You've got a valid question. The service engine soon light only turns on when an OBD II trouble code is present in the engine control computer (ECC). Trouble codes are only triggered when the ECC finds a problem that can cause engine emissions to increase. A trouble code P300 refers to a random miss-fire. It's a very common trouble code. It triggers at the slightest variance between the crankshaft and camshaft RPM ratio and uses knock sensor data to validify the fault. With that said, most often when you take a vehicle in for repairs for this trouble code, the code is cleared by the repair technician and the vehicle owner asked to drive the vehicle in order to see if the fault still exists; this is of course after a preliminary diagnosis is conducted and no obvious engine trouble found.
In California, 2000 & newer vehicles are administered the DAD OIS smog check system. The OIS inspection electronically checks the vehicle's ECC for trouble codes, readiness flag data (emission monitors) and requires the technician to conduct a visual inspection of all emission components, most importantly the catalytic converter and its California Air Resources Board executive order number (EO).
Here's what you might want to do, since you have a scan tool and if your mechanically inclined , you can reset the ECC, get rid of the P300 and turn off the service engine soon light. What you'll need to do next though might consume some time. Resetting the ECC, clears trouble codes but also erases emission monitor data. Without your Chevy having all it's emission monitors set (except for the EVAP) it won't pass the CA smog check. To get the monitors to be ready for the smog inspection, you'll need to drive your truck through what is known as a drive cycle. We've posted the Chevy/GM drive cycle below, should you choose to use it. Ultimately, we always recommend having a qualified smog check station perform smog related repairs, specially when repairs involve trouble code(s) and computer data.
How to Perform a Chevy Silverado Drive Cycle
The Chevy Silverado drive cycle below will "complete" or make "ready" your truck's emission monitors, including the EVAP monitor, Oxygen Sensor monitor, EGR monitor, and Secondary Air Injection. Chevy uses the GM (General Motors) drive cycle.
Take caution when performing drive cycles on public roadways. If you feel you will be unable to complete the driving pattern above safely, seek assistance from a reputable smog check repair station or Chevrolet service center.
Start Here:
COLD START: Begin from Cold Start, and ignition in OFF position for at least 1 hour. Ensure your Chevy Silverado's fuel tank is between 1/4 and 3/4 full. Engine coolant temperature must be below 122°F and within 11°F of the ambient air temperature. Do not leave the key on prior to the cold start or the heated oxygen sensor (O2) diagnostic may not run.
IDLE TIME: Run engine (do not drive) for 2 1/2 minutes with the air conditioner on and rear defroster on. During this period the ECU will be running the Oxygen Sensor Heater self-test, Air Injection System Passive Air test, EVAP Purge "No Flow" test, Misfire Detection.
ACCELERATE: Turn off the air conditioner and rear defroster and begin driving your Escalade. As soon as possible apply half throttle until 55mph is reached. The ECU will now run self-tests on the Misfire Detection, Fuel Trim, and EVAP Purge Flow.
HOLD STEADY: Hold your speed steady at 55mph for three minutes. During this time the Oxygen Sensor O2 response, Air Intrusive (AIS System), EGR Flow, Purge Valve, Misfire Detection, and Fuel Trim diagnostics will be performed.
DECELERATE: During this step slowly let off the accelerator pedal. Do not brake or shift in order to slow down. It is important to let your Chevy coast along gradually slowing down to 20 mph. The ECU will once again self-test the EGR System, EVAP Purge System and Long Term Fuel Trims.
ACCELERATE: Go ahead and accelerate again at 3/4 throttle until 55-60mph is reached. This will perform the same self tests as during the first acceleration step.
HOLD STEADY: Hold steady speed once again. This time at 55mph for five minutes. During this time, in addition to running the self-tests as during the first HOLD STEADY step, the ECU will perform the catalyst monitor diagnostics. If the catalyst (Catalytic Converter) is marginal or the battery has been disconnected, it may take 5 complete driving cycles to determine the state of the catalyst,or in order to Complete the CAT Monitor.
DECELERATE: This will perform the same diagnostics as in the first Deceleration. Remember not to press the brakes or shift gears (don't press the clutch if equipped).
This ends the drive cycle. Performing the drive cycle on your Chevy Silverado at least twice will ensure all monitors run, including the EVAP monitor, and your SUV is ready for the smog check.
Completing Emission Monitors Individually
Catalyst Monitor (CAT):
Prerequisite: No trouble codes present, Intake Air Temperature (IAT) above 32 Degrees (F), Engine Coolant Temperature (ECT) is above 167 Degrees (F); Mass Air Flow sensor is indicating greater than 15 gm/s, engine load is below 63% and steady, engine speed is below 3000 RPM.
1. Idle vehicle 5 minutes (achieve closed loop)
2. Drive 5 min at steady speed above 55 mph
3. Drive 2 min at steady 40 mph
4. Drive 2 min at steady 55 mph
EGR Monitor (EGR):
Prerequisite: No trouble codes present, Engine Coolant Temperature (ECT) is above 75 Degrees (F); barometric pressure (BARO) above 72kPa.
1. Idle vehicle 5 minutes (achieve closed loop)
2. Accelerate and drive for 1 min at steady speed above 50 mph
3. Slow vehicle down to 20 mph. Do not use brake clutch, or shift gears.
4. Repeat steps two and three 5 times.
GM/Chevy/Chevy run their EGR monitor during the deceleration portion of the drive cycle.
Evaporative Monitor (EVAP):
Prerequisite: No trouble codes present, Fuel tank between 1/4 and 3/4 full.
1. Drive 5 min at steady speed between 30 and 60 mph
2. Stop vehicle and idle engine for 4 minutes.
3. Turn engine off. Remove ignition key.
The ECU will run the EVAP Monitor once the engine has been shut down and ignition turned off.
O2 Sensor Monitor (O2):
Prerequisite: No trouble codes present, Intake Air Temperature (IAT) above 32 Degrees (F); Engine Control Temperature (ECT) is above 167 Degrees (F)
1. Idle vehicle 4 minutes (achieve closed loop)
2. Drive 3 min at steady speed above 50 mph
3. Drive 2 min at steady 40 mph
4. Drive 2 min at steady 50 mph
