Just started following this forum and I keep hearing about code readers like they have some value. I'm lost on how they do add value by themselves. Knowing the code will rarely tell you what you should do. For instance, when OBD-I was young, a lot of people would think the computer knew something useful and when it would flash the code for 'O2 sensor voltage too low', people would replace the O2 sensor. They should have looked for a vacuum leak, a plugged fuel filter, bad pump, bad MAF, etc., anything that would have driven the O2 sensor voltage low, indicating a lean condition.

Likewise, a 300 or 30x code indicating misfires doesn't narrow it down much.
It could be triggering for fuel or spark, a plugged injector, weak
secondary, low compression, open EGR, bad MAF, bad O2 sensor, leaking
exhaust manifold, almost anything. EGR codes seem to sell a lot of DPFEs
and EGR valves but the problems are often vacuum or clogged passages.

A minimum requirement for tracking down most problems with a scan tool is
the freeze frame for the code and the ability to graph PIDs over time,
preferrably with the ability to display multiple PIDs for the same time
period. It's a plus if they display on the same graph at the same time.
With this, a scan tool may be the most important diagnostic tool in your
box.

I'm the first to say that most problems can be found with tools other than a
scan tool. Your eyes and ears and sense of feel while driving are more
important. A test light, logic probe, multimeter, lab scope, ignition
scope, low amp probe and gas analyzer along with the normal fuel pressure
guage, vacuum guage and infrared thermometer are as important as a scan
tool. In fact, the best bang for the diagnostic buck in my box is a pair of
ST125 spark testers, maybe $6 each (two because I still see a lot of DIS).

Many common problems like a weak fuel pump, clogged fuel filter, restricted
exhaust, occasional misfires, light pinging, and the list goes on and on,
won't fire a code for a long time after severe drivability problems, if ever. You can still start troubleshooting these with a scan tool (for instance watching the MAP sensor, if available, at idle and 2500 RPM to find a restricted exhaust) or just use the other diagnostic tools in your box.

A code by itself is very rarely enough, maybe not 1 in 50 times, enough
information to buy a part from. If I were AutoZone, I'd do it for free
also. Not knocking AutoZone, just trying to say that a code is amost NEVER
enough to buy a part from.

 

True.

Code readers aren't the be all and end all of diagnostics, but then again, I don't have several grand for a big time computer, either.

I have an EGR "insufficient flow" on my 94 Ranger. Beofre I spend $70 for a DPFE sensor, I made sure that all the EGR related junk was okay.

Found a hose from the DPFE to the EGR tube that had its bottom half cut off. Whew! Thought I had an exhaust leak as much of that smell was under the truck! Replaced both hoses with 5/16" tranny cooler hose and after letting the computer flash back at me for a day or two (got lazy), I pulled the ECM fuse and cleared the code.

For the cost, my multimeter pulled the code fine and I gave myself a big hug for diagnosis!