How Smart is the Auto OBD

Older OBD systems would trigger immediately on sensor fault or failure.

OBDII systems may not indicate a sensor fault/failure immediately. Newer systems may not trigger until the tailpipe emissions are/remain at 1.5 times the federal limit for that vehicle.

I cannot confirm or refute the 50 day figure.

I say that you're both correct, at least from how you've phrased the question. The On Board Diagnostics of an automobile will quickly switch a car into "limp home" mode when it no longer receives a plausible signal from many sensors. Now the OBD may not show a "check engine" warning or even store a short (intermittent) failure status into the limited memory for many miles or days with some sensors but certainly other sensors are considered critical enough that an immediate alert will happen on any interruption of data from a sensor.

I'm certain that the list of critical and non-critical sensors will vary wildly with year, make of a vehicle and whatever environmental control board rules are appropriate. Since you do not offer a make or model of a vehicle nor do you indicate if this vehicle is being used in Rio De Janeiro, Irkutsk, Ottawa, or Des Moines. So I will not guess which failures will generate an immediate warning or not.

It dawns on me now that your question could even include NASCAR and Formula 1 racing cars. I'm certain that in these vehicles any and all detected sensor failures are immediately sent via telemetry to the pit crew computer.

Hi what continueous bad auto sensor would not let the OBD know or record a fault for miles or days? I understand the check engine light may not always lite but is'nt the fault recorded? thanks tom

Your return question reflects the ambiguity of your initial question. Does Jim's status of a fault appearing mean that the check engine light must turn "ON" permanently, or that it is ON for longer than X seconds after the incident (start up) or just that one of the registers inside the OBD recorded an incident?

I usually don't answer a following question without my questions being answered but here is a plausible scenario. A vehicle built for the Irkutsk market might not record an engine coolant sensor failure for a cold temperature alarm for months.

Sensor failure is detected as soon as the sensor signal comes out of the predetermined range. Since it can be an error which disappears after a time it is possible to monitor the time after the failure occurred which time will depend on the importance of sensor output system safety, if the sensor stays out of range for this time then it is considered as out of order and its failure is noticed.

It will register a complete failure "immediately"

Try it. Run your engine and start pulling connectors off the sensors.

Hello.Answer to your question is yes and no.If the currently active sensor is comprehensively monitored (looking for gross faults) it will turn on the light.Example is a disconnected coolant temperature sensor reading -40 degrees.There are also continuous monitored sensors like crankshaft sensors where the system counts the revolutions looking for misfires.If the misfires cause the emissions to exceed the federal limit by 1.5 the light will flash on although it will probably require 2 driving cycles to turn on continuously.Some sensors are only monitored during specific driving cycles like evaporative system sensor monitoring for fuel vapor leaks.Catalytic converter efficiency requires a specific time for the vehicle to be driven along with other parameters. Hope this helps.Wishing everyone happy Thanksgiving.We are joining the crowd and going to Great Granddad's.Cheap out of town vacation.

For a Ford it doesn't take that long...the link below gives the times and distances and such required to initiate the OBDII tests for given sensor circuits.

Under the right operating conditions the same can be said for each vehicle manufacturer. OBDII is a standard set outside of manufacturer guidelines and is nicely outlined (slightly technical) by Ford here:

http://www.obdii.com/drivecycleford.html

You wouldn't think it was this easy to get information:

http://www.obdii.com/

General Motors is fairly accurate and quick on reporting problems. The check engine light is a must on all or any failures, being intermittent or not. If the OBD is not triguring then your cpu has a problem. It is mandated by federal regulation for the main cpu controller to send the signal and the proper flow orders of failures are recorded. Otherwise you will have a faulty main controller.

I think part of the issue is the logic to determine if there is a sensor failure, or simply out of range data. Example; leave your gas cap loose/off after refueling. YOu will trip one or several evap codes indicating failure of eiter the vent soleniod control valve, or the pressure sensor. Clearly neither of those components failed. Resecure your gas cap, and after a few start/run cycles the fault will clear on its own. THere are similar failure codes for the fuel tank level sender card (used for the evap calculations) and many other seemingly insignificant circuits. Just try to reverse engineer or remap a fuel delivery curve on an OBD vehicle. I was just getting into that on my 300ZXTT before I decided to sell it and move on. You'd be surprosed how much you can improve the fuel economy by reducing the minimum ijector pulse width at idle or coast down. During the early 90's some OEMs kept the minimum fuel pulse high to ensure sufficient excess fuel to keep the catalyst hot and functioning. EPA would frown at the burp in emmissions as the cat heated back up.

All this said, I don't know of any codes that take 50 days or 50 miles to set. THough they can take that long to clear. Just like when you manually clear the codes, the vehicle must run at least 50~100 miles before the all OK signal is set. This prevents code clearing immediately before an emissions test. Thus there is your proof that some faults can take a little time to throw the error code.

Have you tried to disconnect any yet?

I can't do it on my vehicle as it's a real diesel. No ECU. OBD is between my ears when I sit in the cab.