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07 trailblazer, 4.2l engine, 125k miles. Initially threw a p0306 code, easy fix i thought. got home, tested coil and sure enough it was bad. replaced it. next day starts throwing a p0300 code with an audible ticking sound; **** me right. so i just decided to do a tune up, replaced all coils and spark plugs. replaced fuel filter and did an oil change. start the vehicle up and still a p0300 code. check plugs and coils again, all good. it does bog down after a minute and there is smoke coming from exhaust. pulled throttle body and cleaned it. still same when i start it up. could it be the vvt or water pump? it doesnt over heat and oil pressure is above 30. i am dreading the possibility its the valves or other engine problem. any help would be welcomed.
 

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I've had this problem happen with a bad upstream (B1 or B2) oxygen sensor.

B1,S1 is the value you should test... the activity may be slow to change or stuck at a flag-able position where it sets P0300 because it affects all cylinders of an inline, I- or L-style engine.
  • This is not quite like the pair of 1st position O2 sensors --- such as a Vee-style engine which will have one in each bank --- eg: B1 & B2
  • AFAIK, there is only a single B1 sensor on our 6 cylinder inline engines and I'm still on a learning curve with these new-to-me engines
  • Typically --- A lazy sensor will raise a Boolean Flag in the ECM, ie: a PENDING CODE if it's not too far below a preset slow value where it would then set a HARD FAILURE and either a Flashing- or Full-time lit SEL/CEL and MAYBE a REDUCED POWER light.
  • A flashing CEL MUST BE FIXED! Mandatory repairs are needed now!
To prove B1,S1 is bad, shut off engine and disconnect the wires to that B1,S1 HO2 sensor. Restart the engine and the ECM will pull up a DLL and will more than likely run better even though the CEL will still be ON. This test works for all pre-cat, or B- designated sensors

SIDEBAR INFORMATION -----> However B1, S2 POST CAT sensor should be fairly inactive at a sweet spot around 50% of it's nominal voltage if it and the catalytic converter are both working decently.

If memory serves me, GM uses the number of times the oxygen sensor moves past the null point as a viable testing (by the ECM) point to test if the sensor is good or too lazy. Low CROSSCOUNT values are valid criteria for popping a code for a slow sensor. This MAY bring up a twin code P0300 at the same time.... so beware of that!
 
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Discussion Starter #3
tested up stream o2 sensor and sure as **** it is bad. unplugged and ran it and she did run better. replaced it today and now she doesnt bog down or smoke. Still getting p0300 code, flashing check engine light and a ticking sound. She also smells lean.
 

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OK - let's review what the P0300 actually means and the causes...

  • Faulty spark plug wires or coils *
  • Distributor failure *
  • Faulty fuel injector *
  • Vacuum leak
  • Low fuel pressure
  • Camshaft sensor defective
  • Crankshaft sensor defective
  • Engine timing off
  • Leaking head gasket
  • Low engine compression
  • Poor quality fuel
However, loose fitting electrical connectors and broken or disconnected vacuum hoses are often overlooked.


In chronological order of things to do to diagnose this code,

1. Remove all the spark plugs and find the one or two that are obviously different in color. because the ECM isn't reporting any particular misfiring plug and it may be a couple or just one of all of them 'at times'.
  • Black or sooty means lack of ignition, but has fuel that is not being burned either completely or at all.
  • Dry-tan spark plugs are normal and there may even be a small amount of modifier deposits on the porcelain. These will usually 'fluff off' as the engine gets to normal operating temp and sees some decent RPM.
  • If too many plugs are wet, dirty or fouled or just plain old - replace them all at this time.
2. Inspect the primary wires going to the individual ignition coils for cracks, corrosion, broken wires, etc.
  • Test the individual ground circuits on each COP unit using a good digital Ohm meter.
  • If you have a master NOID test light you can check each coil for ignition signal from the ECM with it.
  • Ask in case you want to know what a NOID light is.....
The smell you're getting is likely the catalytic converter telling you it is achieving some normal incandescence and trying to burn off excess fuel collected on the rare earth metals inside it.

Since the exhaust is no longer smoking, the cat has collected a lot of unburned/excess fuel and is trying to clear itself out. It may or may not succeed.
  • The FLASHING Check Engine Light is warning you that the cat is in trouble.
That should be enough info for you to be busy for a day or so.

Let me know what you find.
 

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Discussion Starter #5
Thought I'd post since I found the culprit. No pressure in cylinder 6. 125k engine is dead. Took to a mechanic, didn't tell him what I found and he found the same thing. Ty for input.
 

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Sad to hear. Thanks for the update.

I wonder what type of failure the cylinder had.
 
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