Fuel and air
P0175System too rich, bank 2
System Too Rich Bank 2
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Generic OBD-II code
What the computer is measuring
P0175 comes from the same measurement as its bank 1 twin. The computer corrects fuelling based on what the oxygen sensor reports, that correction is fuel trim, and the code sets when the trim runs out of room to subtract fuel while the mixture is still rich.
What is different here is what the bank number tells you, and it is worth more than the code itself.
A bank is one group of cylinders with its own upstream oxygen sensor and its own exhaust before the two sides meet. Bank 1 contains cylinder number one. Bank 2 is the other side. Each bank is fuelled and corrected independently, because each has its own sensor.
That independence is the diagnostic tool. If both banks are rich, the fault has to be something both banks share, and there are only a handful of candidates. Airflow is measured once for the whole engine. Fuel pressure is common to both rails. Coolant temperature is one sensor. The purge valve feeds the whole intake. None of those can produce a rich condition on one side only.
If only bank 2 is rich, all of those are cleared immediately, and what remains belongs to that bank alone. Its oxygen sensor, its injectors, its exhaust and its cylinders. That is a much shorter list, and it is why the first thing to do with this code is find out whether P0172 came with it.
Finding bank 2 before you start
On a V engine, bank 2 is not reliably the side nearest the front, the passenger side, or the side your instinct suggests. It is defined only as the bank that does not contain cylinder one, and manufacturers place cylinder one differently. Confirm it for the engine in front of you before buying a sensor, because a sensor fitted to the wrong bank fixes nothing and the code comes back unchanged.
What you will notice
- Fuel smell and black smoke, the same as any rich condition.
- Poor fuel economy, sometimes with no other complaint.
- Rough idle if plugs on that bank have started to foul.
- A failed emissions test.
- Nothing at all in mild cases, beyond the light.
Likely causes, most common first
Ordered by how often each one actually turns out to be the problem, not alphabetically and not by part price.
A shared fault, when P0172 is stored alongside this
Two banks rich at once is not two coincidences. Look at airflow measurement, fuel pressure, the coolant temperature sensor and the purge valve, and treat the bank numbers as irrelevant.
A lazy or failed upstream oxygen sensor on bank 2
The most common cause when bank 2 is rich on its own. The sensor reports lean, the computer adds fuel, and only that bank is affected.
A leaking injector on a bank 2 cylinder
Extra fuel into one cylinder shows up as a whole bank correction, because the bank has only one sensor watching all of its cylinders.
An exhaust leak near the bank 2 sensor
Air pulled in ahead of the sensor reads as lean, and the computer answers by enriching a bank that was already correct.
Low compression or a burnt valve on a bank 2 cylinder
Unburnt oxygen passing through a weak cylinder reaches the sensor and looks lean, so the bank gets fuel it does not need.
Wiring or connector faults on the bank 2 sensor circuit
Resistance and corrosion shift the reported voltage, and the fuelling follows it.
What to check, in order
Check whether P0172 is stored as well.
Both codes together means the cause is shared and nothing bank specific needs investigating. Bank 2 alone rules out everything common to both banks, which is most of the expensive list.
Compare long term fuel trim on bank 1 against bank 2 at idle and at cruise.
A large gap between the banks confirms a bank specific fault. Similar trims on both banks means the code split is misleading and the condition is engine wide.
Watch both upstream oxygen sensor traces side by side.
A bank 2 sensor that is slower, flatter or sitting lower than its bank 1 twin is the fault. Two sensors behaving identically clears them both.
Identify which cylinders are on bank 2 for this engine, then pull those plugs.
Bank 2 is the side that does not contain cylinder one, and the numbering is not always what it looks like from the front. One sooty plug names an injector, uniformly sooty plugs point back at the sensor or the exhaust.
Check the bank 2 exhaust manifold and its joints ahead of the sensor.
A leak found here explains a false lean reading and the enrichment that followed it.
Where this one turns up most
- V6 and V8 engines where the rear bank sensor runs hotter and is harder to reach, so it is rarely replaced in pairs.
- Transverse V6 layouts, where the rear bank exhaust and its sensor wiring take the most heat.
Codes that often show up alongside it
What a DriveDecode adapter can read
A DriveDecode adapter reads engine and emissions systems: the powertrain and emissions codes the OBD-II standard requires on every US vehicle it covers. It does not read ABS, airbag (SRS), TPMS or transmission-specific manufacturer codes. Those need make-specific decoding that generic OBD-II hardware cannot do.
Reading P0175 on your own car
DriveDecode pulls P0175 straight off the vehicle along with the freeze frame the computer stored when it set: the coolant temperature, engine load and speed at that exact moment. That context is what separates a fault that happens on a cold start from one that happens under load, and it is the first thing the list above asks you to narrow down.
Last reviewed 2026-09-08. Definition sourced from the published standard.