Code
P1155
FIAT
P — Powertrain
Upstream heater resistance 2 fault
Views:
UK: 6
EN: 14
RU: 8
AI status
Completed
Completed
100%
Causes
- Open or shorted heater element inside the upstream O2 sensor (sensor 2)
- Corroded, damaged or disconnected heater circuit connector/pins
- Broken or shorted wiring in the heater circuit (including chafing or rodent damage)
- Blown fuse or faulty relay supplying heater power
- Poor ground connection for the sensor heater circuit
- Faulty ECM/PCM (rare)
Symptoms
- Malfunction Indicator Lamp (MIL) illuminated
- Reduced cold-start emissions control and potential harder cold starts
- Failed emissions test
- Possible rough idle or increased fuel consumption under some conditions
- Diagnostic trouble code stored and freeze-frame data available
What to check
- Read all stored DTCs and freeze-frame/live data with a scan tool; note related codes
- Visually inspect the upstream O2 sensor (sensor 2) connector and wiring for corrosion, damage, loose pins or water ingress
- Check relevant fuses/relays for the heater circuit
- Wiggle test wiring while monitoring heater circuit or live data for intermittent faults
- Measure resistance of the heater element at the sensor connector with the sensor disconnected
Signal parameters
- Heater resistance (typical expected range): approximately 2–15 ohms (manufacturer-specific — consult service data)
- Supply voltage to heater with key ON (fused feed): about battery voltage (~10–14 V)
- Heater current draw when energized: typically 0.5–2 A (varies by sensor)
- ECM monitors for open/high resistance or short/low resistance compared to expected value
Diagnostic algorithm
- Retrieve and record DTC(s) and freeze-frame data. Check for related codes (powertrain, grounds, battery voltage).
- Perform a visual inspection of the upstream oxygen sensor 2 harness and connector for corrosion, bent pins, damage or water intrusion. Repair or clean as needed.
- Check the fuse(s) and relay(s) for the O2 heater circuit; replace if blown or faulty.
- With the sensor disconnected, measure resistance across the heater pins on the sensor connector using a multimeter. Compare to OEM spec. If open or out-of-range, replace the sensor.
- If sensor resistance is within spec, backprobe the harness at the connector with connector plugged or ignition ON to verify supply voltage and ground control from the ECM. Verify voltage feed and switching (if applicable).
- Check continuity between the sensor connector and the ECM pins for heater supply and ground; look for high resistance, shorts to ground or battery, or intermittent opens.
- Repair any damaged wiring, connectors or grounds found. If repairs made, clear codes and perform a drive cycle to confirm the fault does not return.
- If wiring and supply check good but intermittent or unexplained resistance fault persists, replace the upstream O2 sensor (sensor 2) with OEM-specified unit and retest.
- If new sensor and wiring are correct but code remains, inspect ECM/PCM connector and grounds; consider ECM testing/replacement as last resort.
Likely causes
- Open heater element in the upstream oxygen sensor (most common)
- Corroded connector or poor pin contact at the sensor harness
- Supply fuse for O2 heater open or power feed disconnected
- Wiring short to ground or short to voltage in the heater circuit
- Intermittent connection due to damaged insulation or broken wire
Fault status
Status
ECM detected abnormal resistance in the upstream oxygen sensor heater circuit (heater 2). This indicates the heater element or its wiring/connector is open, shorted, or has excessive resistance. Repair wiring or replace the sensor as needed and confirm the fix by clearing the code and rechecking.
Repair difficulty: Medium
Diagnostic time: 0.5-2 hours
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