Bests & Worsts Reviews from Amazon

This is a basic OBD code "reader", not a ScanTool. It can read an OBD code number IF an error has been stored by your vehicle and you can view or reset it. If the check engine light isn't on due to a stored OBD code/value out of spec then it wouldn't have much info to tell you. As an entry level code reader it can't read any of the useful data available via the OBD protocol such as engine temp, rpm, percent engine load, throttle position, oxygen sensor voltage, lean, rich fuel data, MAF sensor, EGR valve position, and so forth. Most vehicles have over 80 sensors and variables that can be displayed and slightly higher end OBD ScanTools (some starting at $65) can read those variables which is very useful in fixing your car. The model 3100 retails for nearly $90 so that isn't the best value considering the limited feature set. This is a "Code Reader" and as such there's no real-time display of engine parameters other than if the O2 sensor (fuel control) is in open or closed loop mode. "Open loop" engine control means the engine runs on stored fuel delivery set points based on throttle position, temperature and other inputs. After the O2 sensors warm up enough to start functioning then the vehicle goes into "closed loop" mode using actual feedback from the O2 sensors to adjust fuel rich/lean mixture as needed. A code reader like this one is useful for translating a check engine light into a number and it provides a useful short text version of the meaning such as "O2 sensor bank 2 is low". That can point you in the right direction of what the issue is. Meanwhile an OBD Scanner can read the code and sensor parameters as well. So if your O2 sensor voltage is slow to respond (for example) a scanner could read that but a reader can't. On scanners you can also see the value change if you unplug the sensor so you know you're replacing the right one. The main downfall of this type of tool is that if the check engine light isn't on then the reader won't have anything to report. It will just say "There are no codes stored". Much of the functionality of OBD tools is provided by the vehicle itself. The vehicle computer monitors engine sensors, checks the data vs known acceptable ranges and triggers the check Engine light if a value is out of range. Then it stores a brief snapshot of sensor data at the time the code is set. If the problem clears up the vehicle will eventually reset the check engine light itself after enough drive cycles of fully warming up the engine have occured over a period of days. If the problem persists then the check engine light stays on. If the check engine light is burned out an OBD tool will still tell you the status. Overall there are some better products available for less money such as an entry level OBD Scanner. Some can be left plugged in like a set of extra gauges that can display MPG, ignition timing, percent load and other parameters that you select such as the UltraGuage for about $65. Some ScanTools can display and store engine parameters for later review or print out using your computer so you can talk it over with your mechanic. Higher end units also interface to other vehicle computers such as the transmission, body control module, ABS and others. If you already own this tool hopefully it served it's purpose and pointing you in the right direction based on the trouble code. If you're in the market for an OBD reader I suggest considering a scantool instead. Those are a better value with more features and some are cheaper than a code reader anyway.