P0173

Code

MITSUBISHI P — Powertrain

Abnormal fuel system 2

Brand: MITSUBISHI

Type: P — Powertrain

Views: UK: 9 EN: 14 RU: 19

AI status

Completed

ready

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Page language: EN

Causes

Faulty or slow oxygen sensor (Bank 2)
Contaminated or faulty mass airflow (MAF) sensor
Vacuum or intake leak on Bank 2
Leaking or stuck-open fuel injector(s) on Bank 2
Excessive fuel pressure or faulty fuel pressure regulator
Wiring harness connector or ground fault affecting sensors on Bank 2

Symptoms

Check Engine light illuminated (MIL)
Poor fuel economy (running rich) or strong fuel smell
Rough idle or hesitation under acceleration
Black exhaust smoke (when rich)
Possible misfire or reduced engine power
Failed emissions test

What to check

Read freeze frame and live data with a scan tool; record STFT and LTFT for Bank 1 and Bank 2, and O2 sensor behavior
Inspect for diagnostic trouble codes related to O2 sensors, MAF, fuel pressure, or misfires
Visual inspection for vacuum leaks, cracked hoses, intake manifold gasket leaks, and exhaust leaks upstream of O2 sensor
Check MAF sensor condition and wiring/connectors
Measure fuel rail pressure and compare to specification
Back-probe Bank 2 O2 sensor (and sensor signal ground) and observe switching/response

Signal parameters

Long Term Fuel Trim (LTFT) Bank 2 — percent (normal ~±10%; codes often when outside ~±10–15%)
Short Term Fuel Trim (STFT) Bank 2 — percent (rapid swings or large steady offsets indicate issue)
O2 sensor voltage Bank 2 (narrowband ~0–1.0 V switching; slow/flat or stuck values indicate fault)
Wideband O2/B1S2 or B2S1 lambda reading if equipped — stoich deviation
MAF sensor output (g/s or V) vs. expected for RPM/load
Fuel rail pressure (kPa or psi) — compare to spec and regulator behavior under vacuum

Diagnostic algorithm

Retrieve all stored codes and freeze-frame data. Note whether only P0173 is present or other related codes (O2, MAF, fuel system).
With a scan tool monitor STFT and LTFT for both banks at idle and under load. Compare Bank 1 vs Bank 2 to isolate bank-specific issues.
Visually inspect intake tract, vacuum lines, PCV and intake manifold gasket for leaks on Bank 2 side. Perform a smoke test if available.
Check MAF sensor cleanliness and wiring; clean MAF per service procedure if contaminated and re-test.
Measure fuel rail pressure at idle and under commanded fuel trim conditions; compare to manufacturer spec. Inspect fuel pressure regulator and fuel return for faults.
Back-probe Bank 2 O2 sensor(s). Confirm sensor switches and responds to induced changes (e.g., snap throttle or introduce slight vacuum leak). For wideband sensors, check lambda values.
Inspect and pressure-test fuel injectors on Bank 2 for leakage or abnormal flow; consider swapping injectors with Bank 1 (if identical) to see if code follows injector.
Check wiring harness and connectors for the Bank 2 O2 sensor(s), MAF, injectors and grounds. Repair any damaged wiring or poor grounds.
If suspicion of exhaust leak or catalytic restriction, inspect manifold and measure backpressure. Replace catalyst if restricted.
After repairs or component replacements, clear codes and road-test while monitoring live fuel trims and O2 behavior to confirm resolution.
If no root cause found after checks, consider ECU/PCM fault or intermittent wiring issues and escalate diagnostics with manufacturer procedures.

Likely causes

Bank 2 oxygen sensor degraded or slow (most common trigger for this code)
Vacuum/intake leak or cracked manifold gasket on Bank 2 causing lean or unstable trims
Leaking fuel injector on Bank 2 causing rich condition
High fuel rail pressure (stuck regulator or weak return) causing rich condition
MAF sensor contamination producing incorrect load signal

Fault status

⚠️ Status

Powertrain control module detected abnormal fuel trim/oxygen sensor readings on bank 2. Emissions control may be affected and engine drivability can be impaired. Further testing required to determine root cause.

🟡 Repair difficulty: Medium

⏱️ Diagnostic time: 1-3 hours

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P0173 — Abnormal fuel system 2