I took my truck (2006 chevy silverado 5.3l v8) to chevrolet dealership due to the fact that it kept sputtering then dying. The check engine light was on the whole 9 yards.

The fuel pump has been replaced; the MAFs replaced, new air filter, new knock sensors, the O2 sensors tested good, new spark plugs, oil change, CATs were tested and came back good. I was out of options, so I took it to Chevrolet and they had it for 2 days saying they've never seen this before.

They finally called me and said my spark plugs on the driver side fouled out. I asked why, given that they were brand new? They said they didn't know, but it's fixed. Lo and behold, that evening it started sputtering again; it died again next day on the way to work while driving 60 down the road.

Monday morning the truck had to be pulled back to the chevy place. They called me next day and said I owe them more money as it had a wire ripped out of the wiring harness. I refused: they should've fixed it the first time and they had it last working on it. If a wire was ripped out, they should have seen that. As I said that, their whole demeanor changed and they claimed that this is now a whole different problem; the first one was due to an O2 sensor that caused my brand new plugs on the driver side only to foul out, but can't have caused the ground wire they found this time.

I don't believe them. I don't believe an O2 sensor being loose the first time caused it; I think it was this ground wire they found this time. They just want me to pay more money because they didn't bother to even look the first time.

What do you reckon?

  • 1
    You say that the O2 sensors tested good. How were they tested?
    – Zaid
    Jul 9, 2014 at 20:11

6 Answers 6


A loose O2 sensor will give a weak signal to its ECU. The ECU will then richen the mixture on that bank, possibly accounting for the plug soiling. If the vehicle is then road tested before release the subsequent faulty wiring would normally be found. Without being hands on with this particular vehicle it would be impossible to be definite about its faults or if to blame anyone. What I would say though is that if a vehicle has persistant and repetitive faults then one good way to get to bottom of things would be to run the vehicle with a data logger connected. This will give you a second by second view of any fault developing and go a long way to nailing it whilst sitting at your computer.


if you have a bad o2 sensor sending a lean signal to the ecu for one side of the engine, then yes is will drive the ECU to create a rich condition to compensate, fouling the spark plugs on that side. an exhaust leak can cause an o2 sensor to read lean, or possibly a short or bare wire in the harness, or defective o2 sensor. or perhaps it really does have a lean condition due to an intake manifold leak.


IMHO, O2 sensor can't cause damage. If signal is faulty - ECU must be working in safe mode. That is why your check engine lamp was light on.

But if you have bad high voltage wires (plugs - coil) then high voltage jumps through any weak points and can damage and even solder parts. It also can damage ECU and its electronic parts like condensers. I suggest you to change wires every 0.5-1 year, it is cheap but very important.

  • changing wires every year? isn't that a bit much?
    – chilljeet
    Feb 14, 2015 at 8:38
  • Just try and see difference Feb 14, 2015 at 8:40
  • 4
    In many cars I have never changed the plug leads, this is completely excessive. Also, many modern cars use COP (coil on plug) where only low voltages run in the wires, making changing them, unless damaged, redundant. Definitely every year is complete overkill unless you run some sort of boosted voltage that can strike through.
    – Elias
    Apr 15, 2015 at 14:38

If you are getting an O2 sensor code, you really need to start at the core of the motor, and have your intake, exhaust, and head gaskets redone. O2 sensors throw codes if there is an air leak, and it could be far from the O2 sensor. A valve adjustment may also be necessary. If it's dying while you're driving down the road, that sounds like your alternator. Also if you are jumping on the highway without letting your car warm up, your spark plugs can actually shatter.

  • I'm just wondering why you would start doing checks away from the O2 sensor for an O2 sensor issue? You should start at the O2 sensor and go from there. What you are suggesting is fixing things which don't need fixed first, then getting back around to the O2 sensor in the process. No way to diagnose ANY issue. Jan 24, 2015 at 18:01
  • Well most people would think that but, that's like replacing a light bulb every time your power goes out actually. Jan 24, 2015 at 21:15
  • 1
    And most people would be right. I didn't say replace the O2, I said to start your diagnosis there. You are willing to rebuild an engine to fix an O2 problem, which is bass-ackwards. Jan 24, 2015 at 22:39
  • I don't think the question contained anything about an O2 sensor code, so this doesn't really answer the question. And Paulster2 has a point, I had an O2 sensor code and the cause was obviously the O2 sensor.
    – juhist
    Jan 28, 2017 at 13:49

When an O2 sensor goes bad the ecu knows it and ignors it so only enextended driving period may cause the plugs to foul but certainley not in the short run . While on the other hand your injectors get + all the time from a common wire and the injections come from - that pulsates from the ecu each injector with its own wire . If these wires are shorted together or to ground (-) the injector (s) may be continuously open causing large amounts of fuel to flow into the affected cylenders and cause one side of the motor to foul while the other side works fine


The ECU will always default to a rich mix if there is faulty O2 readings.

While this can foul the plugs, dilute the oil, etc it will not cause damages while a lean mix could.

Now, cars can run without O2 sensors and the default tables, while rich, will not be rich enough to foul the plugs.

Something else happened: the readings could have been considered valid and the ECU compensated by making the mix very rich, fouling the plugs.

This can happen in a few scenarios:

  • The is a leak by the exhaust manifold. Once exhaust velocity reaches a certain threshold, the leak doesn't let exhaust out, but actually sucks air in; as explained by Bernoulli's Principle.

  • There is an electrical problem; an O2 sensor will have a voltage reading of less than 0.45v if there is oxygen in the exhaust stream. A faulty ground, or a short, can create this condition.

  • Ignition not working on that side; the mix is not burnt and there is still plenty of oxygen left in the exhaust stream. This will foul the plugs very quickly.

  • An injector stuck open; this will make the combustion fail and then you have a lean condition as well; essentially plugs that foul, don't burn the mix and then the O2 sensor reports a lean mix, so the ECU adds even more fuel.

One test you can make is to see if the car behaves differently when cold: O2 sensor readings are used only when the sensors reach their operating temperature, after a few minutes; if the car suddenly behaves differently when switching to what is called 'closed loop' (using sensors for feedback), it is an indication to look at the O2 sensors, if not look at the other options.

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