What does it mean if I have both P0171 and P0174?

It means the cause is shared by both cylinder banks, which narrows the diagnosis considerably. Look at components common to the whole engine: the mass airflow sensor, the main intake pipework and plenum gasket, the crankcase breather system, the brake servo hose, and fuel supply pressure. Two separate bank-specific faults occurring simultaneously is very unlikely, so anything that only affects one side can be set aside until the shared causes have been ruled out.

Can a four-cylinder car set a P0174 code?

No, not legitimately. Bank 2 is the cylinder bank that does not contain cylinder number 1, and an inline engine only has one bank. If the code appears on a four-cylinder car it usually means a generic reader has misinterpreted a manufacturer-specific code, or the car has a non-standard engine or ECU calibration. Have it read again on equipment that supports the manufacturer's own definitions.

Will P0174 fail an MOT?

Yes, through the engine management lamp, which is a major defect on petrol cars first used on or after 1 July 2003. The emissions readings themselves may well pass, because on a V engine both banks combine before the tailpipe and a correct bank 1 tends to mask a lean bank 2 in the sample. A clean gas printout does not overturn the failure while the lamp is lit.

How do I know which bank is bank 2?

Bank 2 is the bank that does not contain cylinder number 1, using the manufacturer's own cylinder numbering. There is no universal left-right rule, and it varies between makes and even between engines from the same maker. Manufacturer data or a workshop manual for the specific engine is the only reliable source, and it is worth confirming before ordering parts because sensors and gaskets for the wrong bank will not fix anything.

Powertrain code · standard

P0174Second cylinder bank running lean

Standard definition: System Too Lean (Bank 2)

Last updated 27 July 2026

Quick answer

The same lean condition as P0171 but reported for bank 2, which only exists on V and flat engines. The most useful thing about this code is not what it says on its own but whether P0171 is stored alongside it, because that single fact splits the diagnosis in two completely different directions.

How the car works this out

Each bank of a V or flat engine has its own oxygen sensor ahead of its catalytic converter, and the control unit trims fuelling for each bank separately. P0174 is set when bank 2's long-term trim has had to add roughly a quarter more fuel than the base calculation and that bank's exhaust still reads lean.

What you might notice

Rough or uneven idle, sometimes with a slight vibration through the car
Hesitation off idle
A hiss from one side of the engine bay
Frequently nothing beyond the lamp

What causes it

Most likely first. A code names the check that failed, not the part that failed it, so more than one of these can produce the same number.

1A leak affecting both banks — the intake plenum gasket, the brake servo hose, the crankcase breather, or a split in the main intake pipe — which is the answer whenever P0171 is stored tooCommon
How to check: Read both banks' fuel trims. If the two are similar and both large, the cause is upstream of where the intake splits between banks. If only bank 2 is high, it is not.
2A mass airflow sensor under-reporting, which by definition affects both banks equallyCommon
3Low fuel pressure or a restricted filter, again affecting both banks togetherCommon
4A vacuum leak specific to bank 2's inlet runner or a hose on that side, when bank 1 reads normallyPossible
How to check: Smoke test with the intake sealed. A leak on one bank only shows itself at that bank's manifold face.
5Bank 2's oxygen sensor aged or contaminated and reporting lean incorrectlyPossible
How to check: The advantage of a V engine is a free reference. Compare the switching speed and range of both banks' front sensors under the same conditions.
6An exhaust leak on bank 2's manifold or downpipe drawing air past the sensorPossible
7Blocked or dirty injectors on bank 2 onlyLess common

What to do about it

1Read both banks before touching anything. P0171 and P0174 together means one shared cause and there is no point chasing anything bank-specific. P0174 on its own means the cause is on that side and bank 1 is your reference for what normal looks like.
2Where both banks are lean, work through the shared components in order of cost: intake pipework and breather hoses, the airflow sensor, then fuel pressure.
3Where only bank 2 is lean, smoke test the intake and check that bank's manifold gasket and exhaust joints.
4Confirm which side the manufacturer calls bank 2 before removing parts, since it is not fixed by convention and getting it wrong wastes the work entirely.

Can you keep driving?

Yes

The car will normally drive acceptably, though a bank running lean while the other runs correctly can feel slightly uneven at idle. The concerns are the same as any lean condition: higher combustion temperatures over time, and the risk that a worsening lean misfire damages that bank's catalytic converter. The lamp stops it passing an MOT in the meantime.

Will it fail an MOT?

The lamp fails the car, as it does for every engine management fault. What is worth understanding about a single-bank fault is why the gas test may not corroborate it. Both banks merge into one exhaust before the analyser probe, so a lean bank 2 blended with a correct bank 1 often produces a lambda figure inside the 0.97 to 1.03 band and gas readings that look perfectly acceptable. Owners then arrive with a failure certificate and a printout that appears to show clean emissions. The failure was the lamp, and the printout is not evidence the fault is imaginary.

The light this turns on

Engine management lightThe engine management system has found a fault it cannot correct and has stored a code for it. The car will usually still drive, but an illuminated engine light is an automatic MOT failure on petrol cars first used from 1 July 2003 and diesels from 1 July 2008.

Common questions

What does it mean if I have both P0171 and P0174?: It means the cause is shared by both cylinder banks, which narrows the diagnosis considerably. Look at components common to the whole engine: the mass airflow sensor, the main intake pipework and plenum gasket, the crankcase breather system, the brake servo hose, and fuel supply pressure. Two separate bank-specific faults occurring simultaneously is very unlikely, so anything that only affects one side can be set aside until the shared causes have been ruled out.
Can a four-cylinder car set a P0174 code?: No, not legitimately. Bank 2 is the cylinder bank that does not contain cylinder number 1, and an inline engine only has one bank. If the code appears on a four-cylinder car it usually means a generic reader has misinterpreted a manufacturer-specific code, or the car has a non-standard engine or ECU calibration. Have it read again on equipment that supports the manufacturer's own definitions.
Will P0174 fail an MOT?: Yes, through the engine management lamp, which is a major defect on petrol cars first used on or after 1 July 2003. The emissions readings themselves may well pass, because on a V engine both banks combine before the tailpipe and a correct bank 1 tends to mask a lean bank 2 in the sample. A clean gas printout does not overturn the failure while the lamp is lit.
How do I know which bank is bank 2?: Bank 2 is the bank that does not contain cylinder number 1, using the manufacturer's own cylinder numbering. There is no universal left-right rule, and it varies between makes and even between engines from the same maker. Manufacturer data or a workshop manual for the specific engine is the only reliable source, and it is worth confirming before ordering parts because sensors and gaskets for the wrong bank will not fix anything.

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