Fuel and injectionGeneric OBD-II codeUrgency: Garage soon
Code P0172: mixture too rich (bank 1)
P0172 means: system too rich (bank 1): the air-fuel mixture has too much fuel or too little air.[1][2] The control unit adjusts the amount of fuel according to what the lambda sensor measures in the exhaust (this is called mixture adjustment or "fuel trim"). With P0172, the sensor detects very little oxygen, meaning too much fuel for the available air, and the control unit has already cut the fuel as far as it can without achieving a correct mixture.[1][3]
What code P0172 means
The control unit adjusts the amount of fuel according to what the lambda sensor measures in the exhaust (this is called mixture adjustment or "fuel trim"). With P0172, the sensor detects very little oxygen, meaning too much fuel for the available air, and the control unit has already cut the fuel as far as it can without achieving a correct mixture.[1][3]
"Bank 1" is the group of cylinders that includes cylinder number 1; P0175 is the same problem on bank 2. Inline four-cylinder engines usually have a single bank, while V6s, V8s and some four-cylinders have two.[2][4]
A negative fuel trim value means the control unit is shortening the injection because the mixture is rich. Normally the trim hovers around zero, within about ±10%.[5]
It is a generic OBD-II code (it starts with P0), so it means the same on any make; what changes from one engine to another is the most likely cause.[6]
Original definition (SAE J2012): System Too Rich (Bank 1)[1][2]
Codes from the same family
They indicate the same problem in a specific cylinder, bank or sensor. Causes, symptoms and diagnosis are those on this page.
Causes of P0172 and its family
From most to least likely, according to the sources.
- 1Dirty, contaminated or faulty mass air flow (MAF) sensor, which gives the control unit a wrong air reading.[1][2][7][8]
- 2Injectors that drip, leak or work badly and deliver too much fuel.[1][2][7][8][9]
- 3Excessive fuel pressure, from a stuck or leaking pressure regulator.[1][2][7][8][9]
- 4Air filter that is clogged or soaked in oil, restricting airflow.[1][7]
- 5Faulty or contaminated front lambda sensor, which reports incorrectly.[1][2][7][8]
- 6Coolant temperature sensor with a wrong reading.[2][8]
- 7Engine oil with the level too high or diluted with fuel (typical of short journeys); on direct injection, a high-pressure pump leaking fuel into the oil, or a badly seated dipstick.[2][9]
- 8Faulty or leaking PCV valve (crankcase ventilation), EVAP purge valve stuck open, or a stuck EGR valve.[1][2][3][9]
- 9Vacuum or air leaks after the MAF sensor (three sources cite them, without detailing the mechanism).[1][8][9]
- 10Less common and cited by a single source: cam timing, cylinder compression, exhaust leaks near the sensors, contaminated fuel, a badly fitted aftermarket intake, or control unit failure.[2][9]
Symptoms
- Engine warning light on in the instrument cluster.[1][3][8]Engine warning light: faults that cause it
- Black smoke from the exhaust.[1][2][7]Black smoke: faults that cause it
- Increased fuel consumption.[1][2][7][8]Uses more fuel: faults that cause it
- Uneven idle.[2][7]Vibrations: faults that cause it
- Hard starting or hesitation when starting.[1][2]Hard to start: faults that cause it
- Strong smell of fuel.[4][7]Fuel smell: faults that cause it
- Loss of power and difficulty accelerating.[4][8]Less power: faults that cause it
- Jerking or misfires.[3][8]Jerking: faults that cause it
- Spark plugs fouled with black soot after running on a rich mixture for a long time.[3][7]
How to diagnose P0172
In order: from the most likely and cheapest to check to the most expensive.
- 1Start with the basics: oil level (if it is high or smells of fuel, it may be diluted), and the condition of the air filter and the intake.[2]
- 2Read the code and the freeze-frame data with a diagnostic tool, and see whether any associated codes are stored, for example from the EVAP system.[2][3][8]
- 3Watch the short-term and long-term fuel trims live: a long-term trim clearly below zero confirms the rich mixture, and on a healthy engine both are close to zero.[2][5][7]
- 4Check the air filter and the MAF sensor (clean it with a dedicated product and refit it completely dry).[1][7][8]
- 5Measure the fuel pressure with a pressure gauge and check the regulator, including whether there is fuel in its vacuum hose.[3][7][8]
- 6Test the injectors with a balance (or current) test to find one that drips or stays open.[2][3][7]
- 7Check the crankcase ventilation (PCV), the EVAP purge valve and the EGR valve.[1][2][3][9]
- 8Leave the coolant temperature sensor and the lambda sensor until last.[1][2][8]
Can I keep driving with P0172?
