P0173

Fuel Trim Malfunction (Bank 2)

P0173 is a generic OBD-II powertrain diagnostic trouble code: Fuel Trim Malfunction (Bank 2). It is logged by the engine control unit when the o2/lambda monitor detects that a specific fault threshold has been exceeded — typically resulting in the malfunction-indicator lamp (MIL / check-engine light) being illuminated.

Code
P0173
Group
Powertrain
System
O2/Lambda
Severity
Warning (MIL on)
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What P0173 means

P0173 is an older generic OBD-II code indicating a fuel trim malfunction on Bank 2 — the engine bank that does not contain cylinder No. 1. It is stored when the PCM determines that both short-term and long-term fuel trim corrections on Bank 2 have moved beyond the calibrated acceptance window (typically ±25% for LTFT, though exact thresholds vary by manufacturer) in either direction: excessively positive (lean condition, PCM adding fuel) or excessively negative (rich condition, PCM removing fuel). Unlike the more specific P0171 (Bank 2 too lean) and P0172 (Bank 2 too rich), P0173 does not distinguish direction — it reports only that the trims are out of range.

Lean-biased P0173 is commonly caused by vacuum leaks introducing unmetered air into the intake manifold downstream of the MAF sensor, a contaminated or failed MAF sensor under-reporting airflow, low fuel pressure from a failing fuel pump or clogged filter, or clogged injectors delivering insufficient fuel. Rich-biased P0173 typically arises from a leaking fuel pressure regulator, saturated fuel injectors, a faulty upstream O2 sensor reporting a falsely lean signal, or a large evaporative emissions leak flooding the intake with raw fuel vapour. Exhaust leaks near the upstream B2 O2 sensor can dilute the exhaust sample and cause the PCM to incorrectly enrich the mixture.

P0173 is closely related to P0170 (same generic malfunction on Bank 1). Because the code covers both lean and rich extremes, live fuel trim data from a scan tool is essential to determine direction before component replacement. Driving with severely out-of-range trims risks catalytic converter damage from unburnt fuel (rich) or elevated combustion temperatures (lean).

Common causes

Most-frequently reported root causes when P0173 is logged.

  • 1
    Vacuum or intake manifold leak on Bank 2 introducing unmetered air, pushing trims positive (lean correction).
  • 2
    Contaminated or failed MAF sensor reporting inaccurate airflow data, causing incorrect base fuel calculation.
  • 3
    Low fuel pressure from a weak fuel pump, clogged fuel filter, or faulty pressure regulator.
  • 4
    Clogged or stuck-open fuel injector(s) on Bank 2 delivering too little or too much fuel.
  • 5
    Faulty upstream Bank 2 O2 sensor (Sensor 1) providing incorrect exhaust gas readings that mislead closed-loop trim.
  • 6
    Exhaust leak near the Bank 2 upstream O2 sensor diluting the exhaust sample with fresh air and causing false lean detection.
  • 7
    Evaporative emissions (EVAP) system malfunction saturating the intake with purge vapour, pushing trims rich.
  • 8
    PCV system fault — excessive crankcase pressure or oil mist entering the intake on Bank 2 side.

Symptoms drivers notice

Check Engine Light (MIL) illuminates with P0173 stored.
Rough idle, engine stumble, or misfires — more pronounced if trims are at their correction limit.
Noticeably reduced fuel economy as the PCM attempts to compensate for the air-fuel imbalance.
Hesitation or sluggish throttle response under load.
Possible black smoke from the tailpipe (rich) or lean surge/backfire under deceleration (lean).
Catalytic converter odour or overheating if a rich condition deposits unburnt fuel over an extended period.

How to diagnose P0173

A typical diagnostic flow when this code is present.

  1. 1
    Connect a scan tool and retrieve all stored codes; note any companion codes (P0170, P0171, P0172, MAF codes, O2 codes, misfire codes) that narrow the direction and root cause.
  2. 2
    Read live short-term fuel trim (STFT) and long-term fuel trim (LTFT) data for Bank 2 at idle and at 2,500 rpm; positive values above +10% indicate lean (missing fuel or extra air); negative values below -10% indicate rich (extra fuel or reduced air demand).
  3. 3
    Perform an intake vacuum leak test using a smoke machine or propane enrichment at idle — a lean condition that improves with propane near a specific location confirms the leak point.
  4. 4
    Inspect and clean the MAF sensor with dedicated MAF cleaner spray; retest trims; replace if trim error persists and the MAF voltage deviates from specification at known airflow rates.
  5. 5
    Check fuel pressure at the fuel rail with a gauge; pressure below specification at idle or during snap-throttle points to a weak pump or clogged filter.
  6. 6
    Inspect the upstream Bank 2 O2 sensor (B2S1) live data for correct cross-count rate and voltage range; a sluggish or biased sensor can cause the PCM to over-correct trims.
  7. 7
    If trims are rich, perform an EVAP system test and check the PCV hose for oil contamination; inspect injectors for leakdown with a fuel-off pressure drop test.

Related powertrain codes

Frequently asked questions

What is the difference between P0173 and P0171 or P0172?

P0171 specifically means Bank 1 fuel trim is too lean (trims forced excessively positive), and P0172 means Bank 1 is too rich. P0173 is the Bank 2 generic equivalent that flags when trims are out of range in either direction, without specifying lean or rich. P0173 pre-dates the more granular P0174 (Bank 2 lean) and P0175 (Bank 2 rich) codes. Live trim data on the scan tool is required to determine the actual direction of the fault.

Can a vacuum leak on Bank 2 cause P0173?

Yes — a vacuum or intake leak downstream of the MAF sensor introduces unmetered air specifically into Bank 2 cylinders. The PCM detects the resulting lean exhaust signal from the Bank 2 upstream O2 sensor and adds fuel via positive fuel trim corrections. When those corrections exceed the calibrated limit, P0173 is stored. A smoke test of the intake system is the most reliable way to find small leaks.

Can a bad MAF sensor cause P0173 on only one bank?

A failing MAF sensor affects the global airflow calculation for all cylinders, so trim faults tend to appear on both banks. However, if the MAF reading error combines with a minor Bank 2-specific issue (small injector variance, slight intake difference), P0173 may appear on Bank 2 first. Cleaning or replacing the MAF is always a cost-effective early step when fuel trim codes are present.

Is it safe to drive with P0173?

Short trips are generally tolerable, but driving for extended periods with severely out-of-range fuel trims risks catalytic converter damage (from rich-condition unburnt fuel or lean-condition high temperatures) and increased wear on engine components. The fault should be diagnosed and resolved promptly; it is not a 'monitor and ignore' situation.

Disabling P0173 in software

RaceTune can permanently disable P0173 — and any other OBD-II diagnostic trouble code — on every ECU family we support. The monitor is disabled inside the ECU itself, so the fault stops being logged: the warning light stays off and the engine never enters limp mode for this code. The change is tied to your exact software version.

Permanent
The monitor is disabled in the ECU itself — not just cleared. It cannot return.
Tailored to your file
Each patch is matched to your specific software version — never a one-size-fits-all file.
Reversible
The original file is always preserved. Reflash the stock to return the ECU to factory state.

Software modifications affect emissions compliance and are not road-legal in many jurisdictions. RaceTune service files are intended for motorsport, off-road, and export use.

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