Mention website for $20 off

Smog Check Coupon

Car dealers can’t use our coupons or discounts. Individual pricing applies to them.

Hensley's Auto Smogs, Brakes, Tires

Hensley’s Auto Smogs, Brakes, Tires

Smog Check PricesSet AppointmentTire Quote
We Can Smog All Vehicles.  Drop In Smogs OK


$20 Off Smog Check. Mention Website For Deal. We Can Smog All Vehicles. Drop Ins OK. Fast Service.

468 Yolanda Ave #915 Santa Rosa – Located In Back

Mention Website Coupon To Get Deal.

Full-service star certified auto repair and smog check Santa Rosa station.

We offer a free re-test if your vehicle fails the smog test. We do all smog checks

including star certified smog checks.

Star smog check and Test Only smog check vehicles are not a problem. We can also smog check motor homes, vans, diesel, and out of state vehicles. If you need a Smog test Hensley’s Auto can do it. We also offer coupons on our coupon page.

Free smog check retest if you fail. We have great reviews. Come on by, so our friendly staff in Santa Rosa can help you.

Lots of people are searching for a good smog check Santa Rosa location. Hensley’s Auto is not only a Smog Check station, we are a STAR Certified smog check station. This means we can smog all vehicles. If your vehicle fails the smog check Santa Rosa inspection we are certified to do smog repairs.

What is involved in doing a smog check in Santa Rosa?

There are three parts to the smog check inspection in Santa Rosa: Visual, Functional, and Emissions

Visual part of Smog Check Inspection

The visual portion of the smog check inspection involves making sure all required smog check emission components are hooked up and not visibly defective, missing, modified, or disconnected. In most cases, aftermarket smog check equipment that is not considered a direct replacement needs to have an Executive Order Number (EO number). The EO number establishes that the aftermarket part in question is, in fact, legal for use in the state of California smog check Santa Rosa program. The smog check technician will then type the Executive Order Number in the Air Resource Database and verify that the smog check part is legal for that specific vehicle being smog checked in Santa Rosa.

Main parts of Visual Smog Check Inspection

Under hood Label: The starting point of the smog check visual inspection is the under hood label. All the smog check emission control components listed on the label need to be installed correctly in order to pass inspection. If the label is missing then the vehicle smog check information will need to be accessed by other sources such as the internet and smog check inspection repair manuals.

Information listed on the label includes:

Under Hood Emissions Label

Certification Type and Year – The under hood label indicates what year the vehicle is. The label also indicates if the vehicle is certified for federal or California smog check emission standards. Most out of state vehicles are certified under federal smog check guidelines. When a federal vehicle is brought into California there is no requirement for them to be updated to California smog check guidelines.

Vehicle Engine Size – This part of the label indicates what size engine is in the vehicle. If the vehicle engine has been changed to an engine that was not offered by the manufacturer for that year, the vehicle will need to go to the smog check referee to be re-labeled. There is a smog check referee center located at the Santa Rosa Junior College.

Required Emissions Components – This portion of the label shows the main smog check emission controls on the vehicle. Every vehicle can have a different configuration of smog check components. For example, some vehicles will require an EGR valve while others will not.

Component Locations – This is a picture of the smog check hose routing and component locations. The arrow in the picture is pointing at the smog check EVAP canister. The smog check technicians job is to make sure that the evaporative canister is installed on the vehicle and hooked up correctly, as well as all inspect other smog check emission components and hoses.

PCV – The Positive Crankcase Ventilation system is used to ventilate the crankcase. The visual part of the smog check Santa Rosa inspection is to check the PCV hoses for cracks or splits. The PCV hose in this picture is defective. You can see that the hose is split open, which will cause fumes to go into the atmosphere. This is a common visual smog check failure during a smog inspection.

Defective Smog PCV hose

Broken PCV Hose

gas cap

Gas Cap Tester

Gas Cap – The gas cap is part of the smog check evaporative emissions system. Its job is to keep the gasoline fumes inside the gas tank from exiting into the atmosphere. The smog technicians job is to make sure the gas cap is installed and can hold pressure. The gas cap is tested during the smog inspection to ensure this.

Gas Cap Check

TAC hose – The TAC, or Thermostatic Air Cleaner hose goes from a stove on the exhaust manifold to the air cleaner. Generally, thermostatic air cleaners are found on older carbureted vehicles. The TAC hose draws hot air from the exhaust manifold and routes the warm air into the engine to aid cold starts and help the vehicle run when cold. Your vehicle will fail the smog check if this hose is missing modified, disconnected, or defective.

EVAP Canister- The Evaporative canister is used to store fumes from the gas tank which are then purged into the motor to be burned. Activated charcoal inside the canister is the medium used for storage.

Evap Canister

evap canister
EGR Valve – The EGR or Exhaust Gas Recirculation valve is used to control NOx emissions. NOx smog check emissions are produced by high combustion chamber temperature. By recirculating inert exhaust gas back into the motor, you are cooling the combustion chamber temperature down. This is similar to blowing smoke back into a fire. This reduces NOx emissions in the process. Not all vehicles have an EGR valve.

