"Check engine" light goes on

Hello, I have a Toyota Corolla 96. The check light is on. I took it to AutoZone and the guy told me everything is Ok. I looked the oil level this morning and it was low. The temperature sensor is normal and the car runs fine. My car experience is almost zero… Anyone to help me? Any idea? Thanks

Was he able to read the code? If the engine light is on, it stores a code that can be read.
Add oil, if it is low.
(that last one may be obvious - just making sure)

 That CEL (check engine light) is just a kid in class waving her hand trying to get you attention because she has the answer. You need to have the codes read. Some places will read them for FREE. Try Autozone or Advanced Auto Parts. Get the exact code (like P0123) not just their translation into English and post it back here. 

Anytime the CEL (Check Engine Light) comes on your car should have a stored error code.

The guy from AutoZone used a device in my car and he told me that he did not find anything wrong with my car. He cannot find any problem using the device. The device showed that the car was OK.

Is the light on now?

If so, there’s something the car’s computer deemed not quite right. The widget he used should tell him what code was stored, Like Joseph mentioned above.
It could be something very benign (like a loose gas cap) or something quite serious that could leave you stranded. If that light is on, get it checked. Perhaps try Pepboys or Advance Auto. Many places will tell you what that code is.

yes, the light is on… I will try to find another place to check the car. Thanks

If there is a CEL there should be a stored code. If there is a CEL and no code, I would say there is a problem with the code reader or a problem you, as a buyer, don’t want to worry about. I would pass on that one.

In addtion to what everyone else stated, it is important to add that there is no trouble code for “your engine is low on oil”.

You need to pull the dipstick frequently in order to determine the oil level, and the object of this exercise is to never allow the level to fall more than 1 qt below the full mark. On an engine with as small an oil capacity as yours, if the oil level falls as much as 2 qts, you are doing damage to the engine.

So–although you do need to have someone more competent or with a better code reader to determine the trouble code(s) that have been stored by the car’s OBD system, you still need to manually check the oil on a regular basis.

I went to Advanced Auto and the sensor indicated the codes P0136, P2270 and P2271.

All 3 codes indicate problems with an O2 sensor in the exhaust stream.
An unscrupulous mechanic might tell you that this automatically means you need a new catalytic converter, when, in fact, it is very possible that you only need to replace one or both O2 sensors.

However, it is possible that the engine is getting an incorrect air/fuel mixture, as this can cause this type of O2 sensor problems.

It is even possible that this is a wiring problem.

Worst case scenario might be a new cat converter, but try the above approaches first.

P0136 means Oxygen Sensor Circuit Low Voltage (Bank1, Sensor2)
P2270 means Oxygen Sensor Stuck Lean(Bank 1 Sensor 2)
P2271 means Oxygen Sensor Signal Stuck Rich (Bank 1 Sensor 2)

It is definitely related to your emission control. Driving with it rarely damages the engine itself but could affect your catalytic converter as that thing can foul up in which case it would eventually need to be replaced. In other words, you don’t want to ignore the codes.
Sometimes those error codes are generated due to the sensors being bad. Other times they flag an actual underlying condition with the engine.
While the car may drive perfectly well, you will likely not be able to pass the emission test with this condition. You may need to bring it to a mechanic that can actually measure what those sensors measure and have him go from there.

The words “Stuck lean” / “Stuck rich” kinda point to a bad sensor because the computer wants to see a change when you start driving it. IT seems to complain that the value isn’t changing and it is low. If I saw one and it was my car, my first instinct would be to replace/check the connections to the sensor behind the catalytic converter but what seems weird is that those error codes are conflicting because “Rich” and “Lean” are opposite conditions.
Perhaps someone else has seen this condition on a Toyota and can comment on it.

“The two latter ones seem conflicting because “Rich” and “Lean” are opposites conditions.”

That’s why I speculated that the engine might not be getting the correct (or consistent) air/fuel mixture.

In any event, the OP needs to get the car to a competent mechanic, which means NOT going to Midas, Meineke, Monro, Sears, Pep Boys, AAMCO, or any other chain operation.

I agree.
Weird that it complains about a low voltage, stuck reading and then a rich/lean reading.
After thinking about it, the system when it thinks it is rich/lean, it complains not about the reading but the ratio of the readings. Maybe with it idling or running low speed, the ratio of sensor is such that it is considered ‘rich’ (or the opposite lean*) and when it runs at higher speeds that ratio is clearly different in that it is considered ‘lean’ (or the opposite rich*)
I bet it is the secondary O2 sensor (the one past the CAT) because performance doesn’t seem to be affected.

Get it checked. If he suggests changing the catalytic converter and wants to charge you an arm and leg, move on and get a second opinion.

*depending on what ratio indicates what condition.

Personally, I think this is a slam dunk, you need a rear O2 sensor. Because of issues with the Toyota computer, I’d suggest that you get one from the dealer based on the vehicles VIN code. It costs more, but the Toyota computer seems to be sensitive to aftermarket rear O2 sensors.