Code
P1170
SCION
P — Powertrain
Fuel system too lean
AI status
Completed
Completed
100%
Causes
- Vacuum or intake air leak (hose, intake manifold gasket, throttle body, intercooler piping)
- Low fuel pressure (weak pump, clogged filter, faulty regulator)
- Faulty or contaminated mass airflow (MAF) sensor
- Leaking or malfunctioning fuel injectors (reduced flow)
- Exhaust leak upstream of the upstream O2 sensor
- Faulty upstream oxygen sensor or sensor heater circuit
Symptoms
- Check Engine Light (MIL) illuminated
- Rough idle or intermittent misfire
- Poor acceleration and reduced engine power
- Hesitation or stumbling on acceleration
- Increased crank time or hard starting (lean at idle)
- Possible poor fuel economy (in some conditions)
What to check
- Read stored freeze frame and pending codes with a scan tool; record fuel trims, O2 sensor voltages, MAF, MAP and fuel pressure data
- Visual inspection for obvious vacuum/intake leaks, disconnected hoses or damaged intake ducting
- Listen for hissing near intake manifold or vacuum lines; perform smoke test if available
- Check fuel level and verify no recent contamination
- Inspect MAF sensor for heavy contamination and wiring connectors for damage
- Measure fuel pressure at the rail with a gauge and compare to specification
Signal parameters
- Upstream O2 sensor voltage low/remaining near 0.1–0.2 V (indicates lean)
- Short-term fuel trim (STFT) and long-term fuel trim (LTFT) large positive (> +10–25%)
- MAF sensor low airflow voltage or low grams/sec vs expected at given RPM/load
- Fuel rail pressure below manufacturer specification (psi or kPa)
- MAP/vacuum higher than expected at idle (indicates extra air)
- Intake air temperature (IAT) and coolant temp should be within expected range for fueling calculation
Diagnostic algorithm
- Retrieve all diagnostic trouble codes and freeze frame; note conditions when code set (load, rpm, temp).
- With a scan tool monitor live data: STFT/LTFT, upstream O2 voltage, MAF g/s or frequency, MAP/vacuum, fuel pressure, engine coolant temp and IAT.
- Inspect the intake system: check clamps, boots, PCV and vacuum lines for cracks or disconnection; perform a smoke test to find small leaks.
- Clean or inspect the MAF sensor and its wiring; substitute known-good MAF if suspect and re-check fuel trims.
- Measure fuel pressure at the rail under key-on and engine-running conditions; compare to spec and test for pressure drop under load.
- Inspect and test fuel injectors for proper pulse and flow; use a balance/tester or perform injector resistance check and waveform inspection.
- Check upstream O2 sensor operation and heater circuit; verify sensor switches and responds when inducing rich/lean conditions.
- Inspect for exhaust leaks upstream of the upstream O2 sensor which can cause false lean readings.
- Check EVAP purge valve operation (stuck open can introduce extra air) and related hoses.
- After repairs, clear codes and perform a road test while monitoring fuel trims and O2 response to confirm the issue is resolved.
- Note: When working on fuel system components take appropriate safety precautions (no open flames, relieve fuel pressure before disconnecting lines).
Likely causes
- Intake vacuum leak or air leak between MAF and throttle
- Low fuel pressure due to pump or clogged filter
- Contaminated/failed MAF sensor giving low airflow signal
- Upstream O2 sensor reading low voltage (lean) due to actual lean or sensor fault
Fault status
Status
ECM detected a persistent lean condition (fuel system too lean). Monitor shows O2/trim values outside allowable range. Check for air leaks, low fuel pressure, faulty MAF or O2 sensor, and faulty injectors.
Repair difficulty: Medium
Diagnostic time: 1.0 - 3.0 hours
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