Code
P0151
MITSUBISHI
P — Powertrain
Oxygen sensor2(front) low
Views:
UK: 15
EN: 29
RU: 27
AI status
Completed
Completed
100%
Causes
- Faulty Bank 2 Sensor 1 (upstream oxygen sensor)
- Open or shorted wiring in sensor signal circuit
- Poor connector contact or corrosion at sensor plug
- Failed sensor heater circuit or fuse/relay issue
- Exhaust leak upstream of the sensor
- Intake vacuum leak or lean fuel condition (fuel pressure, injectors, MAF)
Symptoms
- Check Engine Light illuminated (MIL)
- Poor fuel economy
- Engine may run rough at idle or during acceleration
- Delayed or incomplete closed-loop operation (long warm-up)
- Possible hard cold start or stumbling
What to check
- Read and record freeze frame/pending codes with a scan tool
- Monitor live O2 sensor voltage PID for Bank 2 Sensor 1 and compare to Bank 1 Sensor 1
- Check short-term and long-term fuel trim values for signs of lean condition on bank 2
- Perform a visual inspection of sensor, wiring, and connector for damage or corrosion
- Check for exhaust leaks upstream of the sensor
- Verify fuel pressure and inspect for vacuum leaks and MAF sensor cleanliness
Signal parameters
- Expected upstream O2 voltage: switches roughly between ~0.1 V (lean) and ~0.9 V (rich) when at operating temperature (varies by model)
- Low-voltage fault threshold: sensor voltage stuck below expected lean-level (commonly
- Switching frequency when healthy: typically 1–3 Hz in closed-loop operation
- Heater resistance: typically low (a few ohms to tens of ohms) — consult vehicle-specific spec
- Heater supply: powered by fused 12 V and controlled/grounded by ECM (verify with scan tool/voltmeter)
Diagnostic algorithm
- Connect a capable scan tool, retrieve stored/PID data and freeze frame parameters
- Confirm code and monitor Bank 2 Sensor 1 voltage while engine is warmed up; compare to Bank 1 Sensor 1 behavior
- Inspect sensor connector and harness for damage, corrosion, or disconnection; repair any issues found
- Backprobe the sensor connector: verify reference ground and sensor signal voltage to ECM with key on/engine running
- Check heater circuit: measure resistance at the sensor, verify 12 V supply and ECM control/ground operation; replace sensor if heater open/faulty
- If sensor signal is low but heater and wiring are good, inspect for exhaust leaks upstream and test for vacuum leaks or fuel delivery problems (fuel pressure test, MAF, injectors)
- Repair wiring or replace sensor as indicated, clear codes and perform test drive; recheck for return of code or correct sensor switching behavior
- If wiring, power, and vehicle systems check OK but problem persists, consider ECM diagnostics or replacement by a qualified technician
Likely causes
- Damaged sensor wiring harness (chafed, melted, broken)
- Contaminated or failed oxygen sensor
- Failed sensor heater (no warm-up, sensor stays low)
- Exhaust manifold or gasket leak near sensor
- Vacuum leak causing sustained lean mixture
Fault status
Status
ECM reports Bank 2 Sensor 1 voltage lower than expected. Possible causes include a failed oxygen sensor, open/shorted signal wiring, heater fault, or genuine lean condition (exhaust leak, vacuum leak, fuel delivery issue). Further diagnostics required.
Repair difficulty: Medium
Diagnostic time: 1.0-2.5 hours
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