I have the USB ELM327 module linked to by Gaz above. It worked straight out of the box with the Forscan program. Make sure you have the ignition on (engine doesn't need to be running though). Also give it some time, takes a minute or so to finish scanning and connecting for me.
I've also ordered a $20 bluetooth version to try with my Android phone. Might be able to get one of the existing apps working using custom headers and PIDs, or write my own.
I'm going to make a list of the useful/interesting PIDs that I've tested and worked out the scaling for here for ease of reference. These are all mode 22 requests, so to test them you can use hyperterminal, set the headers as described in my first post, then type 22 followed by the PID. Here I will list the PIDs, the number of bytes returned, and how to make them into useful units. Also note all the respones from the
ECU are in hexadecimal, so have to be converted to decimal first.
1139 - engine coolant/cylinder head temp. 1 byte, multiply by 2 to get degrees farenheit. Then subtract 32, multiply by 5 and divide by 9 to get degrees celcius.
1123 - intake air temp - as above
1141 - fuel injector pulse width - 2 bytes - divide by 100 to get milliseconds
1156 - long term fuel trim percent - 2 bytes - divide by 2.56 then subtract 100
1158 - short term fuel trim percent - 2 bytes - subtract 32768 then divide by 327.68
163C - fueling status - 1 byte - 01 = open loop, 00 = closed loop
116B - timing advance - 2 bytes - divide by 2 to get degrees BTDC
11C1 - vehicle speed - 2 bytes - divide by 320 to get km/h
1165 - engine rpm - 2 bytes - divide by 4
1173 - O2 sensor voltage - 2 bytes - divide by 3200 to get volts
092C - transmission fluid temp - 2 bytes - degrees C (not 100% sure on this yet but it seems about right)
1440 - intake manifold absolute pressure (I6 only) - 2 bytes - divide by 4 to get kPa
1172 - battery voltage - 1 byte - divide by 16 to get volts
More to come when I get a chance to test them!