OBD II (Check Engine Light) Data

I was in traffic court recently listening to the cases before mine and in one a state trooper had ticketed someone for failing to switch into the left lane while passing a police traffic stop (on the right shoulder of an interstate). The officer flagged down the defendant, who stopped and proceeded to open her hood and investigate what she said she thought was an overheating engine. The defendant said they did not switch to the far lane while passing the traffic stop and subsequently stopped because they feared their engine was overheating, supporting that contention by saying that the Check Engine Light and a number of other lights lit up on their dash just before they stopped.

The officer testified that, on the scene, the office challenged the driver’s statement about the check engine light by saying “I have a black box I can hook up to your car that will tell me how long your Check Engine Light has been on. Do you have anything more to say about your check engine light?”

The defendant replied to the officer on the scene and repeated in court that her check engine light had been “on and off” and it came on again together with several other dash lights just before she pulled over.

The defendant ended up being acquitted, but I was curious about the officer’s claim that she could tell using some kind of black box how long a car’s check engine light has been illuminated. I don’t see anything in the OBD II specifications that this kind of information is stored in the car’s computer. I think that officer’s challenge question about the CEL was a bluff. Do any of you experts concur?

Yep, a bluff to make the defendant back down. A code that trips the CEL is either there or it isn’t , with no info as to how long.

You still could have changed lanes even if your engine was overheating. Coolant temp is a value that can be retrived through a OBD2 scanner but theinformation about engine temp is relayed to the driver by either a light or a temp gague. “Check Engine” lights are to tell you if something is wrong with your car that could affect its emmissions. There is a history feature in the OBD2 system. The oil can symbol.the battery symbol,the thermometer symbol these are the systems that warn of immiment engine damage.I am not saying ignore check engine lights they can warn of conditions over time that can damage emissions equipment.If the car was suspected of overheating due to a check engine light or a temp gague your correct course of action is to pull over,in this case continue for half a mile (past the cop) and pull over. Did she think continuing driving was the proper course of action with a suspect overheating condition? The post said the person with the suspect car was “flaged down” by the cop,but later in the post she said she stopped on her own,am I reading this right?

In reality, its not a bluff. I can tell you for sure MY scanner (a Snap-On MODIS) will give that data. The MODIS gives me what is called “Failure record data” It will give me a trouble code, it will tell me how many miles it has been since the first time that trouble code came up, it will also tell me how many miles it has been since the MIL (Check engine light) was requested on. Now, the MODIS is a high dollar computer based scanner/lab scope/DVOM/Graphing meter etc. I cant tell you for sure that the officers scanner, or computer or whatever he had has those capabilities but if my scanner has it then the technology is definately out there. You would be amazed at what data is actually being produced by a vehicle computer and what a good scanner will tell you.


I can also check this with software on my laptop hooked up to the OBDII port, using an interface to go from the ODBII port to USB.

I also think the officer did not have any such equipment on hand to find this information out and was bluffing to try to get the truth from this liar (the defendant) who feels that the law and the safety of an officer is less important than a possible problem (which may not even be serious) with a hunk of metal (the car).

I think the cop was B.S.ing the motorist that he had such a device. He, probably, knows that emissions test stations use OBD II scanners which do have that capability.

What is wrong, in this picture, is this law and/or its enforcement. Someone told me of an instance where the (state highway) cop was stopped on the beginning of the off-ramp lane they (the motorist) had to use. The motorist had to pass the cop and turn into the off-ramp lane to make her exit (What?! X more miles to the next exit?!). The cop chased her down and ticketed her. She took it to the court. The court used that blind old adage in its ruling against her of, " Regardless, the LAW is the LAW ".

The question is, “Do cops and judges sometimes make stupid legal decisions?”

I think the key is being reasonable. Was it reasonable to ticket the motorist in your example? Of course not. It’s not meant for that kind of situation. The point is not buzzing by officers stopped on the side of the roadway with no regard for their safety.

I agree that it is important to change lanes to pass by a traffic stop safely. Too many officers are injured and killed because they are hit by careless drivers passing too close and too fast. My question really was not on the facts or points of law in the case other than the officer’s claim about the CEL data.

In sum, I think it was a bluff, becuase it is highly unlikely that the officer had either the training or equipment to analyze the codes; even if OBD II or its equivalents really do carry date-time of failure information. Also, since such a check would require either a warrant or permission from the motorist to search the vehicle, and certainly such a code check would constitute a search, something for which the officer technically did not have probable cause.

Yep, unless that cop was a real gearhead in his off-duty hours, no way he has the proper equipment. A standard OBD II scanner will NOT retrieve this info.

But I’ve been enlightened about the power of today’s automotive control systems, thanks to transman and Jad. Maybe it’s time to upgrade my test equipment…