Gadgetech

I asked a local wrench to change the oil on the car and give a good check over like look at bushings, hubs, ball joints etc. The CEL was on (always on for something it seems since my Mom had the car) but I have a question about the codes b/c his explanation is different than what I can find on the OBD II codes list.

Here's what he shows:
P0174 - System too lean Bank 2
P1151 - Heated Oxygen Sensor Bank 2 Sensor 1 switch point indicates lean

He suggested that replacing the coil could fix that it didn't seem to run too lean but just his guess. In either case, the only problem I've noticed is when the engine is cold it stutters or doesn't offer full power. Once the car warms up there's no issue. I've only had the car a few weeks and it's in great shape but before I fix the cosmetic stuff I want to address any mechanical issues. I'm mechanically inclined and I just scored some OEM manuals on eBay but I'd like some forum expertise before I dive in.

MikeGuarneri

Does the car have a stock tune or a custom tune on the ECU?

Gadgetech

Stock, nothing like that has ever been changed.

petrock

http://www.obd-codes.com/p0174 - System Too Lean (Bank 2)
http://www.obd-codes.com/trouble_codes/ford/p1151.php - Lack of HO2S-21 Switch, Sensor Indicates Lean

Both are about bank 2 (drivers side). p0174 means the O2 sensors are detecting too much oxygen in the exhaust then expected. The MAF (Mass Airflow) sensor reads the air coming in, and the O2 sensors read the oxygen going out. If they are out of the expected range a code is thrown. A vacuum leak between the MAF and the combustion chamber, or an exhaust leak between the exhaust manifold and the CAT can cause these codes, as well as a dirty MAF sensor can cause this code to be thrown.

P1151 means the computer has detected a problem with the Bank 2 pre-cat O2 sensor. It could mean a wiring issue, or a damaged sensor.

I’d suggest cleaning the MAF first (search the forum and youtube for methods to do this), then clear the codes. If the code come back, replace the O2 sensor and clear the codes again. If the codes come back, then there is some other problem going on. These are all tasks that you can do in your driveway, especially since you have the factory service manuals.

Good luck...

Dooney

Before you go replacing an o2 I would pull it out and bench test it. Cliffy has a good write up on how to do this. Just search for it on the forum

cliffyk

Here is what the Ford shop manual says about P0174 (same possible causes as P0171); and P1151, (same possible causes as P1130)

As it is affecting only one bank (bank 2) concentrate on issues that could affect only on bank. Exhaust leaks before or near the O² sensor or a defective sensor are common. When narrowband sensors fail their output drops to 0.0 V, the PCM interprets this as a lean mixture.

Here is how to bench test an O² sensor using a multimeter and propane torch...

Gadgetech

Would any of this explain the stumbling that occurs when the car is cold?

cliffyk

It very well could. The O² sensors in our cars have 3.3 Ω (60 W) heaters that if operating properly heat up very quickly¹, and the PCM expects them to do just that. If they do not get hot enough their output voltage will be low, telling the PCM the AFR is lean, whether it is or not.

The PCM will then command a richer mix (longer injector pulse) to correct for the perceived lean mix, which when added to the normally richer mix commanded for a cold engine will result in a mix so rich that it will not ignite. This will create the "bogging" you describe...

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¹ - Narrowband O² sensors must be heated to 662°F to work at all, and greater than 1200°F to work optimally. The 3.3 Ω heaters in the sensors in our cars are controlled by the PCM and heated to 1250° to 1400°F.