P0172 Code: Honda Accord (2018-2023) - Causes, Symptoms & Fixes

P0172 Diagnostic Guide for 2018-2023 Honda Accord (OBD-II)

Important data note (limits of the dataset you provided)

• NHTSA data for this exact model/year and code: No owner complaints found.
• Recalls/TSBs in the provided data: No recalls found in NHTSA database.
• Because there is no complaint/recall data specific to this vehicle/year for P0172 in your dataset, this guide uses general OBD-II knowledge plus Honda Accord-specific considerations to inform diagnostic steps and repair options. Data limitations mean model-year specific trends or dealer-issued fixes aren’t cited here.

CODE MEANING AND SEVERITY

• Code meaning: P0172 = Fuel System Rich (Bank 1). The PCM (engine computer) detects that long-term fuel trim is significantly positive, indicating the engine is running richer than commanded.
• Bank designation: For 4-cylinder Honda Accords (typical engines in 2018-2023), “Bank 1” corresponds to the single bank of cylinders in the inline engine.
• Severity and symptoms: Potentially indicates a persistent rich condition. If unresolved, you can see:
  • Reduced fuel economy
  • Rough idle or hesitation
  • Misfire symptoms in severe cases
  • Potential black smoke or strong fuel odor from the tailpipe
  • Possible catalyst overheating or damage if the rich condition is sustained
• Important caveat: A single P0172 may be caused by a variety of issues (air, fuel, sensors, or controls). It’s common to see P0172 alongside related codes (e.g., P0171) if there’s a broader fueling issue or vacuum leaks.

COMMON CAUSES ON HONDA ACCORD (2018-2023)

Note: Causes listed are typical for this model range and engine types (1.5L turbo and 2.0L turbo with direct injection). Use them as a starting point; actual cause should be confirmed with diagnostics.

• Vacuum leaks and related components
  • Cracked or loose vacuum hoses
  • Intake manifold gasket leaks
  • PCV valve or PCV hoses
• Mass Air Flow (MAF) sensing issues
  • Dirty or faulty MAF sensor
  • Air leaks between the MAF and the throttle body allowing unmetered air
• Fuel system delivery issues
  • Excessively high fuel pressure or a leaking fuel pressure regulator
  • Clogged or leaking fuel injectors (especially possible with direct-injection engines)
  • Low fuel pressure due to a weak fuel pump or issues with the fuel pump relay
• Oxygen sensors and emissions sensors
  • Faulty upstream (pre-cat) O2 sensor or its heater circuit
  • Faulty downstream O2 sensor causing mismatched feedback
• Engine and intake sensors
  • Faulty or slow coolant temperature sensor (ECT) can skew fuel trims
  • Throttle position sensor (TPS) or intake air temperature sensor (IAT) faults
• Exhaust and intake sequencing
  • Exhaust leaks before the O2 sensors can skew readings
  • EGR valve issues (stuck closed or dirty) can affect fueling feedback in some scenarios
• Fuel quality and additives
  • Poor fuel quality or ethanol content extremes affecting combustion and trims
• Ignition and misfire considerations
  • Misfires can interact with fueling feedback loops and produce misleading LTFT readings if not resolved
• Turbo-specific considerations (for turbo engines)
  • Boost leaks or faulty intercooler lines can indirectly influence fueling data and trims

SYMPTOMS

• Engine light (check engine light) illumination
• Noticeable reduction in fuel economy
• Rough idle, rough running, or hesitation at low throttle
• Hesitation or lack of power during acceleration
• Possible black exhaust smoke or fuel odor (in severe cases)
• OC/engine indicators: LTFT sitting at a high positive value on scan tool; short-term fuel trims may be fluctuating or elevated
• Noisy or pinging knock is not typical for P0172, but ongoing rich mixture can mask or complicate other symptoms

DIAGNOSTIC STEPS (step-by-step approach)

Prepare with a diagnostic tool that can read live data, freeze-frame data, and fault codes.

A. Confirm code and data

• Read DTC P0172 and any accompanying codes (P0171, P0174, P0101, P0131, P0300, etc.).
• Review freeze-frame data: look at engine coolant temperature, long-term and short-term fuel trims, RPM, MAF reading, and barcode of operation at the time of fault.
• Clear codes, drive under conditions that typically trigger the issue, and re-check if the code returns.

