DiagnosticCodes
Diagnostic trouble codes (DTCs) are you vehicles way of letting you know that something it senses doesn't seem right. It does not tell you, "Hey X part is faulty". Instead it gives you a starting point for further testing. Example, just because you are getting a code saying that your o2(oxygen) sensor isn't working, it does not simply mean that the sensor is bad, it could be a faulty wire/connector/fuse, a faulty sensor, or even another failing part. It's just the beginning of the diagnostics procedure.
96-05 Astro/Safari Vans use OBD2:
The cheapest way to read these is to go to virtually any auto parts store and they will usually do it for free, some shops will do it free as well. However, I recommend getting a cheap BAFX bluetooth OBD2 reader and software on your Android smartphone or tablet, that way you can do it at home, and also have access to live data. Leave the adapter in your vehicle(not connected) for any time you want/need to read/reset the codes or to read the live data. I was pleasantly surprised to find out that Car Gauge Pro (DO NOT BUY)for Android DOES in fact also read the extended DTCs from the Astro, for things like ABS codes.
I used to recommend CAR GAUGE PRO as it handled the extended codes. However, they abandoned the app that we paid for and now are offering a "new" app, which has horrible reviews. That is some shady shit so whatever app you buy into, DO NOT BUY CAR GAUGE PRO.
If you are looking for a tool that will do the ABS automated bleed procedure, the Tech2 is the way to go, or the Tech2Win software that comes with the DXDIAG VX NANO for GM
$7 Adapter with USA shipping http://www.ebay.com/itm/OBDII-Scanner-Code-Reader-Bluetooth-CAN-OBD2-Scan-Tool-for-Torque-Android-ELM327-/400915655460 Note: these seem to have a high failure rate so you may want to look elsewhere, they did refund me for the ones that didn't work, but it took some pressing.
If you search around, you can see there are many suppliers of these devices, and different software(like Torque Lite(or PRO)) available. Quality varies, read the reviews if possible.
If you are having a problem where the device isn't powering up/connecting, check the fuse for the cigarette lighter/power outlets as they are generally on the same circuit, or verify on another vehicle.
Another cool thing about these cheap bluetooth adapters and running the apps on your phone/tablet is the gauges, you can watch many different sensors on a virtual dash board(like RPM, coolant temperature, etc). Oh and graphing can be quite handy as well.
What makes a DTC?
Active/Current DTCs are happening during current/last drive cycle where conditions were met.
History DTCs, will be cleared after 40 trips that meet the drive cycle criteria, 80 for trim and misfire.
Pending DTCs, faults found in the first trip of a two trip DTC.
Permanent DTCs, started in late 2000s, prevents people from clearing codes before smog/emissions inspections
OBD2 Modes:
OBD2 Mode 1 Data Display - Sensor inputs, actuator outputs, PCM fuel trim and misfire counts - NOTE: a circuit fault while DTC is active can subsitute a fail-safe value for missing data.
OBD2 Mode 2 Freeze Frame Data - Saves vehicle speed, engine temp/load/rpm, fuel trim, etc from when DTC was set.
OBD2 Mode 3 Diagnostic Trouble Codes - Displays DTC codes and status
OBD2 Mode 4 Clear DTC - Clears DTCs, erases/resets freeze frame data and monitors, may reset LTFT as well. NOTE: Save FFD before clearing
OBD2 Modes 5, 6, and 7 Monitor Test Results - continuous and noncontinuous monitors ready/not ready
OBD2 Mode 8 Functional Tests - bidirectional component/wiring tests
OBD2 Mode 9 Vehicle Information - Display Calibration/software version/VIN stored in PCM
Some have a 'System Check' mode - Narrow the PCM malfunction thresholds to allow quicker diagnostics.
First Character Letter codes are: P,B,C,U and rare E=P
P = Powertrain (Engine and Transmission)
B = Body
C = Chassis
U = Network codes
Second Character OBD2 code type:
0 - Generic
1 - Vehicle MFG Specific OBD2 codes
3rd Character = System or sub-system
0 Fuel and Air Metering Crank and cam position sensors, HO2S/AF ratio sensor heater
1 Fuel and Air Metering MAP, MAF, ECT
2 Fuel and Air Metering ~injector components, feedback operation HO2S/AF sensors and fuel trim
3 Ignition System or Misfire
4 Auxiliary Emission Controls EGR, EVAP, Cats
5 Vehicle Speed, Idle Control, Engine Timing, braking and PS input
6 Computer Output Circuits
7,8,9 Transmission and Transfer case
A,B,C Hybrid Propulsion
4th and 5th Character = Component/Fault description
11 range/performance problem
12 voltage low(short circuit)
13 voltage high(open circuit)
GM and other manufacturers have added “Symptom Codes” to the ends of the 5 character codes. Symptoms codes are additional 2 digit codes located after the DTC where the first digit is the symptom category and the second digit refers to the subsystem/more info.
There are 16 possible symptom categories available for 0 to 9 and A to F. Currently only 8 categories are used.
Category Name Description
0 General Electrical fault Standard wiring failures
1 Additional Electrical failures More standard wiring failures
2 Frequency/Pulse Width Modulated PCM inputs and outputs
3 PCM internal failures PCM Memory, software, internal electrical failures
4 PCM programming failures PCM software failures
5 Algorithm failures PCM/input comparison failuress
6 Mechanical failures Faults for inappropriate response to a PCM controlled output
7 Bus signal failures Signal integrity or bus hardware failures
8 Reserved codes Reserved for future use.
The second digit of these “symptom” codes refer to a specific problem area.
01 represents “short to battery”
02 represents “short to ground”
Reading codes on the Astro/Safari vans 95 and earlier:
Shorting the A and B pins on the ALDL connector with a jumper(paperclip) located near the drivers left shin causes the SES light to flash stored codes. See here for more info: GM ALDL Interface
12pin US ALDL Connector
Also, there are bluetooth and USB adapters available where you can get live data from them as well, just at a much slower rate, and the cost of entry is higher. RedDevilRiver_USB_ALDL, RedDevilRiver_BT_ALDL, and ALDLdroid for your Android device.
To make things even more confusing, some 94-95 vehicles(often referred to as GM OBD 1.5) use the OBD2 connector, but are actually OBD1(in this case shorting pins 5 and 6). And then some 94 and 95 models(not Astro/Safari) actually can be read by OBD2 scanners. What a confusing time those 94-95 years were.