The Smog Stop
6414 University Ave, San Diego, Ca 92115
Mon-Sat: 07:00 - 17:00

Diesel Smog Inspection Procedures

The procedures for performing a smog check on a diesel vehicle is not much different than a vehicle with a gasoline engine. The inspection is actually is not as intrusive as gasoline vehicles. The following inspection points highlight the general smog check requirement of a diesel engine vehicle.

  • Visual Inspection
  • Smoke Test
  • OBD Readiness.

We will thoroughly discuss the above inspection points so you are completely informed when it comes to a smog inspection on your salvage vehicle.

Visual Inspection

The visual inspection consists of an inspection of the emissions devices that were installed on the truck when it manufactured new. The reason the State of California chose 1998 and newer is because most diesel engine manufactures did not install any emissions devices on older model diesel vehicles. So the smog inspection would have been worthless to perform on these vehicles. The visual inspection is where most diesel powered vehicles owners get in to trouble on the smog check inspection. Many owners of vehicles that are diesel powered want to enhance the performance of their diesel engine and either remove or add on parts that are not legal. This causes you diesel to fail the smog check inspection. Below we will summarize the inspection in detail, so you know what a licensed smog inspection station looks for on you diesel vehicle.

Diesel Smog Check Visual Inspection Procedures

The Diesel Smog Check visual inspection begins with identifying the emissions components that were originally manufactured to be on the diesel vehicle. This is done by checking the vehicles emissions catalyst label. If the catalyst label is missing, then the smog check inspector must look in the emissions guide to determine the original equipment emissions components that were installed on the vehicles. Below list the common emissions devices that are installed on a diesel vehicles.

  • Glow Plugs
  • Crankcase Ventilation System (Similar to a PCV system on a gasoline engine
  • Catalytic Converter
  • Exhaust Gas Re circulation or better known as the EGR system
  • Diesel Particulate Filter (DPF system)
  • Selective Catalyst Reduction or SCR system
  • Turbocharger

If the vehicle came with any of these components and they are listed on the catalyst label, they can not be removed or you will fail the diesel smog check inspection. Additionally, the diesel smog check inspection visual inspection checks for add-on parts, too. So if you decided to change your exhaust down pipe to a larger size or diameter, it must be approved by the California Air Resources Board. Below is a list of common add-on parts that will cause you to fail the diesel smog inspection.

  • Aftermarket Down-pipe that has a larger diameter than the stock downpipe
  • Engine Tuner- This is usually installed in parallel with the engine ECM and is programmable to obtain more horsepower
  • Air Ram Injection – This is better known as the air horn, often these contain heaters to heat the intake air to reduce emissions. Aftermarket air-horns usually omit the air heaters and are not CARB compliant.
  • Aftermarket Turbocharger – If you decide to upgrade your turbo charger it must be CARB approved or be a direct replacement part.


Diesel Smog Check Smoke Test

The smoke test portion of the diesel emissions test asks the diesel smog check technician to verify if there is any visible smog being emitted from the tail pipe during a series of snap tests, engine at idle, and crankcase smoke. If excessive smoke is detected the diesel vehicle will fail the smoke test portion of the smog check Below are the procedures that must be strictly followed when performing the smoke test during a diesel smog check.

  • Ensure the engine is at operating temperature
  • Observe tail pipe for 10 seconds for any indication of smoke
  • Observe the engine from under the hood for 10 seconds to verify no smoke is being emitted from crankcase
  • Perform snap test. A snap test requires the smog technician to quickly snap the throttle to an RPM between 2,000 and 3,000 RPM. Then allowing the engine to return to idle.
  • Perform a second snap test. The smog technician watches for visible smoke during the 2nd Snap Test
  • Perform a third snap test. Visible smoke must be noted on the 3rd snap test, as well

The pass/fail criteria for the snap test is no smoke at idle or from the crankcase. If during the second or third snap test a visible smoke plume that lingers is present between 5-15 feet of the tail pipe the diesel engine vehicle will fail the smoke test. If the smoke dissipates within 3-5 seconds the vehicle will pass the smoke test portion of the diesel smog check.