check engine light

Passing Smog Test when Check Engine Light is Flashing

The smog check is one of those iffy factors that drives car owners nuts. There can be a million and one reasons for that little orange Check Engine light to illuminate. A Service Engine light is usually  illuminated because of a sensor failure or a sensor reading that is out of specification. Your vehicle’s ECU (Electronic Computer Unit) typically operates in a low-performance when the check engine light is on and prevents you from passing emissions. Default fuel-mapping is typically used for your vehicle until the problem is resolved and the computer is able to learn and compensate for your driving style again.

Even if you were to cut the wire to the light or remove the bulb temporarily, you are missing the point. There is a setting monitor stored in the computer of your vehicle that supplies information to the inspection station. Without this information, the inspector does not receive a computer-generated emissions data history to ensure consistent safe levels. It takes the average vehicle 100-200 miles of travel to generate adequate emission data and develop responsive fuel-mapping. As emission standards become stricter by the year, owners struggle to pass aging vehicles.

How to Fix Your Vehicle to Pass the Smog Check

Even newer vehicles that are free of Service Engine light problems can fail the smog test. If your battery was recently disconnected for any automotive service, your default fuel-mapping and emissions history will be lacking. Dirty oil, fouled spark plugs, and an aging coolant temperature sensor can have a dramatic yet obscure effect upon your emissions test results.

Replacing Critical Sensors


The coolant temperature sensor in your vehicle is typically made of a semiconductor material like Germanium that creates greater levels of resistance as it cools. It has the most dramatic effect on your fuel-mapping and emissions systems because your engine dumps fuel when it is cold and runs much leaner once it reaches operating temperature. As these sensors age, the internal resistance becomes higher and the computer keeps dumping fuel in the engine in an attempt to bring it up to normal operating temperatures. Luckily, for most vehicles, these sensors are inexpensive and require anywhere from five minutes to an hour to install (depending upon their accessibility).

Oxygen sensors use zirconium ceramic coated with a porous platinum to detect the amount of oxygen in the exhaust gases. Like the coolant temperature sensor, the ECU will run in open loop (default/limp-mode) until it detects an appropriate oscillating voltage signal from the sensors. Many sensors come with additional wires to preheat the internal element in order to reach closed loop operation faster. Closed loop is the state of the vehicle driving according to sensitive sensor inputs and no longer relying upon its default fuel-mapping. Your vehicle needs to reach closed loop in order to pass the smog test.

Advanced Computer Adjustments

mechanic is using a laptop while repairing car in auto service

Some German vehicles (e.g., Volkswagen, Audi) require factory computer services to fine tune their closed loop operation. For many Volkswagen’s, you will need a throttle body alignment procedure. This is an advanced service that general repair shops often fail to do because they do not have the proper computer equipment. If the power of your vehicle appears to be lacking and you suspect that the battery was disconnected during a repair, your vehicle will require this procedure to pass.  ( This is true for lots of newer vehicles. Pretty much anything that is a Drive By Wire system needs this, Nissan are notorious for this now, you will even get check lights if you don’t do a relearn.  Most cars after you replace the battery needs a relearn and needs to be driven till all of the monitors reset.)

DTC Trouble Codes

OBD2 wireless car scanner

When your “service engine soon” light illuminates, disconnecting the battery is the worst thing you can do. Many websites recommend simply disconnecting the battery to clear the code. Although sporadic codes may appear as weak indications of wear and tear on any aging vehicle from sticky fluids on a cold start or a flaky sensor, they should always be checked and cleared with a proper OBD-II scan tool. OBD-II is the current emissions protocol bus system that is accessible universally throughout all makes and models for emission system testing. You can usually purchase an OBD-II scan tool from any department or automotive store for under $50.

In many late-model vehicles, these codes can appear for something as simple to correct as a loose gas cap. DTC stands for Diagnostic Trouble Codes. These codes are usually prefixed with a P and called P-Codes to denote that they are related to the Powertrain. Most vehicle manufacturers have additional protocols that are accessed through the same OBD-II port or another port that are specific to the brand. These additional protocols allow expensive factory quality diagnostic tools to communicate with everything from your interior preferences to the suspension system in high-end models with adjustable air-ride technology. It may take an experienced technician with factory training and resources for your particular model to interpret why a particular fault code is appearing.

Your only chance of passing the smog test is completing the easy fixes above and taking your vehicle to a factory authorized dealer or independent specialist if it is something beyond the basics. There are some easy DTC code fixes that you may want to go over in addition to the factors above. Misfire DTC codes often indicate that there is a vacuum hose leak or  bad coil, it could also be a  bad Mass air flow or a faulty fouled plug.  

Always make sure that you are driving your vehicle long enough to get the catalytic converter up to temperature for the weeks before a smog inspection. The catalyst honeycomb is there to trap and burn up impurities. It only works when you actually drive the vehicle and bring it up to sufficient temperatures.

Drive Cycle

drive cycle

You need to do a drive cycle to make sure all of the monitors are set. Here is a drive cycle that’s pretty common and shoudl work 90% of the time:

  • Cold Start. In order to be classified as a cold start the engine coolant temperature must be below 50°C (122°F) and within 6°C (11°F) of the ambient air temperature at startup. Do not leave the key on prior to the cold start or the heated oxygen sensor diagnostic may not run.
  • Idle. The engine must be run for two and a half minutes with the air conditioner on and rear defroster on. The more electrical load you can apply the better. This will test the O2 heater, Passive Air, Purge “No Flow”, Misfire and if closed loop is achieved, Fuel Trim.
  • Accelerate. Turn off the air conditioner and all the other loads and apply half throttle until 88km/hr (55mph) is reached. During this time the Misfire, Fuel Trim, and Purge Flow diagnostics will be performed.
  • Hold Steady Speed. Hold a steady speed of 88km/hr (55mph) for 3 minutes. During this time the O2 response, air Intrusive, EGR, Purge, Misfire, and Fuel Trim diagnostics will be performed.
  • Decelerate. Let off the accelerator pedal. Do not shift, touch the brake or clutch. It is important to let the vehicle coast along gradually slowing down to 32km/hr (20 mph). During this time the EGR, Purge and Fuel Trim diagnostics will be performed.
  • Accelerate. Accelerate at 3/4 throttle until 88-96 km/hr (55-60mph). This will perform the same diagnostics as in step 3.
  • Hold Steady Speed. Hold a steady speed of 88km/hr (55mph) for five minutes. During this time, in addition to the diagnostics performed in step 4, the catalyst monitor diagnostics will be performed. If the catalyst is marginal or the battery has been disconnected, it may take 5 complete driving cycles to determine the state of the catalyst.
  • Decelerate. This will perform the same diagnostics as in step 5. Again, don’t press the clutch or brakes or shift gears.

There is no easy answer to passing the smog inspections. Vehicles have become extremely specialized and face road conditions even the engineers couldn’t anticipate. Reduce pollution and pass the smog check by having your vehicle serviced regularly by qualified technicians.

All You Need to Know About a Driver Vehicle Inspection Report
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