Digital Diagnosis
Michael Sean, Warwick, NY: I have a '94 Chevy truck with a bone-stock 350 TBI. I just got back from the repair shop with a fat bill for $700 from work I probably could have done myself. The shop replaced the EGR valve, an oxygen sensor, and the IAC valve, then tacked on the labor. My problem lies in the diagnosis. The dealer had expensive diagnostic equipment that I did not have. Are the handheld machines sold in the auto-parts stores accurate and reliable? Is one type better than another? How much should I expect to spend?
Jeff Smith: You've hit upon the key to making intelligent repairs to virtually any car, Michael. Most anyone can replace a component if he knows which part has failed. The trick is in determining the root of the problem. In effect, a good technician can make very quick repairs if he is good at diagnosing. Most of this comes from experience, but computer-controlled vehicles have introduced a slew of diagnostic tools. There are several tools out there that do a great job and don't cost as much as you might think. We've used the Actron CP9145 scanner for OBD-I and OBD-II (onboard diagnostics I and II) cars before with good results. OBD-I are '84-'95 vehicles, while '96-and-newer fall under OBD-II status. This handheld device plugs into the Assembly Line Diagnostic Link (ALDL) port that is usually located underneath the dash on the driver side. This unit will not only indicate trouble codes and allow you to clear them off the computer, but it also displays a wealth of real-time information, such as rpm, coolant temperature, vehicle speed, oxygen-sensor rich/lean indicators, converter clutch activation, and many more parameters that can be used for diagnosing a problem. The CP9145 sells through Summit Racing for $269.95. If that's a bit much to pay for a tool you may not use all that often, Actron also sells an OBD-I code reader for $82.39 (PN CP9125, Summit Racing) that merely indicates which error codes have triggered the check-engine light. There's also a code reader for OBD-II cars (PN CP9175) that sells for $149.95 through Summit Racing.
Another interesting tool we found in the Actron catalog is a tester (PN CP9087, $59.95 from Summit Racing) that can check TPS, oxygen sensor, MAP, MAF, knock, and even crankshaft-position sensors. If you work on later-model cars that are littered with these sensors, this little tool would quickly pay for itself in terms of diagnosing problems.
The Actron CP9145 scanner tool uses power from the cigarette lighter and a simple connection to the car's ALDL port to access all the diagnostic information from the vehicle's onboard computer. This unit will not only read the trouble codes but allow you to clear them as well.