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P2108 Ford 5.4L engine

Diagnose and fix P2108 Ford 5.4L engine

Owners may see these trouble codes on their Ford 5.4L engine: P2198 O2 Sensor Signal Stuck Rich Bank 2 Sensor 1, P0150 O2 Circuit (Bank 2, Sensor 1), P0130 HO2S (B1) (S1) VOLTAGE. You may also encounter poor performance with a stall or surge. At first glance, the P2108 Ford 5.4L and other oxygen sensor codes might lead you to think the oxygen sensors are the cause. Replacing the oxygen sensors without conducting real diagnostic work will most likely not solve the problem. Replacing whatever sensor is listed in the trouble code is the single most common mistake DIYers make. Worse yet, over 80% of all oxygen sensors replaced by DIYers turn out to be perfectly good. In other words, they were telling the truth and an underlying problem was causing the trouble code.

What really causes a P2108 Ford 5.4L trouble code?

Let’s think about this, a P2108 code is telling us that the oxygen sensor is seeing a constant rich condition. You want to assume the sensor is “stuck” on rich. That’s not how oxygen sensors work. Unless there’s a wiring harness issue, oxygen sensors can’t get “stuck” on either rich or lean. So instead of assuming the sensor is bad, you should assume it’s telling the truth and try to find out why the exhaust is so rich.

Start by checking fuel trims

Yes, you’ll need a real scan tool for this. Stop whining. In the old days if you wanted to do your own car work you’d need a dwell/tach meter, timing light and a distributor wrench. Well, guess what, today you need to own a scan tool. Get over it and just buy one. You’ve probably already spend more money replacing good parts that it would have cost to just buy a scan tool.

What is fuel trim?

It’s really simple: based on several inputs from sensor, the computer adjusts air fuel mixture by varying short and long term fuel trim. The computer can add 25% more fuel than factory programming or subtract 25%. Here’s what that means. Based on the engine temperature, ambient air temperature, accelerator pedal position, engine RPM, and MAF sensor readings, the computer commands fuel injector operation based on a factory programmed look up chart embedded in memory. Then it looks at the oxygen sensor readings to determine how well it did in calculating the proper air fuel mixture. If the exhaust is too lean, it’ll boost short term fuel trim to add more fuel. If short term fuel trims stay at 25% for a while, the computer will determine that the engine has a problem and will change its base calculations by adding the fuel to long term fuel trims and return short term fuel trim to zero, so it can start again.

So examining fuel trim is the FIRST step in diagnosing a P2108 Ford 5.4L trouble code. Watch fuel trims using the live data feature on your scan tool. Check the fuel trims when accelerating, decelerating and at idle. If you see fuel trims hitting 25%, you’ve got a problem and it’s not caused by the oxygen sensor.

What’s causing the P2108 Ford 5.4L rich condition?

Simple, look at the sensor that has the greatest

A bad or dirty maf sensor can cause a high idle

A mass airflow sensor detects the amount and density of air coming into the engine

control over the air/fuel mixture—the manifold airflow sensor (MAF). The MAF tells the computer exactly how much air is entering the computer. Not only the amount of air, but also the density of the air. Your scan tool reports MAF sensor reading in grams per second.

MAF sensor readings at idle should closely match the engine size. So, on a 5.4 engine the MAF reading should be around 5.4 grams/second. Generally speaking, it’s a linear increase, if you double the RPMS to 1,200, the MAF should read about 10.8 grams/second and 21.5 grams/second at 2,400 RPMS.

If your MAF sensor readings don’t match, it’s the MAF that’s causing the problem, not the oxygen sensor.

What causes bad MAF readings?

Crud buildup on the MAF sensor hot wire

Debris clogging the MAF screen (if equipped)

Bad MAF sensor

You can try cleaning the MAF sensor using MAF sensor cleaner. See this post for information on how to clean a MAF sensor. But if the bad readings remain, replace the MAF sensor.

A bad MAF is the most common cause of P2108 Ford 5.4L trouble codes. Once you’ve changed the MAF, disconnect the battery cables for at least 15-mins to clean all the adaptive memory and fuel trim data from the computer so it can start fresh with better MAF sensor readings.

©, 2017 Rick Muscoplat

Posted on by Rick Muscoplat

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