Air/Fuel Sensor

How important is it to replace a faulty sensor responsible for heating exhaust gases headed for the catalytic converter? What’ll happen?
And is it true that if you replace one of these pricy little devices, any others will likely go bad as well?

Ummmm, the A/F sensor does not heat the exhaust. The exhaust is plenty hot due to fuel burn. The heater is to warm up the sensor faster. If the sensor is before the cat, it is very important for proper fuel mixture and good gas mileage. Without it, or if it is malfunctioning, the computer will run the engine rich, wasting a lot of gas. This is preferable to running lean, and damaging the engine.

Are you talking about an oxygen sensor? Upstream or downstream? What kind of car? (year, make, model, mileage). How do you know the sensor is bad? Presumably you have an error code? What is it? Seriously - how do you think anyone can say anything of substance based on what you’ve asked?

If you have an upstream O2 sensor issue your fuel economy will likely suffer, the car may run poorly, and it can cause larger problems the longer it is ignored. If you’re just talking a code for an O2 sensor heater circuit, that mostly means the car will stay in open loop longer. Same possibilities as above but not as bad.

Oops, sorry, let me restate: 2002 RAV4, check engine light on, P1135 code, AF sensor heater circuit malfunction bank 1 sensor 1


Most likely the oxygen sensor’s heater circuit has failed. It won’t greatly affect driveability

But that code won’t go away until you fix the problem. Which means the check engine light will remain on. You’ll have problems when it 's time to get the car smogged

Check the wiring for the sensor. Uplug it, clean the connectors, inspect for damage, clear the code and see what happens. I’d also say that if you can put a scan tool on it to look at what the coolant temp sensor is doing isn’t a bad idea. That’s only if its easy to do since the odds are rather long, but the PCM sets that code when it takes the sensor too long to start switching - the major input to when it “thinks” it should start switching is coolant temp.

If the external wiring for the sensor is all good and you have no reason to suspect a ECT sensor issue then, as db4690 noted then sensor replacement is in order. Aftermarket sensors should not set you back too far.

I’m with db on this. By not repairing the cause of the check engine light, you’re effectively disabling a warning system that could prevent you from a future breakdown…or even save you from total engine destruction.

You want to fix this one. It’s cheap insurance for the future.

@cigroller, Toyota’s are generally very sensitive to the type of sensor you use. For Toyota, I recommend only using Denso sensors, since they supply Toyota.

I can tell you from experience these A/F ratio sensors are expensive parts at the dealer.

@BustedKnuckles, fair enough then. Seems odd to me, but I’m sure you know better than I.

Sounds like your car is equipped with an Air/Fuel Ratio sensor and not a conventional heated oxygen sensor. These sensors are easily diagnosed with the right scan tool but impossible to test using a simple voltmeter or code reader.

Replacing one sensor will have no effect on the other sensors on the car, other than the fact that they are all the same age and may begin to fail soon as well. Ever installed a new 5-bulb light fixture in your bathroom? Have you noticed that all 5 bulbs will start to burn out at about the same age?

There are only 2 options for replacing your A/F sensor, from Toyota or Denso. Anything else is asking for trouble. Denso isn’t that pricey either, I’d think you could get one for $150 or so.

fyi, O2 sensors need to be hot in order to work. The heated O2 sensors are different technology from the traditional non-heated kind. The traditional type (sometimes called narrow band) get their heat from the exhaust gasses. The heated type (wide band) have a little electrical heater built into them, so they heat up more quickly. Helps save gas and reduce emissions. It appears there is something wrong with the heating function on your sensor. Same as if your electrical space heater in your house stopped working. Could be the heater coil in the sensor is kaput. Or a fuse or relay has failed so the electrical power isn’t getting to the heater coil on the sensor.


Sorry to burst your bubble, but you’re mistaken

The terms narrow band and wide band have absolutely nothing to do with heated or not

A heated oxygen sensor is just that, a heated oxygen sensor

A non-heated oxygen sensor functions the same way as an unheated oxygen sensor, except it doesn’t have a heater circuit. It’s usually for older vehicles.

The heater circuit just gets the sensor up to operating temperature sooner

A wide band sensor is also referred to as an A/F sensor, air fuel sensor. It functions completely different than an oxygen sensor, and they are becoming more common

Thanks all for the advice. This AF sensor issue is only very recent, and so far it’s blinky. On and off several times in the few days since it first appeared, and the car seems to run normally. Can I assume it’s reasonably safe to at least wait till it stays on constantly for a week or so?

Yes, the sensor will just take longer to get to operating temps. Until then, the ECU will run in ‘open loop’ mode longer, which it uses to properly warm up the car.

Thanks for the interesting info on the heated vs non-heated O2 sensors @db4690. No, bubble not burst. Just good to get the straight dope from a pro mechanic like you. I have always thought the wideband O2 sensors were the electrically-heated ones and the non-electrically heated ones were the narrow bands. But I was mistaken, you are absolutely correct that the narrow band sensors can be manufactured either electrically heated or just exhaust gas heated. My understanding remains however that all the commonly used wideband zirconia O2 sensors are electrically heated. So if it is unheated it is always a narrow band. If heated, it could be a wideband or a narrowband. Or are there unheated wideband O2 sensors in common use?


Wideband sensors haven’t been out as long as the narrowband sensors

By the time they came out, all new cars were being equipped with heated sensors

FWIW . . . I think any wideband sensor is heated

I found this on YotaTech Forums

—leaving a P1135 unfixed [by replacing the sensor] can lead to severe engine damage and even an internal manifold fire/engine fire from soot burning off.
It is nothing to screw around with. Rear 02 sensor, OK ignore that, but 1135 is an A/F sensor and it is critical for it to always function properly.—



Well, please don’t mistake my direct tone for being snarky, just plain spoken.

You’re car is broken. It’s not working properly. When you go to the dentist and he says you have a cavity, do you have him fill it or do you wait until the tooth turns into an abcess? If your roof has a small leak around the chimney do you wait until a rainstorm floods your attic?

Fix your car. Why wouldn’t you?

@auto-owner, I think the post you found is a bit melodramatic. But, a sensor 1 circuit malfunction can cause other problems, since the ECM uses this sensor to determine the proper fuel mixture. With a malfunction of the sensor, the ECM will run in ‘open loop’ mode, meaning it will run the engine intentionally rich. This can burn out the expensive catalytic converters. If there are other underlining problems and the engine runs lean, that can cause valves and pistons to burn, forcing a valve job or a rebuild. Either way, your gas mileage will suffer as all this happens until something expensive breaks.

If your CEL is on and you know the cause, just get it fixed as soon as you can.