Learn Me: O2 Sensors

1998 Ford Escort Zx2
- First I get trouble code: P1131 - O2 bank 1 Sensor 1 reading too lean.
- Second I get trouble code: P1132 - O2 bank 1 Sensor 1 reading too fat/rich.

Straight out the gate, I know I have an exhaust leak just before O2 bank 1 sensor 1 directly below the manifold due to one of two bolts missing. The stud is broke.

O2 bank 1 sensor 1 last time I checked is reading 0.0-0.4 volts. Sensor 2 is reading ~3.4 volts. In order to get sensor 1 to read at all I had to floor it beyond 4.5k RPM.

Will the exhaust leak cause this issue? Is there a vacuum leak I should be looking for? I just tightened up the intake tube behind the MAF sensor which may have caused it to change from lean to rich.

I'm thinking just fix the leak. Any similar experiences around? Any O2 sensor experts?

start by fixing the obvious then if that doesn't cure the problem, search further.

That means replacing the header. I have been enabled.

It will definitely cause the issue.

O2 sensors measure free oxygen. Therefore its possible for it to be indicating lean or rich based on several factors. For instance, if you pull a plug wire, you may get a lean code because the oxygen isn't getting consumed by combustion.

My guess is that you are getting a lean code during idle and part throttle when exhaust pressures are low. Extra air is getting sucked in and indicating lean. The computer compensates by upping the fuel curve. Then at WOT when exhaust pressures are greater, some of the unconsumed oxygen escapes the leak and doesn't get registered. Hence the rich code.

Thanks Curtis. An unnecessary performance upgrade has been ordered to repair the leak. Those were the lines I was thinking along.

Replaced the manifold and the leak is gone. I'm getting code P1131 O2 upstream sensor indicating lean.

Watching the voltage as I go and the upstream appears to be working properly. The downstream is not changing from 1.3v. Is that going to cause the vehicle to run lean? Ive got a new sensor in hand, but at $75xx, I don't want to be wrong.

Sometimes you can swap the front and rear O2 sensor, take a peak and see if they're interchangeable. If so, swap them and see what happens with the codes. Also, make sure to check the obvious: wiring, connectors, pins, etc. all seem firmly seated, not chaffing, not corroded, etc. Also, goes without saying, but you did clear the codes after installing the new manifold, right?

Bryce

On a 14 yr old car, vacuum hoses get brittle and leak. This a notorious problem on ZX2's. The PCV elbows are the first to go. Check all the connections on theback of the manifold..

N Sperlo wrote: Replaced the manifold and the leak is gone. I'm getting code P1131 O2 upstream sensor indicating lean. Watching the voltage as I go and the upstream appears to be working properly. The downstream is not changing from 1.3v. Is that going to cause the vehicle to run lean? Ive got a new sensor in hand, but at $75xx, I don't want to be wrong.

That is a bit odd. Typically the downstream sensor shouls still cycle a bit, but slower and not quite as much voltage change- probably if a front sensor cycles from .1 to.8, a rear will be more stable in the .4-.6 range. 1.3 is too high- Depending on the code set criteria, it could be that the rear sensor is used to check the front one, and it is saying that your front sensor isincorrectly reading lean, if the rear is seeing rich all the time.

So- could be the front is actually reading incorrectly lean and driving the mixture rich, or the rear is reading incorrectly rich and thinks the front one is broken. If the gas mileage is still where it should be, doesn't smell rich, I'd guess rear. if its kinda rich and stinky, I'd guess front.

Unplug the rear one, clear the codes, drive it and see what codes set. Or just buy two new sensors and try them...

I replaced the rear. I found that the old rear was fine. After I swapped the header out, it took some time for the ECU to get used to the new header. Apparently that is common with the Zx2 when you put an aftermarket exhaust manifold on. A waste of $75xx, but the one on the back had 130,000 miles on it.

I thought 1.3 seemed a bit high, but the pending code went away without me clearing it manually. I guess I'm good to hit the inspection station.

IceRacer, I should have mentioned that I've been through that.

Nascho, yes, I cleared them after changing out the manifold.

Thanks for the help on my very odd problem.