Search engine soon light NEVER goes off

Recently purchased a 1997 Mercury Sable for $1700 cash and all because my Camry after 13 years finally gave up the ghost. “SES” light came on, and I filled up with gas and used Lucas car product as well. The light went off 2 days later. 5 days later, it came back on. Purchased CRC spray and cleaned the air mass flow component. Light goes off. A week later and it’s back on again. I figure an 02 sensor needs replacing. Took car to the mechanic that sold me the car and their diagnostic test stated 1 sensor in bank 1 and another sensor in bank 2. Okay, fine. ordered only 1 sensor and told me that it was the “most imporant” sensor as it relayed to the catalyst. fixed it, paid $45.00 for time, and no more than 7 miles up the road, YES, it came back on. Unhooked the battery cable, light off all day until about 30 minutes ago. Pulled into an Advance Auto Parts store. Diagnostic test showed the 2 errors that the mechanic YESTERDAY was getting before he “fixed the problem.” I AM BEYOND FEDUP!!! Somebody help me as to what I can do and also, who can I trust? Thanks!

You just need to bring the vehicle to someone who can hook a scanner up, read the codes, and interpret what the codes mean. Disconnecting the battery will do more harm than good.


Isn’t that what the diagnostic test is for? I am sorry, but I am ignorant on the majority of car terms, repair codes, etc?

Unless an emissions test is involved, you can put some black tape over the light and drive on…You could also have most any auto-parts store read the codes stored in your computer (free) and post the actual codes back here. How many miles on this $1700 gem?

Actually, only 153,000. Has a different engine in it though with about 125,000. I think the lady that owned it before me was a stay at home mom.

FYI ALL oxygen sensor codes turn on the MIL check engine light.

Replacing only one sensor makes absolutely no sense if you’ve got codes for multiple sensors.

My advice?

Don’t go back to that guy again.

BTW, “insufficient activity” oxygen sensor codes can be caused by exhaust leaks directly in front of the sensor. I’ve run into this before. That is usually caused by deteriorated gaskets or loose exhaust connections which need to be retightened.

Can you look at your paperwork and post all the codes, please?

I didn’t get the codes on paper at Advance. Next one I have done, I will ask for them and then post. Any help is appreciated. Sorry to be so ignorant.

Again, is there a driveability problem with this car? If the car is providing you with the service it was designed to perform, why pour money into it just to make a light go off…Now if an emissions test is looming, that’s a whole different issue…Next thing you know, they will be telling you you need a new catalytic converter…

A single “diagnostic test” won’t necessarily identify the problem. Usually on problems such as this a sequence of parts replacements and further diagnostics are necessary. The O2 sensor code for example usually doesn’t mean the O2 sensor is bad. It generally means there is something wrong with the fuel/air metering, which could be anything from a plugged cat to a clogged air filter or a faulty fuel pressure regulator. There are a lot of O2 sensors replaced for no reason. Which wouldn’t be that big of a problem, except that the aftermarket sensor probablyu is’t as good as the one that was already in there. At this point the OP needs the assistance of a good inde mechanic with experience in this type of diagnostic. Or, if the car runs ok otherwise, the black tape solution as mentioned above.

BTW, SES probably means Service Engine Soon.

@GeorgeSanJose not to burst your bubble, but if you have an oxygen sensor heater circuit fault code, chances are the sensor is faulty. To be specific, the heater inside the sensor is no longer functioning. That is easily tested. Check if the heater circuit is getting battery voltage. If it is, you can go out and buy that sensor.

But I will say that a lot of guys needlessly replace all the oxygen sensors when they get P0171 and P0174 fault codes. They are unknowingly just shooting the messenger. Because they don’t truly understand how things work.

After 153,000 miles and an engine replacement, they obviously don’t work as they once did and they are complaining loudly about it…

so if I got all 4 sensors replaced, should that do the trick? Again, really thankful for the replies.


Unless it has been 100% determined that they are all faulty.

I believe you should find a very competent independent shop to diagnose your problem.

Also, if the O2 sensors have never been replaced, it is likely that they are about just plain worn out. The fact the engine has been replaced may mean that the first one was burning oil or something along those lines. A poorly running engine can definitely lead to fouling of these sensors, the catalyst, etc.

I replaced all 4 sensors at 100k after I did a complete tuneup. I was getting a check engine light with misfire codes as the cap/rotor, wires, plugs, and the coil were worn out and not functioning up to par. I was still having quite a bit of hesitation after I replaced these parts and my exhaust was smelly like it was running rich. My mileage was also still quite poor so I could tell something else was wrong but was seeing no codes of any type with a scanner.

So, I was told that I should replace all 3 of my oxygen sensors which I did. My mileage, performance, and emissions all increased dramatically. Sure, I paid $50 each for the sensors and did the work myself so I had $150 in the repair. My mileage came up significantly so I am sure those parts have long paid for itself by now. I also noticed that my oil was MUCH cleaner on subsequent oil changes. I am guessing that all the soot and unburned gas from poor combustion were going into the engine oil.

My opinion is that if the O2 sensors haven’t been replaced, replacing them is money well spent. Replace any of them UPSTREAM from the catalyst at the very least.

@cwatkin You did well with your car. Your sensors were probably biased

But I still recommend only replacing O2 sensors if it can be 100% determined they are faulty.

Here’s some examples of what I mean

Faulty heater circuit
Biased lean or rich
Contaminated with oil, antifreeze, RTV, etc.
sluggish (slow to respond)

Got my codes. P420 AND P430. New catalytic converter going on this Friday.

@FEDUP I HIGHLY suggest you bring the car to a shop for a proper diagnosis.

P0420 and P0420 codes are often caused by components OTHER than the cat

We would hate for you to buy a cat, only to have the problem be something else.

yes thats what “ses” means service engine soon it means something is out of whack but don’t wait to long to have it looked at.

Those codes are generated by the rear oxygen sensor(s) not the converter…You MAY need a new converter and all 4 oxygen sensors, these parts do not last forever…But start with the sensors since they are cheaper…If money is tight, just do the rear sensors and see what happens…