Diagnose & Clear P0155 with iCarsoft CR MAX BT: Fix O2 Sensor Heater Circuit Malfunction (Bank 2, Sensor 1)
If your check engine light turns on and a scan returns P0155, your vehicle’s oxygen sensor system is signaling a problem with the heater circuit of Bank 2’s upstream sensor. This OBD-II code stands for "Oxygen Sensor Heater Circuit Malfunction (Bank 2, Sensor 1)"—meaning the Engine Control Module (ECM) has detected an abnormal current or voltage in the heater element of the upstream oxygen sensor in Bank 2.
Upstream oxygen sensors (mounted before the catalytic converter) rely on internal heaters to quickly reach operating temperature (600–800°F) after startup. This rapid warm-up ensures the sensor can accurately monitor exhaust gases and adjust the air-fuel mixture within minutes. When P0155 occurs, Bank 2’s sensor heater fails to work properly—either shorting out, failing to draw power, or remaining inactive. This delays sensor readiness, leading to poor fuel economy, increased emissions, and even false readings that disrupt engine performance.
Basic scanners might only label P0155 as a "sensor heater fault" but can’t test circuit continuity or heater resistance. The iCarsoft CR MAX BT—with its wireless circuit testing, real-time heater current monitoring, and sensor resistance checks—solves this. Let’s walk through how to diagnose and resolve P0155 using this advanced diagnostic tool.
iCarsoft CR MAX BT displaying Bank 2, Sensor 1 heater voltage, current, and resistance data for P0155 diagnosis
Understanding P0155: Causes & Key Symptoms
A faulty O2 sensor heater in Bank 2 disrupts sensor performance, leading to noticeable issues. Recognizing these symptoms helps target your diagnosis:
Key Symptoms of P0155
-
Check Engine Light: Illuminates when the ECM detects no current flow, excessive current, or a voltage drop in the Bank 2, Sensor 1 heater circuit for 2+ drive cycles.
-
Delayed Sensor Readiness: The "Check Engine" light may take 5–10 minutes to activate (after startup) as the ECM waits for the sensor to warm up.
-
Poor Fuel Economy: A cold sensor can’t adjust the air-fuel mixture efficiently, reducing MPG by 8–12%.
-
Rough Idle (Cold Starts): Without a working heater, the sensor sends inaccurate data during warm-up, causing uneven combustion.
-
Failed Emissions Tests: Cold sensors increase hydrocarbon (HC) emissions, leading to test failures.
-
Sensor Readiness Monitor Issues: The ECM may flag the "O2 Sensor Readiness" monitor as "Not Ready," preventing emissions testing.
Common Causes of P0155
|
Cause
|
Description
|
|
Faulty O2 Sensor Heater Element
|
Internal damage (broken wiring or burned-out coil) in Bank 2’s upstream sensor stops the heater from working—accounts for ~50% of P0155 cases.
|
|
Damaged Wiring or Connector
|
Frayed wires, corroded terminals, or loose plugs in the heater circuit disrupt power flow to the sensor—worsened by engine heat.
|
|
Blown Fuse or Relay
|
A dedicated fuse or relay for the O2 sensor heater is blown, cutting power to Bank 2’s sensor (common in vehicles with 80k+ miles).
|
|
ECM Malfunction (Rare)
|
A faulty ECM fails to send power to the heater circuit, usually paired with other electrical codes (e.g., P0606).
|
|
Short Circuit
|
A short in the heater wiring (e.g., wires touching the exhaust manifold) causes excessive current, triggering the code.
|
Why iCarsoft CR MAX BT Excels at Diagnosing P0155
The CR MAX BT outperforms basic tools with features tailored to O2 sensor heater circuit diagnostics—critical for pinpointing P0155’s root cause:
Wireless Circuit Testing
Streams real-time voltage and current data from the Bank 2, Sensor 1 heater circuit to your device, identifying shorts or open circuits instantly.
Heater Resistance Checks
Precisely measures the sensor’s heater element resistance to confirm if it’s within manufacturer specs (typically 5–15 ohms).
Fuse & Relay Identification
Accesses vehicle-specific diagrams to locate O2 sensor heater fuses/relays—no more guessing under the hood.
Dual Bank Comparison
Side-by-side data for Bank 1/Bank 2 O2 sensor heaters highlights discrepancies (e.g., Bank 1 works, Bank 2 doesn’t).