Garage soon
The sources agree that you can drive for a while without immediate serious risk, but with moderate urgency. Autodoc Club says that in the short term it puts neither the car nor its occupants at risk, although it should be treated with some urgency to avoid greater damage to the engine; for P0175, Autodoc adds that a prolonged rich mixture can damage the catalytic converter, and Carista that catalytic converter overheating is a real risk if the code persists. MechanicBase classes it as medium severity and advises against continuing to drive without repairing it, because of possible damage to pistons, spark plugs and the catalytic converter.[1][3][4][8]
Documented faults related to P0172
Known faults of specific engines and cars, with their repair cost, the affected years and their sources.
Engine sensors: the most serious faults by engine and car
- High severityOccasionalFrequency: Occasional2.0 DTI and 2.2 DTIVP44 injection pump and air mass meter failureGerman Wikipedia documents recurring failures of the Bosch VP44 injection pump and the air mass meter in the early years of this engine. An owner testimony for this engine describes the pump being replaced at 192,500 km, at a cost the owner calls «prohibitive».
- Medium severityCommonFrequency: Common2.0 TDIClogged particulate filter or faulty pressure sensorThe 2.0 TDI's particulate filter clogs if the car mostly makes short journeys, because it never gets to regenerate; sometimes the fault is just the differential pressure sensor. Cleaning it costs around €200 and replacing it, €800 to €3,000.Repair (garage): €800–€3,000
- Medium severityCommonFrequency: Common1.4 and 1.6 8vCrankshaft position sensor failure (TDC sensor)The crankshaft position sensor (capteur PMH, or TDC sensor) appears repeatedly in the reliability sources for the Clio II, the Kangoo I and the Dacia Logan I with this engine, with hard cold starting or stalling as the main symptoms. It is a cheap part, but the failure can leave you stranded.
- Medium severityCommonFrequency: Common1.4 and 1.6 16vCrankshaft position sensor failureThe crankshaft position sensor (capteur PMH) appears repeatedly in the reliability sources for almost every generation with this engine, with hard cold starting, stalling or the car cutting out unexpectedly. It is a cheap part, but the failure can leave you stranded.
Injectors: the most serious faults by engine and car
- High severityVery frequentFrequency: Very frequentOM651Delphi piezo injector failure (220/250 CDI)Yes: 220 and 250 CDI models built between 2008 and 2011 had Delphi piezo injectors that were prone to failing and seizing in the cylinder head. Mercedes-Benz replaced them under warranty in 2009 and switched the 220 CDI to solenoid injectors from 2010-2011, but the 250 CDI kept them.Repair (garage): €3,600–€5,900
- High severityCommonFrequency: CommonBMW N52 / N53N53 piezoelectric injector failureThe N53's piezoelectric direct injectors fail frequently and cause misfires, rough idle and limp mode; replacing them is expensive and they must be coded.
- High severityCommonFrequency: CommonMZR-CDOil dilution from leaking injector seals and DPF regenerationsOn the 2.0 and 2.2 MZR-CD, injector seals that age badly and incomplete particulate filter regenerations on short journeys can let unburnt diesel into the sump, diluting the oil and leaving the engine with less lubrication.
- High severityCommonFrequency: CommonOM642Wear and failure of the Bosch piezo injectorsThe OM642's Bosch piezo injectors can fail from around 150,000-200,000 km, especially with poor-quality diesel; they cannot be repaired, only replaced. Because it is a V engine, those on the rear bank are harder to reach, which pushes up labour costs.Repair: €4,500–€6,500
Fuel pump and high-pressure pump: the most serious faults by engine and car
- Critical severityCommonFrequency: Common2.0 and 2.5 HybridLow-pressure fuel pump failure: loss of power and fire riskYes: a low-pressure fuel pump with a defective impeller (Denso) affects millions of Toyotas and Lexuses, including Corolla, C-HR, RAV4 and Camry models with these engines. It can cause loss of power or stalling, and in the EU there is also a risk of leaks and fire on 2018-2022 Corolla, C-HR and Auris models.