B. Inspect for obvious issues

• Visual inspection: vacuum hoses, intake tubing, PCV valve and hoses, intake leaks, damaged or loose clamps.
• Air filter: ensure it is clean and not restricting airflow.
• Look for signs of fuel odor or wetness around injectors or the engine bay.

C. Evaluate fuel and air delivery

• MAF sensor: inspect for contamination or damage.
  • If you suspect a dirty MAF, you can carefully clean it with MAF cleaner (follow product instructions; do not touch the sensor elements with solvents or hard tools).
  • After cleaning, test engine stability and re-check fuel trims.
  • If MAF is faulty, replacement is often required.
• Verify air intake path integrity: ensure there are no unmetered air leaks between MAF and intake manifold.
• Check intake manifold gasket and vacuum lines for leaks via smoke test or spray method (spray around gaskets/hoses and observe engine reaction).
• Sensor checks:
  • Coolant temperature sensor (ECT): ensure readings are reasonable and correlate with actual engine temperature.
  • Oxygen sensors: test upstream O2 sensor response and heating circuit if you have access to a scan tool that can read heater circuits; consider swapping or replacing if readings are suspect.
  • Throttle position sensor: ensure smooth, proper throttle response and no binding.

D. Test fuel system

• Fuel pressure test: measure rail pressure with the manufacturer’s specification (check service manual). Compare to spec for your engine. If pressure is high or low, diagnose regulator, pump, or related circuits.
• Fuel injector health: look for stuck-open or leaking injectors (possible with DI engines). Check injector resistance spec and, if possible, perform balance test or professional injector cleaning.
• Inspect for fuel contamination or quality issues; verify no water in fuel.

E. Check for related engine management data

• Monitor short-term and long-term fuel trims (LTFT) while driving. A persistently elevated LTFT with normal or rich O2 sensor readings supports a fuel delivery or air issue.
• Look for misfire codes (P030x). Misfires can complicate fuel trim readings but should be diagnosed separately.

F. Advanced/optional checks (often handled by pros)

• Smoke test for leaks in vacuum/PCV/EVAP systems.
• EGR valve function check.
• Turbo system checks (boost pressure, leak tests) if you have the 1.5L or 2.0L turbo engine.
• Electrical checks for wiring harnesses and connector corrosion to sensors.

G. Determine the root cause

• If vacuum leaks are found, repair or replace leaking hoses/gaskets and re-test.
• If the MAF is dirty or failing, clean/replace and re-test.
• If fuel pressure is out of spec, repair/replace the fuel pump, regulator, or related components; verify regulator return lines and filter.
• If O2 sensors are faulty, replace the upstream O2 sensor first (as it directly influences fueling feedback) and re-check trims.
• If injectors are leaking or clogged, service/replace injectors and re-test.
• If coolant temperature sensor or other sensors are providing erroneous data, replace and re-test.

RELATED CODES

• P0171: System Too Lean (Bank 1) — often a companion code if there’s a vacuum leak or air intake issue.
• P0174: System Too Rich or Lean (Bank 2) — applicable to V-type engines with two banks; for 4-cylinder Accord, Bank 2 is not used, but this code is listed for completeness on engines with multi-bank configurations.
• P0101: Mass or Volume Air Flow Circuit Range/Performance (MAF sensor issues) — can accompany P0172 if MAF is faulty.
• P0131: Oxygen Sensor Heater Circuit Malfunction (upstream O2 sensor) — related to O2 sensor health and fuel trimming.
• P0300-P0304: Random/Multiple misfire codes — can accompany long-term fuel trim issues and should be checked if engine is running poorly.

REPAIR OPTIONS AND COSTS (2025 PRICES)

Note: Prices vary by region, shop, and engine variant (1.5L vs 2.0L turbo). The ranges below are ballpark estimates for parts and labor in 2025 USD.

