Hard start due to bad O2 sensor?

I have a 2000 Chevy Prism (same as the Corolla) which has had the check engine light come on occasionally for over a year now. It started turning on more then normal about 2 months ago so we brought it to the dealer. They told us the car needed a new catalytic converter and O2 sensor. But because the car was running fine and getting great fuel mileage, we decided to just roll with it.

About 3 weeks ago while we were on vacation in CO the car began to have trouble starting. We took it to a mechanic, and he told us that we had a bad O2 sensor which was causing the problem, aggravated by the high elevation, and that it wasn’t a major issue and we should just take care of it when we got home.

Now that we’re back home the problem persists. I’ve done a little research on the problem and I don’t understand how a faulty O2 sensor could be the cause of a hard start. From what I am reading the O2 sensor doesn’t even turn on for at least 30 seconds after the car is on!

Should I go ahead and replace the O2 sensor or is there another problem with the system I should be looking at.

First, find what the error code(s) is(are). That will tell you what part of the system is having a problem, but not specifically what to replace/repair. Post the code back here for help. It will have an ID# like P0123. If it is an O2 sensor code a good mechanic will look at the signal coming out of the O2 sensor(s) to tell what’s what.

You did not state how many miles are on the car but I don’t think the O2 sensor(s) have anything at all to do with the hard starting problem.

By hard starting, do you mean it cranks over for a bit before finally starting? If so, this could be a loss of fuel pressure issue (leaking fuel pump, fuel pressure regulator, injector, etc.).
Since the car runs fine and fuel mileage is great I’d be leaning towards a fuel pump issue rather than the other ones I mentioned.

It would also help to drop by a local AutoZone, Advance, Checkers, etc. and have them scan the car. It only takes a few minutes and they will do this for you free. Post any results back here for discussion. We need to try and determine if the converter/O2 sensor diagnosis is possibly even valid.

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The Prism has 120K on it. And yes, instead of starting right up it cranks over for5-10 seconds before it starts up. Interestingly we have found that if we let it turn over for a few seconds and it won’t start, then we stop turning the key, then it will start up right away… most of the time.

I suppose the fuel pump would not be out of the question. We had to replace the filter and pump when we lived in Flagstaff where there is poor quality gas at many stations which gunked up the fuel system. However, when we had those problems we noticed a significant loss of fuel millage and performance, not so this time. We got 37 mpg on the way out to CO, and 35 on the way back, which is about as high as we have ever gotten.

I will try to top by an auto parts store later today or this weekend and see what error code comes up.

Try this: Turn the key to "ON, wait 5 seconds, then turn the key to “START.” If the engine starts normally the problem is loss of pressure in the fuel line overnight or when the car sits for a time. There is a check valve in the fuel system to maintain fuel pressure at the injectors. If the valve is leaking, and your Prizm is old enough, the pressure seeps away and it takes a few seconds for the pump to pressurize the system when you turn the key.

The CEL is probably on because of the O2 sensor, but I don’t think this is contributing to the starting issue.

Turning the ignition on for five seconds does nothing to reprime a leaked down fuel system. That’s because the fuel pump only runs once for 1-2 seconds each time the ignition is turned to the run position. To reprime a fuel system that has leaked down requires turning the ignition to the run position for two seconds and then turning the ignition to the off position. Then repeating this procedure a half dozen times so the fuel pump runs a half dozen times to reprime the fuel system.


I agree with mcparadise’s suggestion about how to test for loss of fuel pressure and about the fuel pump check valve. Loss of fuel pressure means air in the lines and it takes a bit to purge itself. (Think of a garden hose lying outside, turn the water on, open the nozzle, and water will sputter and cough until the hose is purged of all air. Same thing.)

In the event that the problem is not related to loss of fuel pressure another possibility based on the additional info is a faulty ignition switch (the electrical part). It’s possible the contacts inside could be getting worn and/or burnt and this could be leading to a loss of power to the ignition coil(s) when the key is in the START position.

It’s doubtful that the oxygen sensor is contributing to your hard starting problem. The O2 sensor doesn’t come into play until well after the engine is started. When you start your car, the computer is excluding some of the sensors from the mix, including the O2 sensor. After the engine has been running for a while, and the O2 sensors have heated up to the correct temperature (which they need to be at to give accurate readings), then the computer ‘goes for it’ and puts the engine in “closed loop” mode, where the O2 sensors determine the fuel mixture based on the remaining oxygen in the exhaust gases, indicating whether the combustion process is efficient or not. So to sum up, O2 sensors have no role in staring the engine.

If you can fix the problem first, you can change the O2 sensor later. You may have to try changing the ignition switch first, seeing as no mechanic can do better at finding the problem. Somebody should read the codes.

I had issues with my Pontiac not wanting to start sometimes not at all but sometimes if I could get it revved up and hit the gas quick enough it would stay on but I have replaced a bad O2 sensor now and now it is cranking fine

Other than fouling the plugs I can’t imagine any reason that a bad O2 sensor could cause extended cranking to start. That diagnosis seems a real stretch.

Now This Sound About Right I’m having the same problem

Not working for me.

Yeah most problems go away given sufficient time. I suspect the Prism is a washing machine now.

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