When your Mitsubishi’s check engine light illuminates and you encounter the OBDII code P155, it signals a problem within your vehicle’s emission control system. Specifically, P155 points to a malfunction in the heater circuit of your oxygen sensor, often referred to as the O2 sensor. This sensor is crucial for monitoring exhaust gases and ensuring optimal engine performance and fuel efficiency. For Mitsubishi owners, understanding the P155 code is the first step towards effective diagnosis and repair.
Decoding OBDII Code P155: Oxygen Sensor Heater Circuit Malfunction
The oxygen sensor plays a vital role in your Mitsubishi’s engine management system. Positioned in the exhaust stream, it measures the amount of oxygen present in the exhaust gases after combustion. This data is relayed to the Powertrain Control Module (PCM), which uses it to fine-tune the air-fuel mixture for optimal engine operation and reduced emissions.
To function accurately, the oxygen sensor needs to reach a certain operating temperature. To expedite this process, especially during cold starts, O2 sensors are equipped with an internal heater circuit. Code P155, specifically “O2 Sensor Heater Circuit Malfunction (Bank 2, Sensor 1)”, indicates that the PCM has detected an issue with the heater circuit of the upstream oxygen sensor on bank 2. Bank 2 refers to the side of the engine opposite cylinder number 1, and Sensor 1 is the sensor located before the catalytic converter. When the PCM detects that this heater circuit is not functioning as expected – typically by not reaching the required voltage – it triggers the P155 code and activates the check engine light.
Symptoms of a P155 Code in Your Mitsubishi
While a P155 code can be a cause for concern, it’s important to recognize the potential symptoms. In many cases, the most noticeable symptom will be the illuminated check engine light on your Mitsubishi’s dashboard. However, depending on the severity and specific vehicle model, you might encounter other, less obvious signs:
- Check Engine Light Illumination: This is the most consistent symptom. The light may be constant or intermittent.
- No Noticeable Symptoms: Surprisingly, in many instances, drivers may not experience any discernible changes in vehicle performance. The engine might run smoothly, and fuel economy might seem unaffected.
- Emissions Test Failure: Even without apparent performance issues, a P155 code can lead to failing an emissions test. The malfunctioning heater circuit can affect the O2 sensor’s accuracy, potentially increasing harmful emissions like NOx.
- Elevated NOx Emissions: In some cases, although not always detectable without specific equipment, NOx (Nitrogen Oxides) emissions may be higher than normal due to the inaccurate readings from the oxygen sensor when its heater circuit is faulty.
Common Causes of the P155 Code in Mitsubishi Vehicles
Several factors can trigger a P155 code in your Mitsubishi. Pinpointing the exact cause is crucial for effective repair. Here are the most frequent culprits:
- Defective Oxygen Sensor: The most common cause is a failing oxygen sensor itself. The heater element within the sensor can burn out or become damaged over time, especially with age and mileage.
- Faulty Oxygen Sensor Wiring or Connections: Damage to the wiring harness leading to the oxygen sensor can disrupt the heater circuit. This can include:
- Corrosion: Exposure to road salt, moisture, and engine heat can cause corrosion in the wiring or connectors.
- Damage from Rodents or Heat: Wires can be chewed by rodents or melted due to proximity to hot exhaust components.
- Loose or Broken Connectors: Connectors can become loose or break, leading to an open circuit in the heater system.
Diagnosing a Mitsubishi P155 Code: A Step-by-Step Approach
Diagnosing a P155 code requires a systematic approach to accurately identify the root cause. Here’s a general diagnostic procedure suitable for technicians and informed DIYers working on Mitsubishi vehicles:
- Record Freeze Frame Data: Use an OBDII scanner to retrieve the P155 code and any associated freeze frame data. This data captures engine conditions at the moment the code was set, which can provide valuable clues.
- Clear the Code and Test Drive: Clear the P155 code and perform a test drive under similar conditions recorded in the freeze frame data. Monitor if the code reappears. This helps confirm if the issue is intermittent or persistent.
- Inspect Oxygen Sensor Wiring and Connectors: Visually inspect the wiring harness and connectors leading to the Bank 2 Sensor 1 oxygen sensor. Look for signs of damage, corrosion, or loose connections. Repair or replace any damaged wiring or connectors.
- Test Oxygen Sensor Heater Circuit Resistance: Use a multimeter to measure the resistance of the oxygen sensor heater circuit. Compare the reading to the manufacturer’s specifications. An open circuit (infinite resistance) indicates a faulty heater element.
- Check for Voltage and Ground at the Sensor Connector: With the ignition on, use a multimeter to check for voltage and ground at the heater circuit connector of the oxygen sensor. Lack of voltage or ground indicates a wiring or PCM issue.
- Oxygen Sensor Heater Circuit Activation Test: Use a scan tool to activate the oxygen sensor heater circuit and check for proper operation. Monitor the voltage and current flow to the heater.
- Replace Oxygen Sensor (If Necessary): If wiring and connections are good, and the heater circuit resistance test indicates a fault within the sensor, replace the Bank 2 Sensor 1 oxygen sensor with a new, OEM-quality replacement, especially for Mitsubishi vehicles to ensure compatibility and performance.
- Verify Repair: After performing any repairs, clear the P0155 code and conduct another test drive to ensure the code does not return and that the repair was successful.
Avoiding Misdiagnosis: Common Mistakes with P155
It’s easy to make incorrect assumptions when diagnosing OBDII codes. Here are common misdiagnoses to avoid with a P155 code:
- Replacing the Catalytic Converter Unnecessarily: Do not assume a P155 code automatically points to a catalytic converter problem. The code specifically relates to the oxygen sensor heater circuit, not catalytic converter efficiency. Replacing the catalytic converter is rarely the solution for P155 and is a costly misdiagnosis.
- Replacing the Oxygen Sensor When Wiring is the Issue: Before replacing the oxygen sensor, thoroughly inspect the wiring and connections. Faulty wiring is a common cause of P155, and addressing wiring issues can resolve the problem without needing a new sensor.
By understanding the intricacies of the P155 code, its symptoms, causes, and proper diagnostic procedures, Mitsubishi owners and technicians can effectively address this issue, ensuring optimal engine performance and emission control.
