Fix P0155: Mechanic’s solutions that WORKED

The code P0155: Oxygen Sensor Heater Circuit Malfunction (Bank 2, Sensor 1) points out a fault in the oxygen sensor’s heater circuit for the specified bank and sensor. This issue most commonly arises due to faulty oxygen sensors, wiring issues, or problems with the vehicle’s Engine Control Module (ECM).

P0155 quick overview

Meaning	P0155: Oxygen Sensor Heater Circuit Malfunction (Bank 2, Sensor 1)
Is it serious?	Moderately serious. While the vehicle might still run, it can lead to reduced fuel efficiency, increased emissions, and potential damage to the catalytic converter if not addressed.
Possible causes	– Faulty or failed oxygen sensor (Bank 2, Sensor 1) – Damaged or corroded wiring or connectors in the oxygen sensor circuit – Faulty relay or fuse for the oxygen sensor heater circuit – Issues within the Engine Control Module (ECM) or Powertrain Control Module (PCM) – Exhaust leaks near the oxygen sensor affecting its readings
How to diagnose?	1. Visual Inspection: Begin by visually inspecting the oxygen sensor wiring and connectors for any signs of damage, corrosion, or disconnection. 2. Check for Technical Service Bulletins (TSBs): Refer to manufacturer-specific bulletins that might address the specific fault code and provide guidance. 3. Use a Diagnostic Scanner: Connect a diagnostic scanner to read the exact code and any related codes that might provide additional information. 4. Test the Oxygen Sensor: Measure the resistance of the oxygen sensor’s heater circuit to ensure it’s within the manufacturer’s specifications. 5. Inspect for Exhaust Leaks: Check around the area of the oxygen sensor for any signs of exhaust leaks, which can affect the sensor’s readings. 6. Check the ECM/PCM: If all else seems fine, consider the possibility of a faulty Engine Control Module (ECM) or Powertrain Control Module (PCM) and test accordingly.
What is Bank 2 sensor 1?	Oxygen sensor before catalytic converter on engine head 2. Read the full Bank1 vs Bank2 explanation here.

How to fix P0155 – learn from mechanics

I looked into P0155 service cases at iatn.com website where in the private forums the mechanics seek advice from other mechanics. I looked up a bunch of cases to help you decide how to fix your issue and see what would a real mechanic do to solve it.

Case #1: Chrysler

2002 Chrysler Concorde LX 2.7L

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Vehicle Information:

• 2002 Chrysler Concorde LX 2.7L

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Issue + Repair History:

• MIL light on.
• Initial problem: P0155 B2S1 Heater circuit malfunction.
• Replaced B2S1 O2 sensor.
• Issue returned: P0152 stored, all O2 sensors flatline @ .3V.
• Replaced both upstream O2 sensors and PCM reflash.
• Vehicle later returned with P0300 random misfire and flashing MIL light.

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Suggestions from Community:

• Check for software update for O2 codes.
• Use NTK or Mopar brand O2 sensors.
• TSB 18-031-02 calls for PCM reflash for O2 sensor codes.
• Test for continuity between the PCM and sensors.
• O2 sensor wiring might break down internally; consider hard wiring the O2 circuits.
• Flashing the PCM might solve the problem.
• When one sensor goes bad, it might affect the other sensor.

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Final Fix:

• Found a poor ground connection at the PCM. Cleaned and repaired it.

Case #2: Ford

1996 Ford F-150 Special 5.0L

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Vehicle Information:

• 1996 Ford F-150 Special 5.0L

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Issue + Repair History:

• MIL light on.
• Codes: PO135 HO2S11 heater circuit fault, PO155 HO2S21 heater circuit fault, PO141 HO2S21 heater circuit fault.
• Heated O2 sensor fuse under hood was blown.
• After replacing fuse and road testing, check engine light returned with same codes.
• Customer previously had a free code check at “Autozone”, bought and installed 3 O2 sensors with no change in the check engine light status.

