Driving Cycles are an important part of diagnosing your vehicle, especially when you’ve got phantom OBDII codes or stored codes that must be retrieved. When you own or have a OBDII GM GEN III shortblock in your vehicle, many times these stored DTC’s can be difficult to reveal.
Any vehicle manufactured after January 1, 1996 are OBDII compliant and can require a driving cycle during troubleshooting. Some 1995 model year vehicles are also equipped with OBDII – although some pre-1996 OBD-II cars are not 100% compliant.
Pre-OBD-II cars had connectors in various positions under the dashboard and under the hood. All OBD-II cars have a connector located in the cabin meant to allow a mechanic to plug into the ECU by way of a datalogger or scanner. This J1962 connector should be connected to your scan tool of choice, while GM engines specifically use the SAE J1850 VPW (Variable Pulse Width Modulation)
That’s why our tutorial on performing a GM OBDII Driving cycle is here, to show you how to perform this driving cycle. You must perform this driving cycle to pull all codes from monitored systems and can usually be completed in less than fifteen minutes or less.
In order to be classified as a cold start the engine coolant temperature must be below 122°F (50°C) and within 11°F (6°C) of the ambient air temperature at startup. Do not leave the key on prior to the cold start or the heated oxygen sensor diagnostic may not run.
The engine must be run for two and a half minutes with the air conditioner on and rear defroster on. This Idle portion of our How To Perform a GM OBDII Driving Cycle, is vital to “setting the table” for your driving cycle. The more electrical load you can apply the better. This will test the O2 heater, Passive Air, Purge “No Flow”, Misfire and if closed loop is achieved, Fuel Trim.
Accelerate part 1
Turn off the air conditioner and all the other loads and apply half throttle until 55mph (88km/hr) is reached. During this time the Misfire, Fuel Trim, and Purge Flow diagnostics will be performed.
Hold a steady speed of 55mph (88km/hr) for 3 minutes. During this time the O2 response, air Intrusive, EGR, Purge, Misfire, and Fuel Trim diagnostics will be performed.
Let off the accelerator pedal. Do not shift, touch the brake or clutch. It is important to let the vehicle coast along gradually slowing down to 20 mph (32km/hr). During this time the EGR, Purge and Fuel Trim diagnostics will be performed.
ACCELERATE PART 2
Accelerate at 3/4 throttle until 55-60mph (88-96 km/hr). This will perform the same diagnostics as in step 3.
MAINTAIN SPEED Part 2
Hold a steady speed of 55mph (88km/hr) for five minutes. You want your cataytic converter to light off here, or get your vehicle hot enough for it to activate. This section of our How To Perform a GM OBDII Driving Cycle takes into account the catalyst monitor diagnostics. If the catalyst is marginal or the battery has been disconnected, it may take 5 complete driving cycles to determine the state of the catalyst.
DECELERATION part 2
This will perform the same diagnostics as in step 5. Again, don’t press the clutch or brakes or shift gears.
How To Decipher the GM DTC OBDII Trouble Codes
The GM DTC is made up of 5 digits. The chart below shows the composition of the GM DTC. Armed with this chart, you can now decipher what system to trouble shoot without knowing the exact description of the code.
• X X X X X
• | | | |
• | | | |
• B Body ———-+ | | +– Fault (00 to 99)
• C Chassis ——-+ | |
• P Powertrain —-+ | |
• U Network ——-+ | +—— 1 Fuel & Air Metering
• | +—— 2 Fuel & Air Metering (Injector)
• | +—— 3 Ignition System or Misfire
• 0 SAE ————–+ +—— 4 Auxiliary Emission Controls
• 1 MFG ————–+ +—— 5 Vehicle Speed & Idle Control System
• +—— 6 Computer Output Citcuit
• +—— 7 Transmission
• +—— 8 Transmission
• +—— 9 Reserved
+—— 0 Reserved
There are two categories of DTC’s that apply to OBDII.
1. Emissions related.
2. Requests illumination of the MIL after one failed driving cycle.
3. Stores a freeze frame DTC after one failed driving cycle.
4. Emissions related.
5. Sets a Pending Trouble Code after one failed driving cycle.
6. Clears a Pending Trouble Code after one successful driving cycle.
7. Turns on the MIL after two consecutive failed driving cycles.
8. Stores a freeze frame after two consecutive failed driving cycles.
Have any more questions about the GM driving cycle or retrieving stored or phantom OBDII codes from your GEN III or GEN IV GM PCM? Need the best OBDII scan tool for your GM, but don’t know which one to go with? Leave your questions below!