• Vacuum leak sources (hoses, PCV valve, gaskets)
  • Parts: $10–$100
  • Labor: $60–$250
  • Typical total: $70–$350
• PCV valve and hoses replacement
  • Parts: $10–$40
  • Labor: $50–$150
  • Total: $60–$190
• MAF sensor cleaning or replacement
  • Cleaning: parts $0–$20 (if you do it yourself), labor minimal
  • Replacement: $150–$350 for parts + $80–$150 for labor
  • Total: DIY cleaning <$100; replacement typically $230–$500
• Oxygen sensors (upstream and downstream)
  • Parts: $80–$250 each
  • Labor: $100–$200 per sensor
  • Total per sensor: $180–$450
  • If only one sensor is required, expect one-side replacement costs
• Fuel system (pump, regulator, and lines)
  • Fuel pump replacement
    • Parts: $150–$450
    • Labor: $200–$600
    • Total: $350–$1050
  • Fuel pressure regulator (and related lines)
    • Parts: $40–$150
    • Labor: $100–$200
    • Total: $140–$350
• Fuel injectors (cleaning vs replacement)
  • Cleaning: $50–$150 per injector (DIY or shop)
  • Replacement: $300–$600 for a set (plus labor)
  • DI engines may show higher injector service costs; budget $600–$1200 for full set replacement in some cases
• Spark plugs (if applicable)
  • Parts: $5–$20 each
  • Labor: $60–$150
  • Total: $100–$220 for a 4-cylinder engine
• Catalytic converter and other major components
  • Not a typical P0172 fix unless there’s prolonged rich condition causing converter damage; costs are substantial (often $800–$2000+ for a converter replacement; labor extra)
• ECU reflash or software updates
  • Dealer service charge may apply; cost varies; not guaranteed to fix P0172 unless Honda issues a software patch

DIY VS PROFESSIONAL

• DIY-friendly tasks (lower risk, good learning opportunity)
  • Inspecting and replacing simple vacuum hoses
  • Replacing or cleaning the MAF sensor (with proper cleaner and careful handling)
  • Replacing a PCV valve and hoses
  • Replacing failed upstream O2 sensor if you have the proper tools and experience
  • Checking and replacing a dirty air filter
• Tasks better left to professionals
  • Accurate fuel pressure testing and fuel system diagnostics (requires fuel pressure gauge and correct procedure)
  • Smoke testing for vacuum leaks (requires specialized equipment)
  • Injectors diagnosis/repair, especially on direct-injection engines
  • Turbo system diagnostics (boost tests) and high-pressure fuel systems
  • EGR valve diagnostics and replacement
  • Electrical/wiring harness diagnostics for sensors
• If you are comfortable with basic automotive work and have the right tools, start with non-invasive steps (MAF cleaning, vacuum inspection, sensor checks). If the code persists, consider professional diagnostics to avoid unnecessary parts replacement.

PREVENTION

• Regular maintenance aligned with Honda’s schedule
  • Replace air filter at recommended intervals; ensure intake paths are clean
  • Inspect and replace PCV valve and hoses as needed
  • Use quality gasoline; avoid fuels with unknown additives or inconsistent octane; monitor ethanol content if your area varies
  • Replace MAF sensor only if dirty or faulty; avoid excessive cleaning
  • Replace upstream O2 sensor if sensor performance is degraded (test first)
• Monitor fuel system health
  • Use a high-quality fuel system cleaner sparingly and as recommended by Honda guidelines
  • Ensure fuel lines, rails, and injectors are in good condition; watch for signs of leaks or poor spray patterns
• Turbo engine considerations (1.5L and 2.0L turbo)
  • Allow adequate warm-up and cooldown; check for boost leaks if performance changes
  • Inspect intercooler hoses and fittings for leaks or damage
• Diagnostics habit
  • If a P0172 occurs, address the root cause rather than just clearing the code
  • Monitor long-term fuel trim values after repairs to confirm normalization

Final notes

• Because the provided dataset shows no NHTSA complaints or recalls for this specific 2018-2023 Accord-P0172 combination, there are no model-year specific recall/TSB entries to reference here. The diagnostic guide focuses on typical, widely applicable causes and Honda-appropriate diagnostic logic.
• Always verify the exact engine variant in your Accord (1.5L turbo vs 2.0L turbo) as fuel system and sensor layouts differ, which can influence the diagnosis and repair approach.
• If you’re not comfortable performing diagnostics or repairs, a qualified technician can perform live data analysis, fuel-trim testing, and precise fuel-pressure measurements to identify and fix the root cause efficiently.