Code
P1135
CHRYSLER
P — Powertrain
HO2S Lean Average Bank 1 Sensor 1
AI status
Completed
Completed
100%
Causes
- Vacuum or intake manifold leak upstream of sensor
- Low fuel pressure or weak fuel pump
- Restricted or dirty mass airflow (MAF) sensor
- Faulty or contaminated bank 1 sensor 1 (HO2S)
- Wiring harness or connector damage (open/short/high resistance) to sensor
- Leaking or disconnected EVAP/PCV hose
Symptoms
- Check Engine MIL illuminated
- Poor fuel economy (lean condition)
- Rough idle, hesitation or stumble on acceleration
- Hard starting or long cranking
- Possible failed emissions test (high O2 sensor/fuel trim/NOx readings)
What to check
- Read freeze frame and live data with a scan tool (O2 voltage, STFT/LTFT, fuel trims, MAF, fuel pressure if available)
- Inspect wiring and connector at bank 1 sensor 1 for damage/corrosion
- Perform visual inspection for vacuum and exhaust leaks
- Check for related codes (fuel trim, MAF, injector, EVAP)
- Measure fuel pressure and compare to manufacturer spec
- Smoke test intake and vacuum system if a leak is suspected
Signal parameters
- Narrowband O2 sensor voltage range: ~0.1 V (lean) to ~0.9 V (rich)
- Upstream sensor should switch frequently during closed-loop (~0.2–0.8 V typical switching)
- Long-term fuel trim (LTFT) significantly positive (e.g., > +10 to +25%) may accompany P1135
- Short-term fuel trim (STFT) values and MAF counts should be monitored for correlation
- Heater circuit voltage ≈ battery voltage; heater resistance depends on sensor type (consult service manual)
Diagnostic algorithm
- Retrieve and record freeze frame, pending and stored codes. Clear codes then drive to see if P1135 returns.
- Use a scan tool to monitor bank 1 sensor 1 voltage, switching behavior, STFT and LTFT, MAF, and fuel trim while engine warms to operating temperature.
- Perform a visual and wiggle test of the sensor connector and harness for intermittent faults.
- Inspect for vacuum and intake leaks: check all vacuum hoses, intake boots, EGR, and PCV connections; perform smoke test if needed.
- Check for exhaust leaks near sensor that could cause false lean readings.
- Verify fuel pressure and volume under load; replace fuel filter or test pump if pressure low.
- Clean or test MAF sensor and compare MAF airflow values to expected at idle and known RPMs.
- Backprobe the sensor: verify heater supply and ground, and measure signal voltage with engine running and during induced rich/lean conditions.
- If wiring and system checks are good but sensor signal is slow or stays lean, replace bank 1 sensor 1 with OE or equivalent and retest.
- If problem persists after sensor replacement, inspect injectors, PCM grounds, and consider PCM/software updates.
Likely causes
- Intake vacuum leak (vacuum hoses, intake manifold gasket, throttle body)
- Faulty HO2S (contamination, slow response, heater failure)
- Fuel delivery problem (low fuel pressure or clogged fuel filter)
- MAF sensor dirty/misaligned or faulty
- Damaged sensor wiring or poor connector contact
Fault status
Status
HO2S Lean Average — Bank 1 Sensor 1: upstream oxygen sensor reports a persistent lean-average condition; investigate sensor, fuel delivery, intake/exhaust leaks, MAF and wiring.
Repair difficulty: Medium
Diagnostic time: 1.5 - 3.0 hours
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