Best of Deals Car Reviews Repair Shops Cars A-Z Radio Show

Do code readers work?

are these modules that plug in and read your engine computer, like Micro

Are plugin modules that read your engine computer, like MicroMechanic, worthwhile?

They are great for reading codes. What I do not like about them is some people think that you can fix a car by resetting codes. So the first thing they do is get one of these cheap code readers, erase the codes, find out the car is still broke. Then they will take it to a mechanic and expect him to make a quick diagnosis when they just erased valuable clues to the problem.


Depends on why you want it. Are you going to fix your own car? One of these tools is essential on a modern car to help diagnose a problem. They can provide you with a false sense of intelligence causing you to mis-diagnose your problem or second guess your mechanic who understands better what is wrong.

There are better products, by far, than MicroMechanic. MM reads generic OBD II codes, like dozens of similar products, but not model specific codes or ABS, Airbag, and other computers spread about your car. Those tools are better suited, that is, IF you are going to do your own work.


I have seen some that are only for one make of car as well. Someone had a Chrysler one that read codes specific to their cars. OF course all will read the generic emissions codes.

Now, I have talked with a lot of mechanics while working on their electronic equipment. While I was there some guy came in and told him to replace some part on his car. This shop was literally a block from the nearest AutoZone. The mechanic was like “Why do you want this part replaced.” The guy was like “AutoZone hooked something up to my car and it told me this was the problem.” The mechanic suggested a full diagnostic to be sure but this guy insisted that the error message from AutoZone told him it was this part.

The mechanic made him sign a form acknowledging that he might be having a perfectly good part replaced and that the real problem might be somewhere else and that the mechanic wouldn’t be liable for an incorrect diagnosis or any charges incurred for labor or unnecessary parts being replaced. Basically, he was signing off that there would be no guarantees as to the outcome of the work without the customer paying for the proper diagnosis first.

I was back at the shop a few weeks later and asked what happened with the guy who wanted him to replace certain parts without a diagnostic. He told me that this guy had come right back the next day after the repair was done complaining that nothing was fixed. He got all irate and threatened to take the mechanic to court if it wasn’t fixed correctly and for free. The guy tried to deny he had ever signed the form but never took this to court.

Code readers provide a lot of vary valuable information and can help with DIY repairs. Remember that a bad wiring connection or similar can cause a sensor to trigger a code while the sensor itself is fine. They can be dangerous in the wrong hands as many problems can trigger the same code.

Also, the “As seen on TV” product you mention may not be the best choice. There are many of these so read the reviews on Amazon and order.


Mechanics don’t use code readers for diagnostics. They use scanners.

Most professional scanners are capable of retrieving history codes. A history code is stored in the computer each time the MIL comes on, and has a time stamp and the freeze frame data.


Mine work as intended, and have guided me in the right direction for repairs, saving me a lot of money over the years. However, I also rely on second opinions, like from the professionals and semi-pros here at Car Talk and at


You don’t have to own one. The code will help you decide whether to repair sooner or later. Faulty signal from oil pressure sensor that lets the light stay off for a few days can wait quite a while.

Inexpensive code readers from the auto parts store will read and clear generic, standardized, OBD II codes that relate to the engine and emissions controls. They are very useful for pointing you in the general direction of the problem with your engine, but they rarely correctly identify a specific part that, if replaced, will solve your problem.

More expensive scanners will read current, pending, and historical OBD II codes, and will generally have the capability to download software to read body and drivetrain diagnostics, and perhaps some additional nonstandard engine data, for a specific make of car. These will generally have two-way functions. For example, if your window won’t roll up from the switch in the door, bur your scanner can roll up the window, you know that the problem is the door switch or the connection between the door switch and the computer. You will need such a tool if you need to electronically retract your brake pads for service, or reset an airbag failure light, or register a new AGM battery with the car’s computer.

The most expensive scanners come from the factory, are uniquely programmed to deal with one make of car, and will do all kinds of tests to help you pinpoint problems. These cost several thousand dollars.

Snap-On and a couple of other professional tool suppliers make scanners for independent shops that come with updateable software for multiple makes of cars. I have heard that they run $2000-$3000, so they are a great value for an independent shop.

I bought one. Sitting on the shelf, no codes to read for 5 years…

I thought about buying one, but I never did. Auto parts stores will read your trouble codes for free. Just don’t let them diagnose your car based on the codes. The error codes are a starting point for diagnosis, not the end point, so if you have an auto parts store employee read the codes off your car (or if you do it yourself), get the actual codes, not his armchair diagnosis, and post those codes here for help finding out what they mean.

