Pesky CEL and iffy gas mileage

[HighRev85]

First, the background story:

95' SHO MTX, ~145K

Early in the morning, in temperatures below 35 degrees, my car commutes to work. There's a very short spell on city streets (3 minutes roughly) and then it's all highway. I'm easy on it when I'm accelerating onto the highway, because it's still cold. Lately, it's been throwing the Check Engine Light at me almost every morning. On the way home from work (generally warmer, generally 35-45 degrees) it will NOT throw the code.

Here's what happens when the code is thrown:

- The temperature needle is just coming off the C mark
- Happens only on the highway
- Stays on until the temperature needle reaches the "normal" range and the computer throws heat.

Now I pulled the codes a couple of weeks ago and I got this code

"144 (M) Oxygen sensor not switching Single, Right or Rear HO2S - Fuel control"

I've been monitoring my gas mileage for the past couple of weeks. I have rarely gone WOT (maybe once every fill-up, when engine is warm) and have been relatively easy on the acceleration rates. It's decent (15 minutes trips, mostly highway, cold start-ups) at about 20 MPG.

Now I'm thinking that it's the oxygen sensor, but I'm hoping it's not a thermostat issue. I'm throwing it to you guys and gals so you can hopefully agree with my bad oxygen sensor diagnosis or give me some other things to look at.

Thanks for the help!
-Greg

 

[wood_e]

Replace your O2 sensors and see if that problem goes away.

 

[SHOtimer]

I would repace your o2's with either Bosch or Ford units.

Doug

 

[stangeater]

02 sensors!!!!!!!!!!!!

 

[ChefJ]

Are you sure its 144 and not 114???

 

[HighRev85]

Are you sure its 144 and not 114???

Could be. I'm not positive what the code was, but I knew it related to the O2 sensors.

The reason why I posted this was that I wasn't entirely sure whether it was my O2 sensors or the thermostat. I've heard stories of them staying partially open, which would also cause the problems I'm describing.

So whaddya think? Replace the thermostat (which is significantly cheaper) or replace the O2 sensors?

Note: this morning (45 degrees) the CEL didn't come on (same route, same process)...hence why I'm thinking it could be related to the thermostat.

Again, thanks for the help guys

 

[wood_e]

It all depends... have your O2s been replaced recently? How many miles are on your car? I would guess that you need to repalce your O2s because they probably haven't been replaced before.

 

[SeanMc]

Hmm, interesting. I get a CEL after hard driving, but then it goes away after a few minutes.

 

[SHOtimer]

The thermostat is in no way connected to the computer, so it will never cause a CE light. The manifestations of a bad thermostat is that your car will take an extreme amount of time to warm up, or it will never warm up and will continuously cold.

Your problem is most likely related to the o2 sensors.

Doug

 

[HighRev85]

The thermostat is in no way connected to the computer, so it will never cause a CE light. The manifestations of a bad thermostat is that your car will take an extreme amount of time to warm up, or it will never warm up and will continuously cold.

Your problem is most likely related to the o2 sensors.

Doug

Yep. I agree. But the car does take an extraordinary amount of time to warm up and, when it reaches the normal range, it barely moves out of the lower range. I think that the thermostat may be starting to go bad, but the O2 sensors have 80K on then, I believe.

I'll throw in the O2 sensors and see if it makes a difference.

Again, thanks for the help!
-Greg

 

[e_clouser]

just had the same deal with my car. did the thermostat first... ended up doing the o2 sensors..... cel has not come on since.

Eric

 

[SeanMc]

I've replaced my front O2, I guess it's time for the rear. It's the original from '95.

 

[SHOtimer]

I've replaced my front O2, I guess it's time for the rear. It's the original from '95.

Dang! You have 165k on an original o2 sensor!? FYI - you should replace those in pairs....

Doug

 

[SASHO91]

Yep. I agree. But the car does take an extraordinary amount of time to warm up and, when it reaches the normal range, it barely moves out of the lower range. -Greg

have you replaced the tstat???
if you havent, it could be that the P.O put in a lower temp tstat...

i have a 180* tstat in mine... highway temps run at 182-85* for the most part... with ambient temps being around 85ish...
even going WOT for a minute or two, i wont see temps above 195....