Diagnostics & Repair

P0172 on a 2016-2021 Honda Civic (1.5T): Running Rich? Check the Oil for Fuel First

Check engine light on Lack of power or hesitation

JC Landry, ASE Master TechnicianPublished Oct 10, 2026 · Updated Oct 27, 2026

2016-221 Honda Civic sedan in a garage with a floor jack and tires

P0172 on a 2016-2021 Honda Civic 1.5T means the PCM has pulled fuel until long term fuel trim hit its rich limit. The first thing to rule out is fuel-diluted oil feeding vapor through the PCV system. Pinch the PCV hose at idle; if short term trim comes back up, the oil is carrying fuel. I'd get the cause found rather than just clear the code.

Safety first

Fuel is flammable. Keep sparks and flames away, and relieve fuel pressure before opening a fuel line. Disconnect the 12 volt battery negative terminal before unplugging fuel system connectors.

What does P0172 mean on a 2016-2021 Civic?

P0172 (Fuel System Too Rich) is the PCM's rich-side fuel trim monitor on these engines. It flags more fuel than the PCM can correct for, not a single-sensor circuit fault.

The PCM reads exhaust oxygen content from the A/F sensor (Sensor 1) signal voltage, ahead of the catalytic converter, and runs closed-loop feedback to hold stoichiometric. Short term trim is the instant correction; long term trim, built from it and stored in memory, corrects drift as fuel metering parts age or fail. Normal long term trim is about 1.0 (0 %); lower means the PCM is pulling fuel.

The PCM sets P0172 when long term fuel trim drops to 0.79 (-21 %) or less. All of these must hold:

ECT sensor 1 at 156 °F (69 °C) or higher
IAT sensor 1 at 18 °F (-7 °C) or higher
Engine speed 530 to 4,000 rpm (CVT) or 620 to 4,000 rpm (manual)
MAP value at 16 kPa (120 mmHg, 4.8 inHg) or higher
Fuel feedback in closed loop
P0133, P0420, P219C, P219D, P219E and P219F judged first

The monitor runs once per driving cycle, needs 20 seconds or more of qualifying operation, and clears its fault judgment if a later check under the same conditions reads normal. Constant fuel pull means extra fuel or vapor is getting in, or the air reading is off.

The first detection stores a pending DTC. A repeat on the next drive cycle makes the PCM request the MIL and store a confirmed DTC with freeze data. The MIL goes out after three consecutive fault-free drive cycles under similar conditions.

The lean-side sibling is P0171 (Fuel System Too Lean), set at long term trim of 1.33 (+33 %) or more.

What symptoms does P0172 cause on the Civic?

Check engine light
Rough or unstable idle
Fuel smell at the exhaust
Poorer fuel economy
Oil level above the full mark on the dipstick, or oil that smells of gasoline
Misfire codes setting at the same time, with drivability that feels rough or down on power

What causes P0172 on the Civic, most likely first?

Rank	Cause	Quick confirm	Typical fix
1	Fuel-diluted oil feeding vapor through the PCV system	Pinch the PCV hose at idle; short term trim rises	Find the fuel source, change oil and filter (bulletin path on eligible 2016-2018 cars)
2	EVAP purge valve leaking or stuck open	Vacuum at the valve, engine running	Replace the purge valve
3	Carbon in the throttle body	Carbon accumulation test out of range	Clean it, then throttle position reset and idle learn
4	Faulty injectors	Cylinder air/fuel test Fail, or trim recovers with known-good injectors	Full injector set and fuel joint pipe, then fuel learn
5	Fuel pressure out of range	Low-side pressure out of spec, or high-side test not NORMAL	Fuel tank unit (low side) or P0087/P0088 diagnosis (high side)
6	MAF reading off or air cleaner problem	MAF airflow out of range at 2,500 rpm	Fix the air cleaner, or replace the MAF sensor/IAT sensor 1, then fuel learn
7	Faulty A/F sensor (Sensor 1)	Trim recovers with a known-good sensor	Replace the sensor, then fuel learn

On 2016-2018 Civic 1.5L cars, Honda bulletins 18-137 and 19-027 pair P0172 with misfire codes P0300 through P0304, and their repair starts with an oil drain, then software and A/C control unit work.

How do you diagnose P0172 on a 2016-2021 Civic?

I'd want a scan tool that reads fuel trim and runs Honda's system tests, plus a hand vacuum pump with gauge.

