states offer repair information programs or waivers if you have spent a certain amount on unsuccessful repairs. Contact your state's environmental agency or your local inspection station to ask about options. The specific rules vary by state.