Bi-Directional Heater Activation
Sends commands to activate the Bank 2 heater directly, verifying if the ECM or sensor is at fault.
Vehicle-Specific Guides
Preloaded 3D schematics show Bank 2 layout, sensor location, and heater circuit paths for 80+ makes (Ford F-150, Toyota Camry, Volkswagen Golf).
Step-by-Step: Diagnose P0155 with iCarsoft CR MAX BT
-
1. Connect & Confirm the Code
Plug the CR MAX BT into your vehicle’s OBD-II port and pair it with your smartphone/tablet via the iCarsoft app (Bluetooth setup takes 30 seconds).
Select your vehicle using Auto VIN Scan (reads VIN instantly) or manual entry (make/model/year/engine).
Navigate to Engine > Fault Codes > Read Codes to confirm P0155. Tap Code Details for vehicle-specific insights (e.g., "Ford: Bank 2, Sensor 1 Heater Current = 0A – No Power").
-
2. Identify Bank 2 & Locate the Upstream Sensor
Use the CR MAX BT to avoid confusion:
Bank 2 Identification:
Go to Component Location > Engine > Cylinder Banks. For V-engines, Bank 2 = cylinders farthest from the front (e.g., V6: Bank 1 = 1-2-3, Bank 2 = 4-5-6). For inline engines, Bank 2 = the only bank (rare for 4-cylinders).
Sensor Location:
Navigate to Component Location > Engine > Exhaust System > O2 Sensor (Bank 2, Sensor 1). The app displays a 3D diagram: Upstream sensors are mounted in the exhaust manifold or header, before the catalytic converter, with a 4-wire connector (2 for the sensor, 2 for the heater).
-
3. Analyze Live Heater Circuit Data
The CR MAX BT’s real-time data reveals heater circuit issues:
Start the engine and let it run for 5 minutes (to activate the sensor heater).
In the app, go to Engine > Live Data > Exhaust System and select Bank 2 metrics:
- "O2 Sensor Heater Voltage (Bank 2, Sensor 1)": Normal = 12V (battery voltage). P0155 = 0V (no power) or >14V (overvoltage).
- "O2 Sensor Heater Current": Normal = 0.5–1.5A. 0A = open circuit; >2A = short circuit.
Compare to Bank 1: If Bank 1’s heater works but Bank 2’s doesn’t, the issue is isolated to Bank 2’s sensor or wiring.
-
4. Inspect the Sensor Connector & Wiring
Physical damage or corrosion is often the cause—check with guidance from the CR MAX BT:
Connector Check:
Disconnect the Bank 2, Sensor 1 connector. Look for:
- Corrosion (white/green deposits) on the heater terminals (usually labeled "HT+" and "HT-").
- Bent pins or water intrusion (common in undercarriage locations).
Clean terminals with electrical contact cleaner; straighten bent pins carefully.
Wiring Inspection:
Follow the wiring from the sensor to the ECM (use the app’s diagram). Check for:
- Frayed insulation (from heat or road debris).
- Wires touching hot components (e.g., exhaust manifold) that could cause shorts.
-
5. Test the O2 Sensor Heater Resistance
A faulty heater element is a top cause—test it with the CR MAX BT:
Turn off the engine and disconnect the sensor connector.
Navigate to Special Functions > Sensors > O2 Sensor Heater Test in the app.
Connect the scanner’s test leads to the sensor’s heater pins (refer to the app’s pinout guide).
Measure resistance:
- Normal = 5–15 ohms (varies by vehicle—check the app’s specs).
- 0 ohms = shorted heater (replace sensor).
- Infinite ohms = open heater (replace sensor).
-
6. Check Fuses & Relays
A blown fuse or bad relay cuts power to the heater—diagnose with the CR MAX BT:
Fuse Check:
Go to Component Location > Electrical > Fuse Box to find the O2 sensor heater fuse (labeled "O2 HEATER" or "SENSOR HT").
Use the app’s Fuse Test function to check continuity (0 ohms = good; infinite = blown). Replace if necessary.
Relay Check:
Locate the heater relay (near the fuse box). Use the CR MAX BT’s Relay Test to activate it—listen for a "click" (working relay) or silence (faulty relay).
-
7. Diagnose Wiring for Shorts/Opens
A broken or shorted wire disrupts the circuit—test with the CR MAX BT:
Short Circuit Test:
Navigate to Special Functions > Electrical Tests > Short Circuit Test.
Connect one test lead to the sensor’s heater power wire (at the connector) and the other to ground.