Fix any of these codes stored with P0172 first, then recheck:

P0010, P0011, P0013, P0014 (VTC system)
P0087, P0088 (fuel rail pressure sensor)
P006A, P0102, P0103 (MAF sensor)
P0107, P0108, P2073, P2074 (MAP sensor)
P0133, P1172, P2195, P2237, P2238, P2243, P2245, P2251, P2252, P2A00 (A/F sensor) and P0134, P0135 (its heater)
P0137, P0138, P0139, P2270 (secondary HO2S) and P0141 (its heater)
P0201, P0202, P0203, P0204 (injectors)
P0443 (purge valve) and P2279 (intake air leakage)

Do not clear the code or reset the PCM until a step calls for it; that erases diagnostic data. Check for aftermarket fuel and engine parts.

1
Confirm it.
Idle in P or N (manual in neutral), no load, and read short term trim. Below 0.80 confirms it; move on to the oil check. If idle trim is fine, drive matching the snapshot's engine speed, vehicle speed, throttle and pedal. Still fine means the system is normal for now; reproduce the fault first.
2
Check the oil for fuel.
At idle, gently pinch the PCV vacuum hose. Trim staying below 0.80 means good oil; trim coming back up means fuel in the oil. Find the cause (faulty injectors, for example), then replace the oil and filter.
On eligible 2016-2018 cars (by VIN), Honda's bulletin path drains and refills the oil only, no filter, and resets the Maintenance Minder. In the cold-weather states the PGM-FI, transmission and A/C control unit software are updated. In the other states the A/C control unit is replaced only if its status reads Not Complete, and the PGM-FI and transmission software is updated only if not current. Then clear DTCs, reset the throttle position learned values and run the PCM idle learn (required).
Honda's warranty extension adds a spark plug check: replace the plugs only if fouled.
3
Check the purge valve and throttle body.
Ignition off, unplug the valve, pull its purge line, fit the vacuum gauge and start the engine. Vacuum means it is passing canister vapor; in warm weather that can richen the mix enough to stall the engine in some cases. Replace it if needed. No vacuum, move on. Warm up to fan-on and run the throttle body carbon test. Out of range, clean it, then reset the PCM, clear the throttle position learned value and run the idle learn (required).
4
Check the injectors and fuel pressure.
Clear DTCs, switch A/C and loads off, and run the cylinder air/fuel test three times. Any Fail means all injectors as a set; all Pass, or neither result, continue. Low-side pressure at idle (trim 0.80 or less) out of spec means a new fuel tank unit. For the high side, clear DTCs, hold 3,000 rpm in P or N to fan-on, idle, and run the direct injection fuel pressure test; not NORMAL means the P0087 and P0088 diagnosis. New injectors means the full rail set plus the fuel joint pipe, new clips, O-rings, back-up rings and seals (relieve fuel pressure first), then the fuel learn (required).
5
Rule out fuel and airflow.
A rich code right after a fill-up points at fuel (the MIL distance value and fuel level help judge it). Drain, refill with known-good fuel and drive where trim was 0.80 or less; recovery means bad fuel. For the MAF, coolant above 158 °F (70 °C), P or N and loads off, hold 2,500 rpm for 30 seconds and read airflow. Out of range, check the air cleaner for cracks, damage or a bad fit; if good, replace the MAF sensor/IAT sensor 1 and run the fuel learn (required).
6
Swap-test the A/F sensor, then the injectors.
Fit a known-good A/F sensor and drive where trim was 0.80 or less; recovery means replace it, otherwise refit the original. Repeat with known-good injectors; recovery means a full set. If not, refit the originals, substitute a known-good PCM and recheck.

Get the P0172 fuel trim tests for your Civic

The oil, purge valve, throttle body, injector and fuel pressure checks above follow Honda's own test order. Ask emanuals.ai about your Civic 1.5T and get the exact test steps, specs and relearn procedures from the factory manual.

Ask about your Civic

Specs you need for the Civic P0172 repair

Engine oil type	Honda Motor Oil 0W-20, API certified, "For Gasoline Engines"
Engine oil capacity, change without filter	3.2 L (3.4 US qt)
Engine oil capacity, change with filter	3.5 L (3.7 US qt)
Engine oil drain bolt (new washer, do not overtighten)	40 N·m (4.1 kgf·m, 30 lbf·ft)

What if the obvious fix does not work?

