Will Your Car Pass Emissions? How to Check Before You Go

Your registration renewal is coming up, the notice says you need an emissions test, and you are trying to figure out whether this is a ten minute errand or the start of a bad week. 

Maybe the check engine light is on. Maybe it was on, a shop fixed the problem, and the light is off now. Either way you would rather not pay a test fee, take a morning off, and drive home with a fail slip. 

The good news is that most of what the test looks at, you can check yourself first. 

What the test actually checks 

These days an emissions test is mostly a conversation with your car's computer rather than a probe up the tailpipe. The technician plugs into the OBD2 port and asks three questions. 

Is the check engine light commanded on right now? Are there stored emissions-related trouble codes? And have the vehicle's own emissions self-tests finished running? 

Why a lit check engine light is usually an automatic fail 

If the light is on for an emissions-related fault, the car is telling the tester its emissions controls are not working correctly. There is no judgment call, and in most OBD-based programs that is a fail on the spot. 

This catches people out because the car may drive perfectly. A small evaporative leak or a slow oxygen sensor does not change how the vehicle feels, and the test does not care how it feels. 

So if the light is on, find out why and deal with it before you book the appointment. Our post on what your check engine light really means is a good place to start. 

The readiness trap that fails people who did everything right 

Here is the situation that frustrates people most. You had a real fault, a shop fixed it properly, the technician cleared the codes and the light went out. You drove to work twice, took the test, and failed. 

Not for the original fault. For "not ready." 

Clearing codes resets every one of your car's emissions monitors to incomplete, and disconnecting the battery does the same. Those monitors have to run again and report complete before the car can pass, and they only run under specific conditions. Test programs typically allow one or two incomplete monitors, sometimes fewer on newer vehicles, and beyond that you are rejected whether or not anything is actually wrong. 

The fix is driving, but it has to be the right kind, and it has to happen before you show up. 

What a drive cycle involves, and why yours is different 

A drive cycle is the sequence of conditions your car needs in order to run its self-tests. The details differ by manufacturer and model year, so there is no universal recipe, but the ingredients are similar. 

A genuine cold start, meaning the engine sat long enough to cool completely, often overnight. Some idling. Gentle acceleration up to a steady cruise, then several minutes at a constant highway speed. Some deceleration with your foot off the throttle. Then repeating the mix over a few days. 

The evaporative monitor is the fussiest of the group. It commonly wants the fuel tank in a middle range, roughly a quarter to three quarters full, plus a cold start and a stretch of stable driving, and on some vehicles it refuses to run for days. That is why a car driven only on short trips can take a long time to read ready. 

If your commute is ten minutes of stop and go, plan on a longer drive or two with sustained highway time before you book. 

Check readiness before you spend the fee 

This is the part you can do at home. BlueDriver plugs into the OBD2 port under the dash on the driver's side and pairs with the free BlueDriver app on your phone. 

Open the smog check readiness screen and the app shows what the tester is going to see: whether the check engine light is commanded on, and which monitors are complete versus incomplete. If everything reads complete with no stored codes, you are in good shape. If three monitors are incomplete, you just saved a wasted trip, and the answer is more driving rather than more money. 

The same scan lists stored and pending codes, so you can catch a fault that has failed once and will likely confirm soon. Repair Reports give possible causes and verified fixes for whatever turns up. There is no subscription and no per-scan cost, so checking again tomorrow costs nothing. 

BlueDriver Pro reads the core systems on most vehicles from 1996 on. For 2014 and newer North American cars, trucks, and light-duty diesels, BlueDriver Pro Next-Gen has stronger hardware and supports more of the newer communication methods those vehicles use, so it reaches more of the onboard computer network. 

Common reasons cars fail 

A handful of causes account for most failures. 

Evaporative system and gas cap codes. The evaporative system keeps fuel vapor from escaping, and it is sensitive. A cap that is loose, cracked, or has a hardened seal is the cheapest possible cause. Purge and vent valves and cracked hoses come next. 

Catalyst efficiency. Codes such as P0420 mean the computer does not think the converter is cleaning the exhaust well enough. Sometimes the converter is worn out. Sometimes an upstream problem, a misfire or a bad oxygen sensor, is the real culprit and the converter is a victim. Our P0420 post covers how to tell the difference before you spend converter money. 

Oxygen sensors. These tell the computer how to trim the fuel mixture, and a slow one hurts emissions and fuel economy. 

Misfires. A cylinder that is not burning properly dumps unburned fuel into the exhaust, which fails emissions and can destroy a converter. EGR and secondary air systems, where equipped, round out the list. 

OBD-based tests versus tailpipe tests 

An OBD-based test reads the car's own diagnostics. It is fast, it is what most newer vehicles get, and it passes or fails on light status, codes, and monitor readiness. 

A tailpipe test, sometimes on a dynamometer, measures what is actually coming out of the exhaust and compares it to limits for your vehicle's class and year. Older vehicles may still get one. A car can pass a tailpipe measurement while its computer reports a fault, or the reverse, which is one reason programs differ in what they require. 

What this cannot tell you, and where to confirm 

Checking readiness at home tells you whether your car is likely to pass the OBD portion. It does not guarantee a pass. It cannot predict a tailpipe measurement or a visual inspection, and it will not catch a mechanical problem that never sets a code. A scan is not a substitute for professional diagnosis, and it does not certify a vehicle as safe or trouble free. 

Rules genuinely vary by state and province, and sometimes by county: which vehicles are tested, how often, how many incomplete monitors are tolerated, and whether the test is OBD or tailpipe. Confirm your local requirements before you plan around any of this. 

And if the car is running badly, especially if the check engine light is flashing, do not drive it around trying to complete monitors. Get it diagnosed first. 

Go in already knowing the answer 

An emissions test is one of the few errands where you can see the scoreboard in advance. Confirm the light is off and the monitors are complete, sort out any pending codes, then book the appointment. 

Plug a BlueDriver into your OBD2 port, check smog check readiness on your phone, and turn test day into a formality instead of a gamble. 

Happy safe driving!