A reading <5 ohms = shorted wire (look for damaged insulation touching the chassis).
Continuity Test:
Check continuity between the sensor’s heater terminals and the ECM (use the app’s wiring diagram). >1 ohm = broken wire—repair with heat-shrink connectors.
-
8. Verify ECM Heater Control
Rule out an ECM fault with the CR MAX BT:
Navigate to Special Functions > ECM > Heater Circuit Test.
The app sends a command to the ECM to activate the Bank 2, Sensor 1 heater. Monitor live data—if voltage/current remains 0, the ECM may be faulty (consult a professional for further testing).
-
9. Repair & Clear P0155
Fix the Root Cause:
- Replace the Bank 2, Sensor 1 O2 sensor (use OEM for accuracy—check the app’s Part Lookup).
- Repair shorted/broken wiring with heat-shrink connectors; replace severely damaged harnesses.
- Replace blown fuses or faulty relays with OEM-spec parts.
Clear the Code: In the app, go to Engine > Fault Codes > Clear Codes to delete P0155.
-
10. Validate the Repair
Confirm the heater circuit works correctly:
- Start the engine and check live data—heater voltage should read 12V, current 0.5–1.5A.
- Let the engine run for 10 minutes—ensure the "O2 Sensor Readiness" monitor (in the app’s Monitor Status) shows "Ready."
- Take a 15-minute test drive (city + highway) and re-scan: No P0155 recurrence = successful repair.
Preventing P0155 Recurrence
The CR MAX BT helps maintain reliable O2 sensor heater performance long-term:
-
Sensor Maintenance: Use the app’s Service Reminder to replace upstream O2 sensors every 60,000–100,000 miles (heaters degrade over time).
-
Wiring Protection: Secure sensor wiring away from hot exhaust components to avoid insulation damage.
-
Connector Care: Apply dielectric grease to the sensor plug annually to repel moisture and prevent corrosion.
-
Fuse Checks: Include O2 sensor heater fuses in your regular maintenance (every 6 months).
-
Software Updates: Keep the CR MAX BT updated via Wi-Fi to ensure compatibility with new sensor technologies.
Conclusion
P0155’s O2 sensor heater malfunction in Bank 2 may seem minor, but it disrupts critical exhaust monitoring and engine performance. The iCarsoft CR MAX BT simplifies diagnosis with wireless circuit testing, heater resistance checks, and vehicle-specific guides—ensuring you fix the root cause, whether it’s a faulty sensor, wiring, or fuse.
With the CR MAX BT, resolving "sensor heater circuit faults" becomes a precise, streamlined process—keeping your vehicle’s emissions system efficient and your engine running smoothly for miles to come.
FAQs About P0155 Code
Q: Can I drive my vehicle with P0155?
A: Yes, but fix it within 1–2 weeks. P0155 doesn’t cause immediate engine damage, but a cold sensor reduces fuel economy and increases emissions. Prolonged driving may also prevent you from passing emissions tests due to the "Not Ready" monitor status.
Q: How much does it cost to fix P0155?
A: Costs are moderate: O2 sensor = $70–$150 (DIY); fuse/relay = $10–$30; wiring repair = $20–$50. Professional labor = $100–$200 (if sensor is hard to access, e.g., near turbochargers). The CR MAX BT avoids overspending on unnecessary ECM repairs.
Q: Why does P0155 come back after replacing the O2 sensor?
A: Common reasons: 1) Unrepaired wiring short (still causing excessive current); 2) Blown fuse/relay (not replaced); 3) Corroded connector (not cleaned); 4) ECM heater control fault. Re-run the CR MAX BT’s circuit and fuse tests to find leftover issues.
Q: Will a bad O2 sensor heater affect the sensor’s ability to read exhaust gases?
A: Yes—cold sensors (without heater power) can’t accurately detect oxygen levels until the engine heats them passively (10–15 minutes). This delay causes the ECM to use default fuel trim values, leading to poor economy and rough idle during warm-up. Once heated, the sensor may work normally, but the heater fault still triggers P0155.
Q: Do all O2 sensors have heaters, and can that affect P0155 diagnosis?
A: Most modern O2 sensors (1996+) have heaters, but some older downstream sensors don’t. The CR MAX BT’s vehicle-specific diagrams clarify if Bank 2, Sensor 1 has a heater—avoiding misdiagnosis. If the sensor lacks a heater, P0155 is a false code (likely due to ECM software issues).
