Will a bad o2 sensor throw codes
Elder User
Joined: Nov 2002
Posts: 790
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From: Norwood USA
I think it should. I haven't had it happen on my truck but it did happen on my car (96 jetta). Usually the check engine light comes on only from emissions and sensors, so I would suspect it would. Check out this site- www.fordfuelinjection.com and try the self test-its a lot easier then it may seem. I did it and found 2 codes, and I didn't have a check engine light on.
Lead Driver
Joined: Nov 2002
Posts: 9,557
Likes: 44
From: Concord, NC
I think an O2 code will turn on the CEL. The O2 sensor could need replacing even though it doesn't set a code. I've heard replacement intervals of anywhere from 50-80000 miles, so if it's been that long, you might consider replacing it.
Elder User
Joined: Sep 2004
Posts: 941
Likes: 4
O2 sensors and codes in general are handled very, very differently between post 1996 OBDII cars/trucks and pre-1996 OBDI cars/trucks.
The EPA mandated the concept of a "drive cycle" for OBD2. If the vehicle completes an entire drive cycle with any of the O2 sensors out of "expected" range during a reasonable portion of same, it will light the check engine light. The light will stay lit, even if the condition goes away, until the code is cleared from the computer with a scanner or by disconnecting the battery. The EPA also mandated some standardization in what codes go with what condition and the form factor of the self test connector.
OBDI vehicles typically light the check engine light only WHILE a problem is being experienced. As you speed up and slow down, the problem may go away and the check engine light will go out whenever it does. There was no standardization at all across manufacturers, so the codes and behaviors can vary quite a lot from one make to another.
Both OBDI and OBDII may set continuous memory codes like mad without you ever noticing the check engine light.
The EPA mandated the concept of a "drive cycle" for OBD2. If the vehicle completes an entire drive cycle with any of the O2 sensors out of "expected" range during a reasonable portion of same, it will light the check engine light. The light will stay lit, even if the condition goes away, until the code is cleared from the computer with a scanner or by disconnecting the battery. The EPA also mandated some standardization in what codes go with what condition and the form factor of the self test connector.
OBDI vehicles typically light the check engine light only WHILE a problem is being experienced. As you speed up and slow down, the problem may go away and the check engine light will go out whenever it does. There was no standardization at all across manufacturers, so the codes and behaviors can vary quite a lot from one make to another.
Both OBDI and OBDII may set continuous memory codes like mad without you ever noticing the check engine light.
Junior User
Joined: May 2004
Posts: 51
Likes: 0
o2 sensor
on my 91 f150 5.0 i noticed over a few days time that my mpg was dropping, eventually down to 6 mpg!!! normally 16-18. i also noticed at the same time my speedometer was running about 5-10 mph faster!!! the truck still ran and idled perfect. WEIRD. no check engine light! i put a code reader on it and it came up as a o2 sensor. i installed a new o2 sensor and both problems went away!? i dont know if that will help you or not. has anyone ever heard of this situation?
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96halfton
1987 - 1996 F150 & Larger F-Series Trucks
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Nov 6, 2016 04:31 PM
