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Will my BMW pass smog? There's a light on...

Updated: Aug 10, 2023

BMW check engine light in San Diego
A BMW Check Engine light and his friends


Believe it or not, here at Driven SD we get this question quite often. And the answer is no. Not with that light on it won't, at least not in California. The dreaded "Check Engine" light or "Service Engine Soon" light. This light comes on when your car's engine computer (or DME in BMW lingo) thinks there is a problem that could affect, or is affecting, your car's emissions control system. This is one of the main things checked by the smog machine during a test. If it's on, it won't pass.

This light can come on for very many reasons. From a simple loose gas cap, a misfiring spark plug, to an expensive engine or transmission failure.

BMW E46 Service Engine Soon Light in San Diego, CA

What if I just clear the codes?

You could, we have a handy check engine light reset guide, but... if the failure is permanent (like a bad sensor or shorted wiring) the fault code will come back pretty quickly, if not instantly. If it's an intermittent issue the fault may stay away, however your engine computer is really smart.

When you clear the fault codes out the computer also resets what are called "readiness monitors". These are system test that the computer runs. The purpose of readiness monitors is to self-test the car’s emission control systems. If the monitors are not set, you won't pass smog.

Here's a list of the monitors:

  • Misfire

  • Fuel System

  • Comprehensive Component (malfunctions in any powertrain electronic component or circuit that provides input or output signals to the computer that can affect emissions and is not monitored by another OBD II monitor.)

Spark ignition vehicles (Gas)

  • Catalyst (CAT)

  • Heated Catalyst

  • Evaporative (EVAP) System

  • Secondary Air System

  • Oxygen (O2) Sensor

  • Oxygen Sensor Heater

  • EGR (Exhaust Gas Recirculation) and/or VVT System

Compression ignition vehicles (Diesel)

  • NMHC Catalyst

  • NOx/SCR Aftertreatment

  • Boost Pressure

  • Exhaust Gas Sensor

  • PM Filter

  • EGR and/or VVT System

That's a lot of stuff!

So, what should I do?

You could check codes with a scanner and get lucky putting parts in the car, but from our experience this usually turns into a waste of time and money. Your best bet is to get the car properly diagnosed by a professional. Speaking of professional mechanics check out our fun article about the difference between a BMW Autohaus and a BMW Werkstatt.

BMW smog check

Schedule your BMW to get diagnosed properly at Driven SD and we'll get your faults fixed, monitors set and ready to pass smog.

And don't forget to check out some of our other BMW articles like "Why Using the Right Engine Oil Matters for Your BMW" and "Where Can I Get BMW Suspension Repair in San Diego?"


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