sowellfan

I've got a '97 Mercury Sable, with DOHC engine. It has had a couple of lean codes for some time, and people had said that the O2 sensors may be going out. I left it alone, because it was running ok. Then about 2 weeks ago, it started running really rough, so when the parts store ran codes on it, it ran a couple lean codes, plus it showed that 3 of the 4 O2 sensors were bad (both of the sensors between the engine and catalytic converters, and one of the two sensors between the catalytic converters and the muffler). So I decided to replace all four O2 sensors.

So I replaced two O2 sensors last weekend at my parents house, then drove it ten minutes home and parked it, until this weekend, when I drove it to the parents again and replaced the other two O2 sensors. When I started it up after that, it seemed to be running pretty smooth (I only let it run for 10-15 seconds, then I shut it down to do clean up), and the "check engine" light had gone out.

So on the ride home, the "check engine" light comes back on, and it starts running fairly rough (though not as bad as it had before). I took it directly to the auto parts store, and the codes came out like this:

P1132 - something about Fuel Metering malfunction (on internet search, it shows up as "Lack of HO2S11 switch - sensor indicates rich)

PO303 - Cylinder 3 misfire detected

PO155 - O2 heater circuit bank 2 sensor 1

PO161 - O2 heater circuit bank 2 sensor 2

So I figure that the Cylinder 3 code is probably because my engine has been running the wrong fuel/air mix because of the O2 sensor problems, and my spark plugs have been dirtied. So perhaps new spark plugs can fix that. But what's up with this O2 heater circuit stuff, and the Fuel Metering thing? I don't see that there's anything wrong I might've done on the installation. You just screw the new O2 sensor in and connect it up to the wires that the old O2 sensor was connected to.

Any ideas? I'm getting frustrated, because a couple of those O2 sensors were a real pain in the butt to replace, and I thought I was finished.

SHOZ123

Seldom do the O2 sensors go bad. More than likely you have a vacuum leak, shorted wire somewhere or a dirty MAF. When a O2 sensor fails it will set a completely different code.

sowellfan

I talked to a mechanic I trust, and he said that the cylinder misfire could be causing the fuel mix issue, so I went and changed the spark plugs and wires. I haven't run codes on it again yet, but it seems to be running like it's supposed to now. He also added that the two sensors in Bank 2 (after the catalytic converters) don't really have anything to do with how the engine runs, so that makes me feel better.

Thanks for the reply.

SHOZ123

Indeed the rear O2s only check the efficiency of the cats.

sowellfan

I wish I'd known that before I bought those two sensors - they cost ~$60 each.

sowellfan

I took it to Autozone today at lunch, and had them run codes. The same 4 codes came up (they read the codes with key on, engine off). I asked the guy to erase the 'Misfire on Cylinder 3' code, and see if if came up again, but he said that they aren't allowed to erase codes, and that the code would go away when the problem went away.

The car is running fine now, though, so I'm thinking/hoping that he's wrong. Either that, or the misfire is slight enough that I can't hear or feel it. I asked if things would be different if he read the codes with Key On, Engine Running, but he said no, so I gave up for the moment.

SHOZ123

I've got two plugs where my rear O2s should be. :)

If you disconnect the battery for a minute the codes will be erased.