Lean and rich mixture: P2177, P2187, P2195 vs P0171
These codes do not merely say «the mixture is wrong» — they say where and when: at idle or under revs, on bank one or bank two. That detail cuts the search down dramatically.
The familiar P0171 means «lean in general». The codes in this article mean «lean right here and right at this moment» — and that is exactly why they are more useful.
How to read the number
They all follow one grid. Two things are enough to remember.
When:
Where: the odd code is usually bank one, the next one bank two. P2179 and P2180 are the same as P2177 and P2178 but on bank 2. An inline engine has one bank; a V or flat engine has two.
And a third pair: P2195, P2196 — the pre-catalyst oxygen sensor is stuck in one position. That is not about the mixture but about the sensor itself; in detail in oxygen sensor.
Why «when» matters more than «lean or rich»
An air leak and a weak pump both produce the label «lean», but they behave differently — and these codes separate them.
Lean only at idle (P2187), normal off idle — almost certainly an air leak. At idle the engine takes in little air, and an extra hundred millilitres through a cracked hose changes the mixture noticeably. Off idle the same hundred millilitres vanish into the general flow.
Lean only off idle (P2177), normal at idle — the opposite, not enough delivery: a weak pump, a clogged filter, a tired injector. There is enough fuel for small demand and not enough for large.
That separation is the main value of the P21xx codes: it answers, for free, the question people usually pay a workshop for.
How to confirm it in the app
Watch short-term and long-term fuel trims — how much the unit is forced to add or take away. How to read them is covered in fuel trims.
The procedure is simple:
- Warm the engine and note the trims at idle.
- Hold 2000–2500 rpm and note them again.
- Compare.
Plus 20 % at idle and plus 5 % off idle means a leak. Plus 5 at idle and plus 20 off idle means delivery. Equally bad everywhere means a common cause: the mass airflow sensor, fuel pressure, or the oxygen sensor.
The full leak-hunting procedure, including the places leaks appear most often, is in how to find an air leak.
About «rich»
A rich mixture (P2178, P2188, P2196) is rarer but nastier: excess fuel washes the oil off the cylinder walls and kills the catalyst. The signs are visible without instruments — black smoke, a smell of petrol, fuel use up, misfiring when cold.
Common causes: a leaking injector, excessive rail pressure, a lying intake air temperature sensor. Sometimes simply bad fuel.
What this means for the emissions test
While such a code is active the self-tests do not complete, and the car is turned away even with clean exhaust — covered separately in readiness for the emissions test. Clearing the code before driving to the station is pointless: the self-tests reset along with it.
