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O2 sensor or fuel pump?

My brother in laws brother in law has been driving and fixing my car in return for using it while I’ve been out of town. He tells me that either the O2 sensor or the fuel delivery system(fuel pump?) is bad. It was missing on a cylinder with the check engine light on,both intermittently right before I gave it to him to borrow/fix. Also previously even before my sister in law gave us this car it had a slow acceleration issue while cold and only from a dead stop. I’ve felt like checking the O2 sensor before this but got sidetracked and it’s not making the telltale whine of a bad fuel pump. My ex mechanic brobroinlaw hasn’t had time or money to do the repair and I need it fixed quickly. I’m no stranger to a wrench, what should I do? Can the sensors just be cleaned? I’m low on cash myself.

The symptoms are: misfiring on X cylinder(s), and slow acceleration, with check engine light on, yes? And the model year of the car is? And it’s a 4, or 5, or 6 cylinder?
Get the trouble code read at an auto parts store (etc.) and let us see it. Then, someone might have an idea. OK?

Follow hellokit’s advice and scan for codes. It sounds to me like you are blindly guessing, and this gets to be a very expensive way to go about diagnosing a problem.

Though I would say that if any of the basic maintenance items on the car aren’t up to date (plugs, wires, filters) you should just do those as a matter of course.

It’s a 99 model toyota corolla 4 cyl. I’m trying to get a hold of my brobro to get him to take it to be put on a computer. Clarification:slow acceleration from a dead stop mostly while cold for over a year, past two weeks missing on a cylinder with the check engine light on. No noticable whine like a bad fuel pump (if that’s actually not an urban legend). Thanks for the quick replies.

Missing on “a cylinder” as in one? IF you’re sure it’s just one cylinder, which cylinder is it?

How many miles?

When were the spark plugs, plug wires, distributor cap last changed?

If your fuel pump were going out, you’d be missing on ALL cylinders. And a bad 02 sensor would cause higher fuel consumption and eventually trash the catalytic converter.

Find out which P030X code is being thrown and scrutinize the plug, wire, and compression in that cylinder.

Plugs and wires checked out OK. Has had high fuel consumption just in the past 2 weeks. Brobro is supposed to get it on the computer today. The car is in another town altogether right now and I can’t check it out personally. I’ve had a gut feeling to check the O2 sensors because of the slow acceleration issue anyway. Hopefully we can turn some wrenches on saturday, thanks guys and I’ll keep you posted.

Some of the chain auto parts stores will scan the engine computer, for codes, for free; or, they will lend the scanner for the customer to do it. Ask. Bring the codes, like P0303, here for comments/advice.
A sudden, rich-running engine? Check the fuel pressure regulator. Having the repair manual, such as Haynes or Chilton’s, is a real bonus, to you.

I don’t know if this matters but we had an extreme cold spell immediately before it started acting up. I’m wondering if the extremes of hot then cold created a flaking condition from expansion and contraction fully fouling the sensor. If I’m off base let me know.

OK, changed the front o2 sensor, spark plugs, and air filter. Performance has improved but not back to normal. I’m about to buy new wires in the morning (store was closed). The list of codes before replacements is: P0300,P0420,P0203.

Do I need to reset the computer by unhooking the battery? Could it reset something that could be triggering a counteractive setting causing it to still act up? It still bogs down noticably with a light/medium miss in the lower rpm’s but clears up the higher it runs, I do feather the accelerator a bit if that’s helpful info. The less you push it the more it went, and now the more you push it the more it wants to go.

P0203 is a problem with the circuit for the 3rd cylinder fuel injector - bad injector or bad wiring/plug to injector. I would track down the source of that problem before doing anything else.

Do yourself a favor and test the electricals for both the signal to the 3rd injector & the injector itself rather than just replacing the injector.

The computer will turn the light off after certain number of drive cycles if it doesn’t find any more problems. Or any code reader can clear the codes. Or, yes, you could disconnect the battery - which you really should have done before doing the other work anyway.

But until you deal with the injector issue the light will most likely come back.

On your 1999 Toyota Corolla, 4 cylinder, those DTC trouble codes (P0203, P0300, P0420) brought a lot of useful information. The cause of code P0203 could be causing the other codes. Fix it, and the other ones might be fixed.
P0203 concerns there being a problem in the circuit of the #3 fuel injector, which could include the fuel injector, itself.
Really, you need the repair manual for these problems. You need to know HOW to check the fuel injector with an electric multimeter and a 'NOID Test Light. You still need to check the fuel pressure. You can follow the repair manual, and do some tests, or you can throw parts at it.

ADDED later: If you need an adapter for your fuel pressure gauge to fit your car, there are several sources online:, or, etc.

You don’t need to reset the computer, but resetting it won’t do any harm and will clear the check engine light – temporarily at least. If you have an anti-theft radio (a truly lunatic concept) make sure that you have the reset code.

Checking the DTC codes that are associated with the check engine light might save you a lot of trouble. They aren’t always useful, but when they are, they can keep you from changing parts that don’t need changing and maybe breaking something else in the process.

Before you replace the fuel pump, buy or borrow a fuel pressure gauge (around $60 I think) and make sure the pump needs replacing. I doubt a Corolla will have a Schraeder valve in the fuel system, so you won’t be able to use the cheapest low end fuel pressure gauges.

OK, I threw a couple hundred bucks at the car. New plugs, forward O2 sensor, and a fuel injector. In my defense, my first reaction would have been plugs and wires first but Bobro had a pocket full of codes and I had to get it rolling fast so I wasn’t thinking clearly enough obviously. First it took a week to get my car back! No sweat, Brobro’s girl is in the hospital right now and I’m still on the call list for an emergency ride, bad timing stuff. Well in a fit of desperation I found myself swabbing contact grease on the injector pins, didn’t help. 15 mins later I was squeezing it in the spark plug boots and voila. She runs like a dream now. I’ve changed my opinion on that 2 dollar grease by the register at the auto parts store. Just thought you’d be interested, thanks for all the help.