- High severityCommonFrequency: CommonBMW N54/N55High-pressure fuel pump (HPFP) failureThe N54's high-pressure pump is one of its best-documented failures: most units will have had it replaced at some point. It warns of itself with long cranking and loss of power; the 2010 campaign was US-only.
- High severityCommonFrequency: Common2.2/2.5 dCiInjection pump or injector failureThe 2.2 dCi's common rail injectors fail fairly often on the X-Trail and Almera, with cases of seizing and leaks; on the Almera with the older YD22DDT, the documented fault is in the injection pump electronics, not the injectors.
- High severityCommonFrequency: CommonVW 2.0 TFSI EA113Wear of the high-pressure pump cam followerYes: the 2.0 TFSI EA113's high-pressure fuel pump is driven by the camshaft through a follower (tappet) that wears; if it wears through it can damage the pump or the camshaft, so it should be checked and replaced as a preventive measure.Repair (garage): €250–€900Appears at around 90,000 km
Frequently asked questions
Can I drive with code P0172?
Yes, for a while and without pushing the engine, but it should be repaired soon: a sustained rich mixture can damage the catalytic converter, foul the spark plugs and, according to one source, affect the pistons.[1][3][8]
Why does black smoke appear with P0172?
Because the mixture contains more fuel than the available air allows to be burnt. Black smoke from the exhaust, together with higher consumption, is one of the typical symptoms of a rich mixture.[1][2]
What is the difference between P0172 and P0175?
It is the same problem on different banks: P0172 refers to bank 1 (the one that includes cylinder number 1) and P0175 to bank 2. On a single-bank engine only P0172 can appear.[2][3][4]
How do you know whether the mixture is really rich?
With a diagnostic tool you look at the fuel trims: negative values, and a long-term trim clearly below zero, indicate that the control unit is removing fuel because the mixture is rich.[5][7]
Does P0172 mean the same on all makes?
Yes. Codes that start with P0 are generic (defined by the SAE) and have the same definition at every manufacturer; what varies is the part that tends to fail on each engine.[6]
Will I pass the MOT with code P0172?
If the code has switched on the engine warning light, no: with that light on, the inspection records a serious defect, and on Euro 5 and Euro 6 cars the ITV (Spain's roadworthiness inspection, similar to the MOT) also reads the OBD diagnostic system. Repair the fault before you go; clearing the code just before the inspection does not help either, because it counts as a serious defect.[10]
Not sure what is wrong with your car? Describe what you notice in the symptom diagnosis or see all fault codes.
Sources
Every fact on this page links to its source. If you find an error, let us know.
- 1.Código de avería P0172 - guía paso a paso para su diagnóstico y resoluciónAutodoc Club · Technical documentation · consulted 2 October 2026 · in Spanish
- 2.Code P0172 - System Too Rich: Symptoms, causes, common problems, diagnosticSamarins.com · Technical documentation · published 25 June 2023 · consulted 2 October 2026
- 3.P0175: OBD-II code P0175 means the PCM is pulling significant fuel out on bank 2Carista · Technical documentation · consulted 2 October 2026
- 4.Código de avería P0175 - guía paso a paso para su diagnóstico y resoluciónAutodoc Club · Technical documentation · consulted 2 October 2026 · in Spanish
- 5.Fuel Trim InfoRoss-Tech Wiki · Technical documentation · consulted 2 October 2026
- 6.On-board diagnosticsWikipedia · Other source · consulted 2 October 2026
- 7.P0172: Fuel Trim System Too Rich (Bank 1)Carista · Technical documentation · consulted 2 October 2026
- 8.P0172 – System Too Rich (Bank 1)Mechanic Base · Technical documentation · published 7 January 2023 · consulted 2 October 2026
- 9.16556/P0172/000370 - System Too Rich, Bank 1Ross-Tech Wiki · Technical documentation · consulted 2 October 2026
- 10.Desmontando mitos: ¿puede pasar mi coche la ITV con el testigo de «avería motor» encendido?Diariomotor · Motoring press · published 8 March 2021 · consulted 2 October 2026 · in Spanish