In and out emissions testing is a two-part inspection that checks your vehicle's exhaust system when the engine is cold and again after it warms up
The "in and out" format — sometimes called a "cold and hot" test — measures how your vehicle's emissions control system performs under different operating conditions. The first measurement happens when you drive into the testing station with a cold engine. The second happens after the engine reaches normal operating temperature, usually after a short drive or a brief idle period at the station. This two-stage approach catches problems that only show up once the engine is warm, like a failing catalytic converter or a thermostat stuck in the open position.
Not every state uses this format. Some states run only a single hot test, while others use a dynamometer (a machine that simulates driving) instead of a tailpipe test. Your state's emissions program determines which method applies to your vehicle, based on your vehicle's model year, weight, and fuel type.
Key Takeaways
- In and out testing measures emissions twice — once with a cold engine and once after the engine warms up — to catch problems that only appear under different conditions.
- The cold test happens when ready when you arrive; the hot test typically follows after a few minutes of driving or idling at the station.
- Your state's emissions program decides whether your vehicle gets tested this way, and the rules vary significantly by state and vehicle type.
- A vehicle can pass the cold test but fail the hot test, or vice versa, because emissions control systems behave differently as the engine temperature changes.
- If your vehicle fails either stage, you will need repairs before you can retest, and some states allow you to retest at the same station on the same day.
How the cold test works
You drive into the testing bay with your engine at or near ambient temperature — ideally, the vehicle has not been driven for at least four hours. The technician connects a probe to your tailpipe and records baseline emissions readings for hydrocarbons, carbon monoxide, and nitrogen oxides. This cold-start measurement is important because a vehicle's emissions control system is least effective when the engine is cold; the catalytic converter has not yet reached its operating temperature, and the engine runs a richer fuel mixture to start smoothly.
The cold test typically takes two to five minutes. You do not need to rev the engine or drive anywhere during this phase — the technician straightforward records the readings while the engine idles. If your vehicle passes this stage, you move on to the hot test. If it fails, you will see the results when ready, though some states allow you to attempt repairs and retest the same day.
How the hot test works
After the cold test, you either drive the vehicle around the parking lot or on a short route specified by the testing station, or you idle at the station for several minutes until the engine reaches normal operating temperature (usually around 180 to 200 degrees Fahrenheit). The technician then runs a second emissions reading under these warmer conditions. A properly functioning emissions system should produce lower emissions once the engine is warm, because the catalytic converter is now at full efficiency and the engine's fuel mixture is leaner.
The hot test is where many vehicles that passed the cold test will fail, because it reveals problems with the catalytic converter, oxygen sensors, or the engine control computer that do not show up when the engine is cold. A vehicle that idles smoothly and passes the cold test but has a degraded catalytic converter will typically show high emissions during the hot test.
Why states use two tests instead of one
A single hot test alone would miss vehicles with cold-start problems — engines that run too rich or have faulty fuel injectors that only misbehave during startup. A single cold test would miss catalytic converter failures and other warm-engine issues. By testing both conditions, the state captures a more complete picture of whether the vehicle's emissions system is working across the full range of driving conditions a vehicle actually experiences.
This approach also reflects real-world driving: most vehicles spend time idling in traffic and warming up in the morning, and they also spend time at highway speeds with a fully warm engine. Testing both conditions means a vehicle that passes has demonstrated it can meet emissions standards in both scenarios.
What happens if you fail one test but pass the other
If your vehicle fails the cold test but passes the hot test, the problem is usually a cold-start issue — a faulty fuel injector, a stuck choke (on older vehicles), or a malfunctioning cold-start valve. These repairs are often less expensive than catalytic converter work. If your vehicle passes the cold test but fails the hot test, the problem typically involves the catalytic converter, oxygen sensors, or the engine control module. These repairs are usually more costly.
In either case, you will need to have the vehicle repaired before you can retest. Some states allow you to retest at the same station on the same day after repairs, while others require you to return on a different day or at a different station. Check your state's emissions program website or call the testing station to learn the retest rules in your area.
Preparing your vehicle for in and out testing
Before you arrive, make sure your engine oil is at the correct level and your tires are properly inflated — both affect emissions readings. Do not drive the vehicle for at least four hours before the cold test; if you must drive it, keep the distance short. Make sure your gas cap is tight and undamaged, because a loose or missing cap will trigger a check engine light and cause an automatic failure in many states.
If your vehicle has a check engine light on, it will fail the emissions test in most states, regardless of the actual tailpipe readings. Have the vehicle scanned for diagnostic trouble codes before you go to the testing station. Many auto parts stores will scan the codes for free, and you can then decide whether to repair the issue before testing or after.
Retesting after repairs
After you have had the vehicle repaired, you will need to clear the diagnostic trouble codes — either by disconnecting the battery for several minutes, by using an OBD-II scanner to clear the codes manually, or by having a technician do it. Some states require the codes to be cleared before you retest; others do not. Ask your repair shop or the testing station what the requirement is in your state.
When you return for the retest, the same two-stage process applies: cold test first, then hot test. The vehicle must pass both stages. If it fails again, you will need additional repairs. Keep records of all repairs and receipts, because some states offer emissions repair information programs or tax credits for vehicles that fail and require expensive repairs.
Frequently Asked Questions
Can I retest the same day if I fail?
Some states allow same-day retesting at the same station after repairs, while others require you to wait a certain number of days or return to a different station. Contact your state's emissions program or the testing station directly to learn the retest rules where you live.
What if my vehicle passes the cold test but fails the hot test?
This usually points to a catalytic converter, oxygen sensor, or engine control module problem. These repairs tend to be more expensive than cold-start fixes. Have the vehicle scanned for diagnostic codes to narrow down the cause before you pay for repairs.
Do I need to drive the vehicle to warm it up between the cold and hot tests?
That depends on the testing station. Some stations have you drive around the parking lot or a short route; others let you idle at the station. The technician will tell you what to do when you arrive. Either way, the goal is to bring the engine to normal operating temperature.
Will a loose gas cap cause me to fail?
A loose or missing gas cap will trigger a check engine light, which causes an automatic failure in most states. Make sure your cap is tight and undamaged before you test. If the light is already on, have the vehicle scanned and the code cleared before you go to the station.
What should I do if my vehicle fails both the cold and hot tests?
Have the vehicle scanned for diagnostic trouble codes to identify the problem. Common causes include a faulty oxygen sensor, a failing catalytic converter, or an engine control module issue. Get a repair estimate and decide whether to fix it yourself or use a shop. Some states offer repair information for low-income vehicle owners.