If the EGR valve is not working correctly your vehicle can fail the smog inspection. Sometimes the EGR will be working fine but the passages will be restricted or plugged due to carbon build up.

EGR Valve

EGR Valve

The picture below is an electronic EGR valve. There are vehicles that have EGR valves that are vacuum operated as well.

EGR valve


The functional portion of the smog check inspection involves checking to make sure individual smog check components are working. Not all smog check emission components are functionally checked.

EVAP System – The purpose of the smog check EVAP test is to make sure there are no leaks in the gas tank, filler neck, or associated hoses. Older vehicles (1995 and older) require a functional evaporative emissions test. Newer vehicles perform their own EVAP test. If there is a problem with the EVAP test the vehicles computer will turn the check engine light on. Below is a picture of the smog check EVAP test machine.

Evap Test

Check Engine Light – The check engine light (MIL-Malfunction Indicator Light) needs to turn on when the ignition key is in the run position. If the light remains on while the vehicle is running there is a smog-related problem which will cause the vehicle to fail the smog inspection.

The check engine light is also a signal that there could be something wrong mechanically with the vehicle.

If the check engine light is flashing the vehicle should not be driven. This indicates a very bad misfire.

Check Engine Light

check engine light
OBD2 system check – (Vehicle 1996 and newer) have a diagnostic port that the smog machine plugs into.

OBD2 diagnostic port

The vehicle’s computer checks all the smog components on the vehicle while driving. Each system has a test monitor that the smog machine will check to ensure completion of each test. If the test monitors are incomplete, the vehicle will fail the smog test. Clearing the check engine light or having a dead battery will cause the monitors to reset and the vehicle will need to be driven to run all the tests needed to complete the test monitors.

You are allowed to have 1 incomplete monitor. For 2000 and up the only incomplete monitor you can have is for evap.

The monitors are set by driving the vehicle in specific conditions set by each vehicle manufacturer.

List of common monitors

O2 sensor – Used to monitor the oxygen content in the exhaust in order to maintain the correct air fuel ratio. The air-fuel ratio is important for smog and drive ability concerns. Basic voltage reading for an oxygen sensor is .5v for rich and .45v or lower for lean. The computer is constantly changing the mixture while driving and idling. The voltage goes above and below .45 volts (cross counts) When the computer detects voltage readings that are inconsistent with this, the check engine light will come on

O2 sensor heater – In order for the oxygen sensor to work properly, it needs to be hot. Usually, the heat from the exhaust is enough to heat up the oxygen sensor, allowing it to work. This can take a while sometimes so newer cars have an electric heater inside the oxygen sensor which helps it heat up faster. If the computer detects a problem in the oxygen sensor heater circuit, the check engine light will come on.

EGR Valve – The EGR valve reduces NOx emissions. The valve does not come on at idle, only at cruising speeds. There are various sensors the vehicle uses to check if the EGR valve is working. Once a problem is detected the check engine light will come on.

EGR Valve

AIS system – The air injection system extends the combustion process by blowing oxygen into the exhaust manifold. This causes the exhaust manifold to get hotter, heating up the catalytic converter in the process. It’s like blowing air on an open fire to help it stay lit. The computer uses various sensors to determine if the air injection is working.

Electric Air Injection Pump

Exhaust Catalytic Converter – The catalytic converter stores oxygen in the exhaust and continues the burning process.

Catalytic Converter

It can be considered a second combustion chamber. Further eliminating any pollutants. Catalytic converters work best at high temperatures and need to have the correct balance of air and fuel going through it. When the converter is working the inlet temperature is cooler than the outlet temperature by at least 50 degrees. The computer checks the converter by monitoring the oxygen content after the converter with the rear oxygen sensor.

On a good converter, the rear O2 sensor activity will be slower than the front.

Good Catalytic Converter

On a bad converter, the front and rear O2 sensor activity will be the same.

Bad Catalytic Converter

EVAP – The evaporative system stores fumes from the gas tank to be burned in the engine. Through various sensors, the computer is able to determine if there is any leaks or problems in the system.


Air Fuel Emissions Chart

The first part of emission diagnostics is determining the Air/Fuel Ratio. Complete burning of the fuel mixture is key to having low emissions and optimum catalytic converter efficiency. The above Air/Fuel Ratio Chart is a good guide to help understand the correlation between different emissions. There is a give and take correlation between the emissions.

1) To do accurate emission diagnostic the exhaust needs to be free of leaks.

2) Carbon dioxide and oxygen readings are the best to use for diagnostics.

In order to get good at emission diagnostics experience helps. In our blog, we will be sharing our experiences with you. Just look at our emission repair blog posts.

The emissions smog test involves checking HC, CO, and NOx. Each vehicle has different emission requirements. The Smog inspection machine will automatically select the proper emission requirements when the technician enters the vehicle information.

Share this Post

Share this Post

No Printing Needed
Mention Website Coupon
Smog Check Coupon