How to Make Sure Your Car Passes the Smog Check

Mia Bevacqua
June 12, 2018

smog check

Depending where you live, you might have to begrudgingly head over to your local repair shop or state inspection station for a smog check every year or two. It's a pain, but not too complicated — unless your car doesn't pass.

Cars pollute — we know this. Smog checks, also called emissions tests, help to minimize this pollution. The procedure involves inspecting a vehicle’s emissions-related equipment to ensure it’s working correctly. Doing this reduces the amount of pollutants that enter the atmosphere, allowing us all to breathe better.

Let’s look at what exactly gets tested, or you can jump ahead to learn what it means if your car fails the smog check, reasons your car can fail, and what to do if that happens.

What’s involved in a smog check?

Smog test requirements vary from state to state — and sometimes even within a state. Unless your state has free emissions testing, the exam typically costs anywhere from $30 to $90, depending where you take your car. Call a few shops or do some online research to find prices before heading over.

To maximize your chances of passing the test, make sure to keep up on your vehicle’s maintenance — change your oil regularly and make sure the battery is in good shape — use quality gas and drive the car for 10 minutes or so before heading to the testing site.

The two basic types of checks are onboard diagnostic and tailpipe. In most cases, a visual inspection is also performed.

Onboard diagnostic

All vehicles built after 1996 are equipped with an onboard diagnostics system, called OBD-II. As part of this, the engine’s computer monitors emissions-related functions. If it detects a problem, it turns on the check engine light and stores a diagnostic trouble code (DTC).

Since OBD-II is reliable, some states only require testers to hook up to the car’s system during inspection. A technician uses a scan tool to check for codes and determine if the computer has run all its monitoring tests — essentially meaning it has finished checking all the parts it monitors.


This type of inspection is used on vehicles built before 1996, though some states require it on all vehicles. The test involves attaching an exhaust gas analyzer to the tailpipe. It measures two or three harmful pollutants: hydrocarbons and carbon monoxide and, in some cases, oxides of nitrogen. Although not considered pollutants, the greenhouse gas carbon dioxide and regular old oxygen are also measured. 

A technician uses an exhaust analyzer to measure each of these gases as they’re emitted from the tailpipe. If the vehicle exceeds the allowable limit for one or more emissions gases, it fails the smog check.

Visual inspection

Typically, a visual inspection is performed in addition to onboard diagnostic or tailpipe testing. A technician will check items like the gas cap and fuel filler neck. He or she will also look to make sure all emissions equipment is intact.

What does it mean if the car fails?

In states that require smog checks, your car will have to pass before you can renew your vehicle registration. If it doesn’t pass in time, you won’t legally be able to drive the car or truck. 

Tip: If your car is having a hard time passing the test in California, and possibly other states, it's best to pay the registration fee before the expiration date to avoid late fees. 

A car or truck will automatically fail a smog test if the check engine light is on. Similarly, missing or modified emissions equipment will cause a failure. Beyond these two scenarios, whether your car passes largely depends on the type of testing being performed.

With onboard diagnostic testing, a failure indicates the car’s computer has a stored or is preparing to store a DTC. If the vehicle is simply “not ready,” that means the car’s computer hasn’t completed one or more of its monitor tests. What this typically means is that the car needs to complete a “drive cycle” — typically this requires having at least a quarter tank of gas and driving at a set speed for a certain amount of time. 

This does NOT mean the vehicle has failed. Typically, the vehicle will need to be driven until the tests have run. Then the vehicle can be retested. This is why smog tests can take a little bit of time to conduct, because the car’s computer needs to “warm up.”

After a tailpipe test, the vehicle owner is usually presented with a printout of the results. This document indicates which emissions gases exceeded specification, causing the vehicle to fail. The readings can be interpreted as follows:

HC: Test results that show excessive hydrocarbons indicate unburned gas is passing through the engine. This points to incomplete combustion (typically causing an engine misfire).

CO: Too much carbon monoxide points to a engine that’s running rich — that is, there’s too much fuel in the air-fuel mixture that the engine burns.

HC and CO: If both of these readings are too high, the problem is usually an emissions device outside the engine, such as the catalytic converter.

NOx: Excessive oxides of nitrogen imply extreme combustion temperatures. This is usually a problem with the exhaust gas recirculation system. 

Get it diagnosed by a professional

Common reasons for failing a smog check

Generally, a smog check failure indicates a problem the vehicle’s engine, engine control system or emissions equipment.

Engine: Anything that causes a loss of engine compression can result in increased emissions and a smog failure. Examples include improperly seated valves, worn piston rings or cylinder walls, and blown head gaskets.

Engine control system: The engine’s computer receives input from dozens of sensors. It then uses this information to control outputs, such as the fuel injectors and ignition coils. A problem with the computer, its input or its outputs can cause a smog failure. This could be anything from worn spark plugs to a damaged mass air flow (MAF) sensor.

Emissions equipment: Vehicles are fitted with components designed specifically to reduce emissions. A problem with any of these parts can result in a failed smog test. Examples include a failed catalytic converter, air-injection pump or exhaust gas recirculation (EGR) valve.

» LEARN MORE: Get an estimate for your car repair

How to fix the problem

Smog check failures can be triggered by a lot of different causes. Possibilities include things that are easy and inexpensive to fix or replace — say, a new gas cap — or things that will cost significantly more, like a faulty catalytic converter.

This diagnosis is best left to a professional mechanic. Some locations can do both the testing and the repair, but others can only run the check. Either way, having repairs done by a RepairPal Certified shop will ensure you don’t overpay for any fixes needed.

After the car has been fixed, you can have it tested again. You’ll likely have to pay for this second test, as well, unless your state does it for free. 

Once it passes, your can complete your registration with the department of motor vehicles and know your car’s emissions system is doing its job.


Mia Bevacqua

About the Author

Mia Bevacqua is an automotive expert with ASE Master, L1, L2 and L3 Advanced Level Specialist certification. With 13-plus years of experience in the field, she applies her skills toward writing, consulting and automotive software engineering.