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Suggestions from Community:

• Check the connections in the electrical harness and related pins at the PCM for loose pins and signs of heat.
• Measure the voltage at the pig tails from the loom as they might share the same power source.
• Consider replacing the PCM, but also check for common wiring issues near the battery.
• Check for corrosion in the connector where the trans harness plugs into the main.
• Use a breakout box to monitor power and ground with a voltmeter.
• Check potential problems in the area of the battery and chafed wiring at the accumulator.

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Final Fix:

• Replaced the ECM with a used one.
• Sealed the firewall cover rubber surfaces due to signs of moisture and corrosion on the outside of the ECM cover.

Case #3: Mercury

1996 Mercury Sable LS 3.0L

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Vehicle Information:

• 1996 Mercury Sable LS 3.0L

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Issue + Repair History:

• MIL light came on during a long trip, went off after refueling.
• Codes: P0135, P0155, P1137, P1127.
• After testing, codes P0135 and P0155 persisted.
• Checked O2 sensors, heater circuits, and other related components.
• Previous repair: MAF sensor replaced at 75K miles; both front O2 sensors replaced at 60K miles.

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Suggestions from Community:

• Check for high resistance in ground circuits.
• PCM controls the ground circuit for the O2 heaters; problem might be internal to the PCM.
• Ensure the correct PCM part number before replacement.
• Check for cracks in the exhaust pipe in front of the O2 sensor.
• Monitor the inline amp draw for the O2 sensor heaters.
• Monitor the heater drivers for the O2 sensors.

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Final Fix:

• Replaced the PCM, re-ran tests, and passed.

Case #4: Nissan

2001 Nissan Sentra GXE 1.8L

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Vehicle Information:

• 2001 Nissan Sentra GXE 1.8L

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Issue + Repair History:

• MIL light on, cuts out, poor fuel economy.
• Code: P0155 bank 2 sensor 1.
• All 4 oxygen sensors have been replaced.
• Issue with oxygen sensor connectors being swapped.
• Swapped connectors and oxygen sensors, but the code persisted.
• Scoped both banks, both appeared similar.
• Checked power to the sensor and wiring from connector to the PCM.
• Swapped the oxygen sensor from the good bank to the bad bank, but the code remained.

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Suggestions from Community:

• Check for high resistance in the heater circuit.
• The ECM might not be grounding the heater circuit properly.
• Check for exhaust or intake leaks.
• Check the power and grounds to the O2 sensor heater circuit.
• There might be a defect in the ECM.
• Bank 1 is always cylinder one, and Bank 2 is always cylinder 2.
• There might be a recall on the ECM.

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Final Fix:

• The wiring diagram from ALLDATA was incorrect.
• The connectors were backwards.
• The scan tool data and scope readings did not match.
• The PCM might be providing a substitute signal.
• Verified the issue by switching the pins at the back of the PCM.
• Concluded that the PCM was faulty and needed replacement.

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Additional Notes:

• There were recalls for the crank sensors and ECM case foam for this vehicle.
• The front bank 1 has a black connector, and the front bank 2 has a blue connector.
• The heater circuit should have 12v and drop to 0 after 3000rpm.

Case #5: Toyota

2000 Toyota Tundra Limited 4.7L

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Vehicle Information:

• 2000 Toyota Tundra Limited 4.7L

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Issue + Repair History:

• MIL light on.
• Toyota TSB states that certain 2000 – 2002 Tundras may exhibit MIL light on with O2 sensor codes.
• Vehicle falls within the VINs listed in the TSB.
• Vehicle is out of the 3/36K warranty.

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Questions:

• Is there a hidden warranty on this?
• Should all four O2 sensors be replaced at once or only as needed?

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Final Fix:

• The dealer replaced both front O2 sensors under an extended warranty (7 years or 90,000 miles).
• The part used was an OXYGEN SENSOR KIT part# 04003-34150.
• The dealer’s charge to replace one sensor would have been $419.

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Additional Notes:

• There is a warranty extension on the 2000-2002 Tundras for the p0135 & p0155 codes, which is 8 years/80,000 miles.
• The original owner should have received a letter about this.
• The warranty bulletin is PRO04-12, which extended the warranty for these specific codes to 7 years/90,000 miles.
• The front O2 sensors are A/F sensors and can only be replaced at the dealer.