What happens when an Oxygen sensor goes bad? What happens if it's disconnected? My pickups gas mileage is dropping. At todays prices, I don't need that. The engine, a 350, is in excellent shape and runs beautiful. Someone told me to check the oxygen sensor. Any clues?

don't think you can disconnect it - but, as I recall, if you are careful you can clean it - it gets covered in carbon

The oxygen sensor is a feedback sensor. It "reads" the amount of oxygen(or other components on some models) in the exhaust it sends that signal back to your fuel system ecu(electronic control unit) with uses that signal to "adjust" the fuel system to try to maintain the stoichiometric fuel ratio.

If it is disconnected then, 1) your check engine light should come on and stay on. 2) your engine will fall into a "failsafe" operational mode. Which is richer than normal and will use more gasoline.


Gm 02 sensors are usually not that expensive, especially for older 350's... So cheap insurance is to replace it(do a little flaps price shopping for your best purchase price) and fix the system.

As far as cleaning the 02 sensor, usually a pointless procedure. The carbon will coagulate on the outer casing of the 02, the "reactive" components are inside that shell and inaccessible to proper cleaning. If you do spray a can of carburetor cleaner through the open holes in the casing, you've oversaturated the sensor wires.....

Said in the other thread: It's a cheap first strike to just replace the O2 sensor. If it is fouled to the point of inoperability it'll send the ECU a signal (as Ron says, above) to go "fat". If it continues to run rich after the sensor replacement, take it to a local parts store offering the free service of pulling the codes to find what's gone off.

We think we found the problem Doc. I posted it on the Pub. Diagnostic check pinned it on a bad spark plug, even though it was still firing and upped the heat range 1 number on the new ones. Wrong signal going to the computer. We'll see.

One thing to remember about o2 sensors for cars that have carbs and o2 sensors (my old '85 Dodge Colt)- if you disconnect the sensor the car will run so LEAN that the resulting piston burn-thru will give you melted aluminum all over the cylinder walls. Really strange mode of failure. (You don't want to ask me how I know this!)
Bill

eeewwwww.

That is precisely why the ECU's now will go rich in "failsafe" mode. Limp home without cooking the engine.

New plugs, new O2 sensor and it seems to run very well. Someone told me that the T body injector jets can wear. Is this true? If so, does the computer make allowances for this and adjust the mixture to compensate? I hate these computer controled devices as I don't know a thing about them.

Should, at least under cruise conditions. That's the point of the O2 sensor, to get the mixture just right (for catalyst operation) during cruise. Unfortunately, most cars go "open loop" during WOT/max power.

Dunno about the injectors getting worn, but they sure can get clogged with deposits and flow less fuel.

I had to replace a TBI injector on Herself's Dammed Dodge due to a case of th' drips. Went rich and it seems the stoopid thing was leakin' from the seat area. I didn't do an autopsy on it as that car and I were locked into a hate/hate relationship and I didn't care to know much about it. I did R&R the O2 sensor at the same time out of prudence... and the desire to NOT wrench on the hateful thing anytime soon again. This malevolent machine was "gifted" to 'smits by her mum, ~I~ got to deal wifit when th' eeevile thing malfunctioned... it finally left my life on a Jerr-Dan as a tax deduction.
One of my fondest memories is of the butt-end of that K-car thing disappearin' off into th' sunset atop that flatbed.

Horsemits called it her Dodge "Insipid".


...is my prejudice showin'?

Word of caution ! When you replace it make sure you don't contaminate it! Dont get any thing on it or it will fail in short order. Check with your parts sales guy on a O2 safe sealent before you install it and use sparingly.

Dont contaminate *it*?

How's about ... don't contaminate *them*

Wife's X5 needs 4 replaced in a few thousand miles (maintenance item)
Two pre cats and two post cats, 4 sensors in total.

Actually, the "Throttle Body" injectors used on GM 350 ("K" engine code) 8the digit in VIN #.... are peak hold injectors. They use 2 very heavy duty "driver" transistors in the ECM, and they get tired after so many years of use, and hash (not the kind you smoke!LOL) can be seen on the voltage line with an Oscilloscope. What to do? Go thee to Radio Shack and purchase 2 0.1 microfarad capacitors and solder each one across the injector right at the connector under the air cleaner. I did this on my 1990 Suburban 4 years ago. She has idled much smoother since, gets 2 MPG better mileage, and passes emissions with flying colors! And with almost 185,000 on the odometer! All original,too!

As far as the oxy sensor goes, new ones are cheap... and fairly easy to replace. ALL Electronically Controlled engines go out of closed loop at WOT... This is the clear flood function whereby the fuel pump is disabled during crank. It is also how I can perform a clear diagnosis on driveability using a scope. Snap throttle to wide open takes the computer out of the loop and enables me to find bad injectors, etc. You can also scope the oxygen sensor and "see" bad injectors..

Cheap?

$137.21 ea for the Precat (x2)
$135.21 ea for the Postcat (x2)

So, thats $544 and change; from a discount house.

In keeping with the vehicle which this man inquired, a new sensor (it only has one) is 20 bucks at AutoZone....The prices you quote for a BMW are proportionately in keeping with the price and emissions control system on the car... no way can you compare them.... your is OBD2 and his is NOT. Furthermore, Toyota sensors go for just a tad over 200 a pop times 4! (They call theirs "Air Fuel Ratio Sensors) Guess they can snarf more money from the unwary public's pocketbook thataway! CHEERS!

Most likely, the $20-30 sensors are "narrow band"; while the $100-200 ones are "wide band". The wide band sensors are better in that they allow the ECU to extend closed-loop operation into the WOT region, but (obviously) cost more.

But "easy to replace" may be relative ... myself and 3 different shops tried to remove the one on a turboed Dodge Caravan without success. No room to swing a box-end wrench, and a "sensor" socket would just spread and slip.

4th shop had a "slugging wrench" which proved to be the right tool for the job.

I take issue...., EEC system vehicles do NOT have computer (oxy sensor) control at WOT including wide-band sensors....period. This is the very reason snap-tests show problems on an ignition scope that the o2 control is so good at masking. Such as a bad injector, carbon on a valve, etc... but, ya hafta know how to diagnose these new-fangled cars and read a dang-fooled scope, Doncha see!

I'm sure you're right, of course.

But look up the Toyota Prius sometime. It runs closed-loop at WOT even with a narrow band sensor.

Toyota( and others) sensors designed in the past few years are a lot more complex than just ordinary 02 sensors from the mid70's... Complexity costs more.

Yeah, just look at th' Hubble.