For example, there is one error code that indicates you have one of several possible issues. The error code that indicates you need a new catalytic converter might also indicate you need a new oxygen sensor (or two, or more if you have a V-6 or V-8 that has them on both sides of the engine), or some other problem with your air/fuel mixture, or incomplete combustion in one or more of your cylinders. The code itself only indicates that the downstream oxygen sensor is getting a reading that is out of the normal range, so don’t throw parts at a problem like this without an expert diagnosis that goes beyond reading the trouble codes.

The only issue I have with you buying one is: Will you have the technical knowledge to get what you need from the codes it gives you? If not, it’s probably not worth the price.

1 Like

You can get some really cheap code readers. They will get you the code and maybe allow you to clear it. Remember that clearing it can be a bad idea if you really want to get to the bottom of the problem.

On the other hand, my GF used to have a car that would get random codes all the time. I would clear a code and that code would never return. It was something electrical because the codes would be something completely unrelated. It might go months without one coming back. Of course that car had lots of problems so eventually there would start to be a pattern and then I knew the code actually meant something.

But yes, often people here will ask “What codes are present?” when someone here posts that their car has a check engine light and is running like crap. It might be worth $20 to go out, read the codes, and just come back in any post them here. Sure, parts store will do it for free but these are not expensive.

There are codes and there are codes. Most people on this forum are only familiar with “P” (Powertrain) codes relating to the engine, emissions, and transmission. That’s all the cheap and middle-range units will report and reset.

There are other codes that are only read by the professional units: “B” (Body) and “C” (Chassis) codes. DW’s Yukon XL has the Level-Air suspension. An error showed up on dash. Rather than check every part of the system, I visited the local auto-electric shop and asked them to read the complete codes. The guy said “There are no “B” codes set.” I asked him to check the “C” codes and sure enough, one was set. His tool even gave the text for the number, something like “Problem at right rear gas lift.” I crawled under the car right there on the spot and sure enough the plastic clip that hods the air hose on had sheared and the hose was off.

The guy refused to take any money for doing the test and the part (replacements were metal) cost $0.75. I have a couple of standalone scanners and two that interface to my smartphone and tablet. One of these days I’ll write some tablet software so I can check or set the “B” and “C” codes myself.

You bought the code reader in case you had OBD-II related issues in the future. That prevented anything from going wrong. Clearly, a worthwhile purchase. :wink:


When both my kids (one of each variety, now 31 and 24) were in college (7 years apart) I made sure they were supplied with an Actron code reader and were well versed in using it. They sometimes drove home and back, a several hundred mile trek, and sometimes in more “experienced” cars which can add drama to a road trip.

Should a CEL (check engine light) arise they knew how to check it, research it, and get with me if there were questions as to whether to continue on their merry way or not (We didn’t raise any dolts). I actually can’t recall a problem they had while travelling, but I was always a bit nervous and protective. It’s what dads do, I guess, and with the reader they felt more secure and believe me, so did I. It’s tough to help somebody from a couple hundred miles.

They were a hit on campus, too. When other students were at a loss with a CEL my kids could help them out as well and offer advice.

Now, they both drive nicer, newer cars than I have. I drive a 1,500 mile road-trip twice each year in “experienced” cars. I always have a code reader on board.

There are smartphone based readers that will indeed read chassis codes, abs code, airbag codes… the full slate. Usually those versions just work for particular manufacturers. I have Forscan for my Ford 9and Lincoln) and OBD Eleven for my Audi (and VW products).

With the exception of a very brief CEL episode in 2002–with a previous vehicle–I haven’t had to deal with a CEL.

Those who frequently have to deal with lit-up CELs are almost always the same folks who fail to maintain their vehicles according to “factory” guidelines.

I’m not sure I’d completely agree with a blanket statement like that

Partially agree, yes . . . but not completely agree

I’d like to think the vehicles in our fleet are well maintained, as per factory guidelines . . . with an obvious exception being the guy that put in 85w140 conventional instead of 75w90 synthetic . . . and our vehicles get the occasional mil on, due to evap or O2 sensor heater problems, to name some of the more common examples.


@db4690, your fleet is different than our vehicles. Your employer keeps them for probably more than 200,000 miles, maybe much more. When they are surplussed, they are pretty well worn out. It is not surprising they throw codes, even though they get regular maintenance from you and your associates.

I don’t know about that fleet, but the fleets I’ve worked with tend to be older low mileage vehicles. They sit more than they get used or they make a lot of short trips and spend a lot of time idling. They age out of their warranties and get surplused with relatively low miles but a lot of visible wear.

what you just described sounds like a lot of vehicles in our fleet

Unfortunately, pencil pushers make all the decisions, in regards to which vehicles get phased out

There are some older vehicles that obviously aren’t worth much, but they’re in excellent condition and have many good years and miles left, but to the guy behind the desk, it seems like a good candidate for replacement

In any case, the vehicle operators and mechanics are never spoken to . . . and in many cases the wrong vehicles are sent out to pasture, whereas the bad ones are kept