The California Air Resource Board uses OBD scanner for emission testing. (https://www.arb.ca.gov/msprog/obdprog/obdprog.htm)

In other places like India, 'tail pipe exhaust gas analyse' method is still used to certify a Pollution/Emission test. I am curious to know the steps that are followed in California and other places where OBD is used for emission test. Can OBD alone determine if the vehicle is passing the emission test (Can there be cases where the vehicle is throwing smoke, while OBD monitors can't determine it?). Which all parameters (OBD modes and PIDs) do OBD scanner check to certify it as a Emission pass test? Compared to the exhaust pipe gas analyser which accurately determines the composition of exhaust gases, how reliable is using OBD data for emission check?

Reference to the wiki page: https://en.wikipedia.org/wiki/OBD-II_PIDs

  • I cannot answer all your questions, but realistically, no. The OBD-II system only reports when the system is out of bounds or throwing a code. If the parameters are changed within the ECU programming to where the code is never thrown, the OBD-II system won't report it and it would pass emissions. This is done all over the he place to "tune out" the after cat O2 sensors. Oct 31, 2016 at 21:23

2 Answers 2


OBD2 style emission testing makes one very large assumption.

If every system is working properly then the car is not polluting.

This is verified in two ways.

  1. If the computer has any hard fault codes in it. Hard fault codes equal something not working.
  2. The monitors that test the specific emissions systems have run. If a monitor runs and fails it will set a code the see 1 above.

That said, there is a whole lot of ways this type of emission goes wrong.

  1. The car is allowed to have one monitor that has not run yet. This monitor may flag a problem after the car has been emission tested. If your clever, the car can be fooled into not running a monitor to get it passed emission testing even though you know something is wrong with the car. This is very common cheating.
  2. A pending code does not equal hard fault. Only hard faults turn on the check engine light. While there is a pending code the monitor will still show not run and then see above.
  3. Other things can inhibit monitors from running like the outside temperature.

OBD data is not reliable at all or what so ever in predicting what is actually coming out of the tailpipe.

  • I am sorry but I didn't get the 1st point of - "whole lot of ways this type of emission goes wrong" If a monitor has not run, i.e. its not ready, the emission test wont happen for the vehicle, I guess. All monitors must run/be ready, right?
    – Soumya Sen
    Nov 1, 2016 at 15:49
  • @SoumyaSen The vehicle is allowed one monitor that has not run but has not failed and hard faulted.
    – vini_i
    Nov 1, 2016 at 15:51
  • Thanks man! Didn't know that. Also, with pending fault code active, will the continuous monitors show not ready? Also can you please share some links where I can get to know about the whole process
    – Soumya Sen
    Nov 1, 2016 at 15:55
  • @SoumyaSen That is correct, it will show not ready.
    – vini_i
    Nov 1, 2016 at 15:56
  • ODB is not used to test that a car is not polluting. ODB is used to validate that the car's emission control systems are functioning. Nov 2, 2016 at 6:58

In California, the emissions test (aka SMOG test) is performed with a combination of instruments: ODB-ii reader, tail pipe exhaust gas analysis, and rolling road.

There is no assumption that your car will pass emissions if there are no ODB faults or codes.

ODB is read to validate that the car's emission systems are in working order not prove that the car is not polluting. Any error codes from the ODB reader are recorded and if those are for any part of the engine or exhaust system, then the test is an automatic fail. If you had any of those codes, you'd probably already have a check engine light illuminated on your dashboard. If that were the case, you wouldn't even have the rest of the test performed.

The real test is performed with the ODB reader and exhaust reader hooked to a computer while the car is "driven" on a rolling road. The readings from ODB are used to verify that the engine is put through the required load (typically first at idle and then 25-30mph) with the measurements from the gas analyzer correlated to the vehicle speed, engine temperatures, etc. As long as both the gas analysis and ODB result in valid and are within allowable output tolerances, then your vehicle will pass.

The allowable emissions depend on the year of the engine manufacture, so if you swap engines your emissions requirements may change. As regulations become more restrictive on emissions, newer cars emit less and have less variance in what they may emit. Older cars are only expected to meet the emissions standards from when they were manufactured. Cars made before 1974 are test exempt, unless you put a newer engine in the car. So, if you put a 2016 Chevy LS crate engine into your 1920 Duesenberg, you'll now need to get your Duesenberg SMOG checked.

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