I had to have an automobile emissions test the other day for the first time in many, many years (I used to cycle through cars like some change their socks and thus never had a vehicle long enough to require an emissions test.)
Anywho, back in the day, the vehicle emissions test used a “tail pipe snifffer” and gave you an estimate of the amounts of the various filth your vehicle was belching into the sky (apologies to The Police) and whether your vehicle was within tolerance.
Nowadays, if your vehicle is 1996 or newer, they simply connect to your vehicle’s OBD and if it detects no trouble codes, you pass.
I’d sure like to have an estimate of the amounts of the various pollutants my vehicle is emitting, though.
Now to the question: is OBD really a better gauge of vehicle pollution and why?
in theory, if the PCM does not have any DTCs set, it’s safe to assume the car’s emissions are within specification.
back in the day when cars had carburetors or early fuel injection systems, the self diagnostics weren’t nearly as capable as they are now. You could, for example, have a completely non-functional catalytic converter and the car wouldn’t have a clue about it.
on modern cars, the sensors that control the engine practically all have sensors monitoring them For example, older cars had one oxygen sensor per exhaust outlet, and it was placed upstream of the catalytic convertor. modern cars now have twice as many O2 sensors, with one upstream of the catalyst and one downstream. The downstream O2 sensor monitors whether the catalyst is doing its job correctly.
Actually there is more to it than just no DTCs.
The deal is not all DTCs are continuously monitored. Some DTCs can take a particular type of driving to set. For example I owned a car where the evaporative system would only test after 17 minutes of driving and then only when idling.
The system has to have preformed a number of monitor tests and if they all pass a readiness flag is set.
I don’t if I can recall all the monitors off the top of my head but they include:
Evaporative system
Oxygen sensor heater
Cat converter
Oxygen sensor (?)
At least one other I can’t recall
Comprehensive component
When all of the monitors have run successfully the readiness flag is set.
Some earlier cars only require 1 or 2 monitors for a pass, newer cars require the whole enchilada with the flag.
Once set monitors stay set until codes are erased or power is lost to the ECM/PCM.
Without monitors that car I mentioned above with the evap test 17 minutes in? I could kill the code take it in for a test and I would be down the road before the light came on. The presence of monitors prevents that.
You can have a car where there are no codes but will not set monitors ( boy is that a pant load of fun to fix)
An example of this is a car with a thermostat that is stuck open. Car warms up too slow and monitors refuse to run as engine temp is either too low or took too long to reach normal temp.
For example, older cars had one oxygen sensor per exhaust outlet, and it was placed upstream of the catalytic convertor. modern cars now have twice as many O2 sensors, with one upstream of the catalyst and one downstream. The downstream O2 sensor monitors whether the catalyst is doing its job correctly.