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Larry, Technical Editor
Category: Ford
Satisfied Customers: 1139
Experience:  Automotive tech editor & author, 30 years experience, ASE certified
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1998 Ford Escort, P1131 code Fuel Trim error, O2 #1 sensor ...

Customer Question

1998 Ford Escort, P1131 code Fuel Trim error, O2 #1 sensor constant at 0.245v and O2 #2 at .89v. Can you suggest repair? Is it the O2 #1 sensor or a vacumn leak? How do I find a vacumn leak?
Submitted: 10 years ago.
Category: Ford
Expert:  Larry replied 10 years ago.
Your Code P1131 O2 sensor code can actually have a variety of causes. I have included the diagnostic chart for this code below.

If the O2 sensor is reading low, indicating a LEAN fuel condition, the fault could be a bad O2 sensor, a vacuum leak, LOW fuel pressure or an EGR leak.

Chances are the O2 sensor has reached the end of the road. But before you replace it, look for any obviously loose vacuum hose connections (PCV hose, EGR vacuum hose, vacuum hose conenction to the power brake booster, any otehr vacuum lines). If you do not ehar any whistling or sucking noises while the engine is idling, and the engine idles smoothly, I doubt the problem is a vacuum leak or low fuel pressure.

If the engine has been hard to start or has not had normal power, the fault might be low fuel pressure. If you have a fuel pressure gauge, attach it to the service port ont he fuel rail and check pressure. Here's the spec:

Fuel Pressure (Key on engine off) 517-862 kPa (55-85 psi)

If fuel pressure is low, the cause might be a plugged fuel filter, leaky fuel pressure regulator or weak fuel pump.

If fuel pressure is okay, replace the O2 sensor.