Problems with o2 sensor

The car is an Opel Tigra 1.6 16v, 2000. yr, xel engine. Diagnosis shows codes for o2 and tps and the symptoms that are showing are next: When cold, sometimes, it takes a bit more cranking to start and the ignition is not clear if you understand me, also, sometimes, when cold, while changing gears usually from 2nd to 1st or 3rd to 2nd it drops revs to about 500 and gets back up or shuts down completely, but that is rarely the case. I changed spark plugs two months ago and put 4 new NGK ones with two electrodes if that has any difference. What I have seen with old ones which were put last year at the beginning of the year (cheapest bosch ones) is that all 4 of them were black, and the one in the third cylinder was soaked wet (about a month before changing these my mechanic cleaned fuel injectors and told me that one of them was only cleaned at 98% which shouldn’t be concerning by his words). What I hadn’t mentioned is that the engine is consuming oil but at low levels which I don’t have money to fix or check because of studies. My question here is, could the o2 sensor be the cause of random stalls and rev drops or longer cranking times when cold?


The O2 sensors don’t come into play until the engine warms up. The computer goes into the closed loop mode when that happens and then starts adjusting the air/fuel ratio from the signal(s) from the O2 sensor(s)


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Thanks for answering.
Any idea what else could be causing these?

When the engine is cold, the computer uses the signals from the coolant temp sensor, crankshaft position sensor, throttle position sensor, and the MAP/MAF sensor.

The hard starting when cold could be caused by a leaking anti drain-back valve on the fuel pump.

To test for this, the next time the car has sat for some time and the engine is cold, turn the ignition switch on so the dash lights turn on for two seconds and then turn the ignition switch off.

Repeat this a half dozen times and then try starting the engine.

If the engine starts right up, the anti drain-back valve is leaking as the vehicle sits.


Since I get tps code, that will be the first thing I fix, thanks for that part of the answer I didn’t knew that.
For the second part I already tested that and no difference, I think I read somewhere that on my car the fuel pump doesn’t get primed on ignition, but while cranking and I think that that is true because I can’t hear no humming or sound from the pump when turning the key on ignition

When the ignition switch is turned on before starting the engine. the computer commands the fuel pump to run for 1-2 seconds and then shuts the fuel pump off. This is to build the proper fuel pressure.

Then when the ignition switch is turned to the start position, and the computer see’s the crankshaft is rotating from the signal from the crankshaft sensor, the computer turns on the fuel pump so that engine starts and runs.


No experience w/you car, but presuming n/a gasoline engine w/automatic transmission. “Black” to one person might be “grey” to another. Suggest to post a photo of all four plug tips, focused on the electrode area. Are you able to tell if the “wet” appearance is caused by gasoline vs oil? I’m thinking you may have some oil getting past the valve stems. Do you notice an unusual amount of smoke out the tailpipe on initial start-up after the car has been sitting overnight, then it gradually goes away over 5 minutes of driving? This “wet” appearance on number 3 would be the concerning finding to me. However it is possible this is just the nature of this particular engine as well, the way the internal passages are oriented and the slope of the engine may be causing more oil to drain past number 3 valve stem than the others.

Another possibility is injector 3 is leaking fuel into its cylinder when the engine is off. That could cause hard starting and a period of engine sputtering after it starts. Wouldn’t explain the weird engine behavior during shifting though. A fuel pressure test would confirm/disprove.

The most bang-for-your-buck diagnostic test is probably a fuel trim test. Once you get that info, you are welcome to post it here for more ideas.

Other than when the fuel rail is purposely depressurized, I never hear any sound from my Corolla’s fuel pump before the key is placed in “start” either.

Yes, tester is spot on. The only additional thing I would add is a leaking injector can cause extended cranking too by bleeding off the fuel pressure. They cleaned the injectors? Why?

Giving the specific codes that you are getting is helpful. There are quite a few codes for O2 sensors and TPS sensors. There are also codes (like P0171 and P0174) that people often think are about O2 sensors - and they very well might be, but aren’t necessarily so.

The fouled spark plugs are causing the engine to run poorly, find what is causing the spark plug to be fouled.
You have a vague description of the computer faults, a precise recall of the faults will help in diagnosing the engine performance issue.

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Thank you guys for your answers, the code for tps is P0120-2, the one for o2 I don’t exactly know as how my mechanic told me, his diagnostic tool broke while he was doing a dagnosis haha

They did that on my request

I’d like to post the picture, but currently I’m not in the same state as my car is. To me it looks like gasoline but not to my mechanic who is telling me to take off the head and replace all gaskets

Car is 24 yrs old.
Is in another state?
So, u aren’t driving it? Who is?

Does that information have any connection with the solution to my problem?

These engines are known to consume oil, but since my last oil service, it didn’t consume any. I passed maybe about 1200km since then. If I got you all right, the source of my problem would be fouled spark plugs. I’m thinking about opening the engine up in the future because I would really like to make this thing reliable. I also noticed that I could smell some petrol on the oil-level stick, but this oil was in the car before I cleaned the dirty injectors so could that be the cause of those dirty injectors or I’m looking at possible bad piston rings. What I’m trying to say is that if I’m meeting with “only” bad valve seals I’d be ready to take the head off and change the seals but if there is a possibility that piston rings need replacing that would be a whole other problem to me.

It may be possible — depends on the engine configuration — to replace the valve stem seals without removing the cylinder head. The valve stem seals on my 4-banger 1.6L VW Rabbit were replaced without removing the cylinder head. Something to ask you shop. If you feel he piston rings may be problematic, ask you shop if there’s a test they can do to confirm/disprove that idea; i.e. before trying to effect a repair. If the test indeed shows the piston rings need fixing, also ask the shop if it would make more sense to replace the engine , used from the wreckers, rather than repairing the existing engine.

I expect you already know that w/you separated from your car, and unable to provide the requested info & photos, any help here is limited as best. Best of luck.

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You don’t need to remove the heads to replace the valve stem seals.

Use the ole rope trick.



Is the head gasket leaking or is this speculation? If the head gasket is leaking, there is no reason to discuss the oxygen sensor.

No it doesn’t leak