Harley diagnostic trouble codes make identifying a particular problem in your motorcycle easier. Therefore, hundreds of these fault codes exist, indicating problems with the carburetor, engine, battery, sensors, and fuel injector.
But what is the Harley code P0151? The code displayed due to a faulty rear oxygen sensor. Typically, this code will appear when the engine control module detects low voltage for the bank 2 sensor 1. This could be due to an exhaust leak or a shorted O2 sensor circuit.
So then, how do you diagnose the Harley P0151 code and fix it? Read the complete guide below to know this trouble code’s causes, symptoms, diagnosis and solutions.
What the Harley Code P0151 Means
This is a diagnostic trouble code that pops up when the ECM detects a low voltage at the rear oxygen (O2) sensor. The low voltage issue is due to excess oxygen in the exhaust.
However, this code is related to the bank 2 sensor 1 and not the Bank 1 sensor 1, which usually triggers Harley code p0031. Bank 2 sensor 1, also known as B2S1, can be found upstream from the engine in the exhaust stream.
This sensor is designed to analyze the amount of oxygen in the exhaust system before it enters the catalytic converter. The Engine Control Module (ECM) then uses this information to adjust the bike’s air-fuel ratio.
Generally, the voltage generated from the oxygen sensor should be from 0.1 to 0.9 volts. 0.1 volts is on the lean side, and 0.9 volts is on the rich side. If the readings are low, the P0151 code may show up.
Causes of P0151 Code
Similar to Harley 114 engine problems, many causes of the P0151 code exist. These include:
Exhaust Leaks
The common cause of the p0151 code is a leak near the sensor. Even the tiniest leak usually leads to excessive oxygen in the exhaust, causing the ECM to run rich.
Defective Oxygen Sensor
The ECM uses the signal from the oxygen sensors when calculating your Harley’s air-fuel mixture. Therefore, when the sensor is faulty, the ECM may give false readings like low voltage output in the O2 sensor. A defective sensor can be in the form of a loose connection or damaged wiring.
Shorted O2 Sensor Circuit
The oxygen sensor circuit may short to the ground due to a blown fuse, affecting the air-fuel ratio on your Harley. Consequently, this may cause low voltage in the Bank 2 sensor 1.
Faulty ECM
The engine control module may malfunction due to a shorted oxygen sensor circuit or bad connections. As a result, it may detect incorrect data, such as low voltage for B2S1.
Symptoms Of The P0151 Code
So, how do you tell that the oxygen sensor of your motorcycle is bad without scanning for the Harley code P0151? Some of the visible symptoms you may notice include:
- Check engine light illuminating
- Excessive fuel consumption because of poor air-fuel mixture ratio
- Hard starts when cold
- Too much black smoke from the exhaust system
- Exhaust leaks before or near the bank 2 sensor 1
- Engine stalling under hard acceleration
- Engine runs lean at lower RPMs
- Unusual popping sounds from the exhaust
Diagnosis of the P0151 Code
Below we will discuss the different methods you or the mechanic can use to diagnose the P0151 code in Harley bikes.
Use OBD-II Scanner
Get a scanner or code reader tool like the OBD2 scanner and scan for the codes. Then, use the scanner to clear the codes to verify if the O2 sensor is faulty.
Check the Voltage
Using a multimeter, you can also diagnose the P0151 code by monitoring the voltage for the bank 2 sensor 1. If the sensor works correctly, the voltage should range between 0.1 and 0.9 volts.
Look for Exhaust Leaks
There are different ways to check for exhaust leaks, including:
- A strong smell of exhaust
- Abnormal discoloration on the exhaust (blue)
- Put a rag or paper towel at the tip of the exhaust. Turn the ignition key on and check if it flaps due to a leak
- Noisy exhaust
Test the O2 Sensor Circuit for Resistance
To check if the O2 sensor is shorted:
1. Turn your bike’s ignition off
2. Disconnect the ECM harness connectors
3. Measure the resistance between the O2 sensor circuit and the ground. If the resistance is below 100 ohms, the circuit is shorted
Check for Physical Damage
Additionally, you can diagnose the P0151 code by checking for any damage on the O2 sensor wiring. Also, look for any loose connections or corrosion in the wiring.
Common Mistakes When Diagnosing The P0151 Code
When diagnosing the P0151 code, there are common mistakes you must avoid, including:
- Failure to test the oxygen sensor voltage for bank 2 sensor 1 before concluding it needs to be replaced
- Replacing the oxygen sensor before repairing exhaust leaks
- Failing to fix any coolant or oil leaks that could contaminate the oxygen sensors
- Failure to test the resistance of the O2 sensor circuit for a short before doing the replacement
- Clearing the P0151 code with the scanner without fixing the actual cause of the problem
How To Fix the P0151 Code?
Let’s look at the different methods of fixing the P0151 code on your Harley motorcycle.
Method 1: Repair the Exhaust Leaks
- First, remove the exhaust pipe and look for any leaks or holes
- Also, check the O-rings, gaskets, and seals. If these components are damaged, they could lead to exhaust leaks. So, replace them
- If you spot holes in the exhaust pipe, seal them using epoxy putty or a heat wrap
- However, if the exhaust leak is due to a broken pipe, you must replace the entire exhaust system
Method 2: Repair or Replace Damaged Wiring
- Once you have fixed the exhaust leaks, replace any damaged wiring to the oxygen sensor for bank 2 sensor 1
- Also, replace the O2 sensor circuit if it is shorted
- If the wiring is corroded, clean the wires using baking soda or vinegar
Method 3: Replace the Faulty O2 Sensor
Once you test the O2 sensor and confirm it is faulty, you must replace it immediately. In this case, you only replace the upstream sensor for the bank 2 sensor 1.
You can watch this video to understand how to replace the oxygen sensor:
Method 4: Replace the ECM
Lastly, you may have to replace the engine control module of your Harley if it is faulty or damaged. The cost of replacing the ECM ranges between $300 and $500.
FAQs
Before concluding, check out these commonly asked questions regarding the Harley P0151 code.
Q: What happens if you don’t fix the P0151 code?
First, you may have to spend more on fuel. In some cases, it could result in severe damage, which can be expensive to repair. For instance, the catalytic converter and the engine control module may get damaged.
Q: Can you ride your Harley without clearing the P0151 code?
Yes. You can still ride your motorcycle even when the oxygen sensor is faulty. However, even though the P0151 code is not as severe as most engine codes, you should not ignore it for long.
Q: Can you fix the Harley P0151 code by yourself?
Yes, if you are mechanically inclined and have the right tools. You just need an OBD2 scanner to scan for the code and a multimeter to check the sensor’s voltage and resistance.
Conclusion
If you get a P0151 code, this only means the oxygen sensor for B2S1 of your Harley is faulty. The causes of this code may range from exhaust leaks to a bad O2 sensor, shorted wires, and faulty ECM. The good news is that many symptoms of the P0151 code exist.
If that’s not enough, there are many ways to diagnose the P0151 code, including using an OBD-II scanner and a multimeter. And once you have diagnosed the code, you can fix it yourself or get a mechanic to help you. But there are some common mistakes you must avoid making during the diagnosis.