P1131 — HO2S / Fuel Trim Lean — Bank 1 Sensor 1
What it means: HO2S / Fuel Trim Lean — Bank 1 Sensor 1.
How serious / can you keep driving: Caution. Limp to the next safe stop.
Check first: Read short- and long-term fuel trim at idle and at 2500 rpm. Lean at idle that cleans up at speed is a vacuum leak, near enough every time
Don't risk it — look up listed providers near you and give them this code when you call.
Listed providers nearest Macon, GA
Closest first. Tell them P1131 and what the truck is doing. RoadService.app lists these providers and does not employ them or carry out the repair.
- 0.0 mi away — based in Macon, GA · (478) 538-3245
- 0.0 mi away — based in Macon, GA · (478) 216-8330
- 0.0 mi away — based in Macon, GA · (706) 536-3376
Seen on: Light/Medium Duty · Mixed Fleet
What P1131 means
HO2S / Fuel Trim Lean — Bank 1 Sensor 1. Common causes: Unmetered air: cracked intake boot, split PCV hose, leaking intake gasket, or a clamp left loose after a previous repair | Dirty or drifting MAF sensor under-reporting airflow | Weak fuel delivery — tired pump, clogged filter, or restricted injectors on that bank | Exhaust leak upstream of the sensor pulling in fresh air and faking a lean reading | Failed HO2S heater circuit keeping the sensor out of closed loop | An actually failed O2 sensor — real, but well down the list.
How serious is it?
Caution. Limp to the next safe stop.
Most likely causes
- Unmetered air: cracked intake boot, split PCV hose, leaking intake gasket, or a clamp left loose after a previous repair
- Dirty or drifting MAF sensor under-reporting airflow
- Weak fuel delivery — tired pump, clogged filter, or restricted injectors on that bank
- Exhaust leak upstream of the sensor pulling in fresh air and faking a lean reading
- Failed HO2S heater circuit keeping the sensor out of closed loop
- An actually failed O2 sensor — real, but well down the list
First moves (roadside)
- Read short- and long-term fuel trim at idle and at 2500 rpm. Lean at idle that cleans up at speed is a vacuum leak, near enough every time
- Smoke-test the intake from the throttle body back. Ten minutes here saves a parts order
- Check the MAF reading against expected grams per second, and look for oil film on the sensor wire from a soaked air filter
- Check fuel pressure under load, not at idle. A weak pump holds pressure right up until you ask it for fuel
- Inspect the exhaust upstream of the sensor for leaks before condemning the sensor itself
Common misdiagnoses
- Replacing the upstream O2 sensor first. It is reporting the lean condition, not causing it — the classic money-waster on this code
- Clearing the code after a MAF clean without smoke-testing, then watching it return the next day
- Skipping the PCV and brake-booster hoses because the leak only opens up once the engine warms and the rubber moves
- Assuming a failing fuel pump when an exhaust leak ahead of the sensor was pulling in fresh air the whole time
FAQ
HO2S / Fuel Trim Lean — Bank 1 Sensor 1. Common causes: Unmetered air: cracked intake boot, split PCV hose, leaking intake gasket, or a clamp left loose after a previous repair | Dirty or drifting MAF sensor under-reporting airflow | Weak fuel delivery — tired pump, clogged filter, or restricted injectors on that bank | Exhaust leak upstream of the sensor pulling in fresh air and faking a lean reading | Failed HO2S heater circuit keeping the sensor out of closed loop | An actually failed O2 sensor — real, but well down the list.
Caution — Limp to the next safe stop.
The most common cause is: Unmetered air: cracked intake boot, split PCV hose, leaking intake gasket, or a clamp left loose after a previous repair. Other possibilities include Dirty or drifting MAF sensor under-reporting airflow; Weak fuel delivery — tired pump, clogged filter, or restricted injectors on that bank; Exhaust leak upstream of the sensor pulling in fresh air and faking a lean reading; Failed HO2S heater circuit keeping the sensor out of closed loop; An actually failed O2 sensor — real, but well down the list.
Limp to the next safe stop. When in doubt, get a qualified mobile diesel tech on the truck before continuing.
Related codes
Codes that commonly appear alongside P1131, or share the same subsystem.
- Injector Metering Rail 1 Pressure - Data Valid But Below Normal Operating Range (XPI common rail fuel pressure too low) (SPN 157 FMI 1)
- Fuel Delivery Pressure — Low (SPN 94 FMI 1)
- Injection Rail Pressure — Below Normal (SPN 157 FMI 18)
- Injector Metering Rail 1 Pressure - Data Valid But Above Normal Operating Range (XPI common rail fuel pressure too high) (SPN 157 FMI 0)
- Injector #1 open circuit (SPN 651 FMI 5)
- Injector #2 open circuit (SPN 652 FMI 5)
- Injector #3 open circuit (SPN 653 FMI 5)
- Injector #4 open circuit (SPN 654 FMI 5)
Same symptom: Fuel delivery
Other codes on the tank-to-injector path — pressure, filters, water and the pump.
- Fuel System Leak Detected - Large Leak (P0093) · Fuel System
- HP Fuel Rail Pressure Low During Power Enrichment (P1093) · Fuel System
- High-Pressure Fuel System Relief Valve Open (P2BAD) · Fuel System
- Injector #5 open circuit (SPN 655 FMI 5) · Fuel System
- Injector #6 open circuit (SPN 656 FMI 5) · Fuel System
- Fuel Rail/System Pressure Too Low (P0087) · Fuel System
Find a mechanic or related guides
General diagnostic guidance for diesel engines (Cummins, Detroit, PACCAR, Volvo, Mack and more). Not a substitute for a scan-tool diagnosis on your specific truck.