Judging oxygen sensor condition

1996 Dodge Grand Caravan 3.3L V6, 155k

If there’s no CEL, does that assure that O2 sensors are good? Do they / can they deteriorate slowly over time without causing a CEL, but causing a slight impact on fuel economy? Or are they 100% until they fail? I have no idea how old the sensors are, could be original, or not.

I’ve just noticed a decline in gas mileage on a 500 mile trip which is probably caused by old spark plugs (which will get replaced this week or next). But I wonder about the O2 sensors. The cooling system is mostly new, and the temp gauge sits rock solid right in the middle, so I don’t think it’s running cold. Last trip was 25mpg, this one looks more like 19+, driving mostly around 55-60mph, light load.

No CEL, new t-stat, rad cap, water pump, radiator, air filter, passed CA smog test very easily 5000 miles ago (tech said “as good as it gets”).

Thanks for any thoughts…

If there’s a problem with the O2 sensor the Check Engine light would be on. The OBDII system monitors the O2 sensor pretty closely.


That’s what I expected, but wasn’t sure. Thanks Tester.

Oxygen senors have a design life of 100K miles…They can get tired without popping a CEL…The front ones are the most critical as they control the fuel mixture…The rear ones are just emissions tattle-tales…

There’s some u-tube vdos on bench testing O2 sensors. Maybe do a look-see on Google. I watched one vdo, looked fairly simple, involves a propane torch and a DVM. Unsure how accurate of a test it is, but it looks like it would identify one with a bad problem at least.

Note: There’s at least two basic types of O2 sensors. One (usually this type is electrically heated) is sensitive over a range of O2 levels. The other (usually heated by the exhaust itself, not electrically) is very sensitive, but only at a certain O2 level. Make sure the testing technique you use is for the type of O2 sensor you are testing.

Well passing a CA smog test is a good sign since they test both tailpipe emissions and OBD2 systems. But that doesn’t mean that the O2 sensors are good, just that they’re not bad enough to trip a fault code. O2 sensors deteriorate with age. I don’t know of any way a layman can do a comprehensive test of the O2 sensors, since it involves having a pretty decent graphing/recording scan tool. They’re not to expensive, and if you are at all mechanically inclined easy to replace. Pick up a couple of upstream O2 sensors and change them on a Saturday afternoon. Then see if your mileage improves.

Edited: I think your car only has 1 upstream O2 sensor. So you’ll be out $50 and a scraped up hand.

"But that doesn't mean that the O2 sensors are good, just that they're not bad enough to trip a fault code. O2 sensors deteriorate with age."

Ahhh, exactly what I wanted to confirm. If that’s indeed correct, then I have no problem justifying a replacement, regardless of whether there’s a CEL. I don’t yet know yet where the sensors are on this car, but I’ve got the complete factory manual, and not worried about doing the work.

Thanks gentlemen.

Just had a Toyota sequoia set a downstream code when the upstream was bad. Wide band type. Yes the get lazy…

In general I always recommend testing before replacement,
but it can be difficult to find competent troubleshooters.

Like anything else, O2 sensors have a tolerance. As long as they are in tolerance, you won’t set a code, but they may deteriorate slightly over time and have a small impact on your fuel economy, but it will be very slight.

Your van should be OBDII compliant. If it is not, if it is only OBDI compliant, the O2 sensor may not be monitored. 96 was a transition year between OBDi and OBDII, most vehicles were OBDII though.

A scope can give some great insight into monitoring and diagnosing automobile electronics. I really regret selling my lab scope because it could often predict failing sensors like the O2. Some O2 sensors will deteriorate and nearly flat line at idle and even low cruise but come to life as engine load increases. Eventually that condition will set a code and turn on the CEL but in does go unnoticed for a while.

Thanks. My good friend has a fascination with buying, fixing, and then selling engine analyzers and smog test equipment etc, and knows how to use them. It’s an ebay venture which he does for fun and a little profit. He’s also an advanced DIY motorhead with lots of cars with a newly built shop with a lift that he loves to use. In other words, it won’t cost me anything for diagnosis, and I’m due for a visit.

I’ll check the sensors there, but will almost certainly replace the upstream O2 sensor regardless. I could just replace the sensor, but there’s a benefit in knowing if there’s any detectable problem with the current one first. It’s probably the original one and replacing it will be cheap and easy, and Caddyman has already pointed out that they are designed for 100k, I’m at 155k.

Not sure if this is OBDII, but my guess is that it is. Here’s a picture of the connector.

I appreciate the comments.

The EPA mandated that all vehicles sold in the U.S. be OBDII compliant in the model year 1996.


Thanks Tester.

WesternRT, when you do the replacement, don’t throw your old sensor away. It may come in handy. If nothing else, you can do some experiments with it, heating it with a propane torch, keeping the flame at the detection spot so the O2 level is low there, then removing the flame so the O2 returns, and seeing what the voltage produced is for both conditions.