Updated September 2025 — by Gianni Tritella . The error code P0172 – Fuel Mixture Too Rich (Bank 1) means that the engine is receiving too much fuel or too little air . Here you will find symptoms , causes , step-by-step diagnoses and solutions based on real workshop cases.

Read Also: FREQUENT OBD-II DTCs – P0299, P0171, P0172, P0300, P030x, P0106, P2195, P2196: MEANING, SYMPTOMS, DIAGNOSIS AND SOLUTIONS

Last updated: 08/09/2025 • Based on workshop diagnoses and OEM technical bulletins

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P0172 – Rich Mixture (Bank 1): A Practical Guide to Symptoms, Diagnosis, and Solutions

✅ Introduction

When P0172 appears, the ECU detects that the air/fuel ratio is too rich on Bank 1. In practice, the fuel trims (STFT/LTFT) are negative because the ECU removes fuel to get back within the target. Ignoring the problem leads to high fuel consumption , fouled spark plugs , diluted oil , and the risk of damage to the catalytic converter due to excess unburned fuel.

ℹ️ What is P0172 (too rich mixture)

Operational definition : Long-term trimmers ( LTFTs ) become negative beyond a threshold (e.g. < −10÷−15%) because the ECU is compensating for excess fuel or air deficit. Causes range from leaking injectors , too high fuel pressure , EVAP purge stuck open , to implausible MAF/MAP and air-side blockages .

⚠️ Symptoms on the road

• Smell of gasoline from the exhaust, black smoke when accelerating.
• Consumption is clearly worse, idling is irregular when hot.
• Long hot start (dripping injectors), black spark plugs .
• Check engine light and sometimes related codes (P2196, P0106, P0175, misfire).

🔧 Most common causes (sorted by actual frequency)

• EVAP purge stuck open → extra fuel vapors at idle/partial throttle.
• Leaking injectors or injectors with an altered spray pattern ; on GDI, even tip fouling.
• High fuel pressure (faulty regulator, FPR without reference, blocked return; on returnless: rail sensor/pump driver).
• Implausible MAF/MAP (MAF overestimates flow; MAP out of whack → enriches).
• Air side obstructions : saturated air filter, dirty throttle/ducts, misaligned throttle body.
• Incorrect ECT/IAT (engine “felt” cold → excessive enrichment).
• Lambda pre-cat “stuck rich” or slow that drives the correctors towards rich (be careful: it is often an effect , not a cause).
• On LPG : map too rich, high reducer pressure, saturated filters.

🔎 Step-by-step diagnosis (operational checklist)

1. Read DTC + freeze frame → rpm, water/air temp, load, speed. Know when it triggers (warm idle? highway?).
2. STFT/LTFT → at minimum and at 2,500 rpm. Indicative thresholds: LTFT < −10÷−15% with STFT on average negative.
3. EVAP purge → pinch test of the hose towards the manifold at idle (diagnostic only): if the correctors return to 0, the purge is leaking .
4. Fuel pressure → idle and load, compare with OEM data. If high: FPR/regulator, rail sensor, pump control, blocked return.
5. Injectors → balance and leak-down test; inspection of spark plugs (black/wet) and petrol smell in the oil.
6. MAF/MAP plausibility → MAF: snap-throttle responsiveness, wiring; MAP KOEO ≈ barometric, consistent with vacuum at idle.
7. Air side → filter, ducts, dirty throttle, PCV stuck closed (less air at idle).
8. Lambda pre-cat → if “stuck rich”, first exclude enrichment causes; replace only if slow or defective and documented.

🧪 Essential practice tests

1) Fuel trims: how they move

• At warm idle : very negative LTFT (e.g. −12%) + negative STFT → suspect EVAP , dripping injectors or clogged air.
• At 2,500 rpm : if the trimmers improve, the richness is more evident at idle → EVAP/injectors. If they remain negative → high pressure/sensors.

2) EVAP purge – quick test

• Temporarily isolates the purge at idle . If the rpm rises and the trims return to 0, the valve will not hold .

3) Fuel pressure

• Measure with a pressure gauge/scanner. If the pressure is above specification or does not vary with vacuum on the FPR → faulty regulator or clogged return line.

4) Injectors – balance/leak

• Uniform pressure drop between cylinders. If one drops more than the others or the pressure drops with the engine off → injector leak .

5) MAF/MAP plausibility

• MAF overestimating flow → negative trims. Clean with a suitable spray and check connectors/ground/5V.

🛠️ Remedial interventions (from simple to complex)

• Air side : new filter, clean throttle body/ducts; check PCV and throttle alignment.
• EVAP purge : replace solenoid valve and check canister/hoses for leaks.
• MAF/MAP : clean MAF with specific spray; replace sensors only if defect is evident .
• Fuel pressure : reset FPR/regulator; on returnless, check rail sensor/pump driver and any software updates.
• Injectors : ultrasonic cleaning on the bench with flow rate check; if they drip → replace (consistent set).
• Lambda pre-cat : replace only if “stuck rich”/slow confirmed after excluding upstream causes.

💰 Indicative costs

Intervention	Description	Fork*
Diagnosis P0172	OBD, STFT/LTFT, purge test, rail pressure	€60–€120
Air side restoration	Filter, throttle, lines, PCV	€70–€200
EVAP purge	Solenoid valve + pipes	€70–€180
MAF/MAP	Cleaning/replacing the sensor and restoring the wiring	€40–€70 / €120–€220
Fuel pressure	Regulator/FPR, rail sensor, pump driver	€120–€380
Petrol injectors	Bench cleaning/replacement (each)*	€40–€80 / €90–€180*

*Indicative prices excluding VAT; prices vary by area, spare parts, and model. Injector costs are generally higher on GDI.

🛡 Prevention & best practices

• Good air filters and clean intake (no oiled filters on sensitive MAFs).
• Quality fuel and periodic EVAP/PCV maintenance.
• Punctual checks to avoid sludge that alters pump pressure/actuator.
• On LPG : check filters and fine-tune the map to avoid chronic enrichments.

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❓ FAQ – Frequently Asked Questions

📌 Conclusions

P0172 shouldn't be "turned off" randomly. With a thorough checklist (freeze-frame → trims → EVAP → pressure → injectors → MAF/MAP → lambda), you can find the cause without unnecessary replacements. Fixing it early protects the catalytic converter and restores fuel economy and smooth running.

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