An oil change alone can clear the symptoms for a while, and the code can return if the fuel source is still there. Injector trouble on this engine can show up as rich running with misfires. Don't replace the A/F sensor first; it may only be reporting a rich condition from elsewhere.

The stop point: a direct injection fuel pressure test that does not read NORMAL moves you to the P0087 and P0088 diagnosis. If trim still does not recover with known-good injectors, refit the originals, substitute a known-good PCM and recheck.

How do you confirm the repair?

Clear the codes (scan tool or 12 volt battery disconnect) and run the required relearns. Fuel learn follows a new injector, A/F sensor or MAF sensor/IAT sensor 1, and any PCM update or reset. Idle learn follows a PCM reset, update or replacement, and throttle body cleaning or replacement. The bulletin path requires the throttle position reset and idle learn too.

For the confirmation drive:

1
Hold 3,000 rpm, no load (P or N, manual in neutral), until the radiator fan comes on.
2
Drive a steady 15 to 75 mph (25 to 120 km/h) for 15 minutes or more, watching long term trim. Near 1.0 means the repair held or what remains is very minor; a real fault moves it up or down.
3
With freeze data stored, drive those conditions instead.
4
After clearing, drive a steady 15 to 55 mph (25 to 88 km/h) for 40 minutes or more so long term trim relearns.
5
Then recheck for stored DTCs.

Questions mechanics ask about P0172 on the Civic

Can I drive with P0172 on a 2016-2021 Civic 1.5T?+

Get the cause found rather than just clearing the code. On these cars Honda ties P0172 to drivability problems and misfire codes, and Honda's owner notice to 2016-2018 owners points out that a lit check engine light can fail a state or local emissions inspection. A leaking purge valve can also richen the mixture enough to stall the engine in warm weather in some cases.

Will an oil change fix P0172 on the Civic 1.5T?+

Only if fuel in the oil is the cause, and only for a while unless the source is found. Pinch the PCV hose at idle; if short term trim comes back up, the oil is carrying fuel. Find what put it there (faulty injectors are one source), then replace the oil and filter. With the filter, this engine takes 3.5 L (3.7 US qt) of 0W-20.

Is there a Honda bulletin for P0172 on the 2016-2018 Civic 1.5T?+

Yes. Bulletin 18-137 covers cars in the listed cold-weather states and 19-027 covers the rest, both for 2016-2018 Civic 1.5L cars with P0172 or misfire codes P0300 through P0304. The work is an oil drain and refill without a new filter, a PGM-FI and transmission software update (on the other-states bulletin, only if not current), and an A/C control unit update (on the other-states bulletin, a replacement, and only if its status reads Not Complete), then the throttle position reset and PCM idle learn. Dealers check eligibility by VIN, and Honda's owner notice describes the update as done at no cost.

Does Honda's warranty extension cover P0172 on the Civic?+

That coverage has run out for these model years. Warranty extension bulletins 19-033 and 19-038 stretched powertrain coverage on 2016-2018 Civic 1.5L cars from 5 years/60,000 miles to 6 years with unlimited mileage from the original sale date, for camshafts, rocker arm assemblies and spark plugs damaged by the misfire, P0172 and whirling-noise concerns. It still tells you where Honda found damage on this engine, so inspect the plugs and replace them only if fouled.

Should I replace the A/F sensor first on a Civic with P0172?+

No. On the Civic 1.5T, the A/F sensor gets swap-tested near the end, after the oil, purge valve, throttle body, injectors, fuel pressure, fuel and MAF checks. Fit a known-good sensor and drive where trim was 0.80 or less; if trim does not recover, the original goes back in. If you do replace it, the fuel learn is required.

Do Honda's P0172 bulletins apply to a 2019-2021 Civic?+

No. Honda's bulletins and the warranty extension list 2016-2018 Civic 1.5L cars only. On a 2019-2021 Civic, work the full P0172 diagnosis above, starting with the trim check and the PCV pinch test.

Sources

Honda Civic Service & Repair Manual, section DTC Troubleshooting: P0172 and DTC Advanced Diagnostics: P0172OEM Manual
Honda, TSB 18-137, "Product Update: 2016-18 Civic: Software and A/C Control Unit Update; DTC P0300, P0301, P0302, P0303, P0304, or P0172", 2016-2018, March 5, 2019Service Bulletin
Honda, TSB 19-027, "2016-18 Civic 1.5L: Software Update with A/C Control Unit Replacement; DTC P0300, P0301, P0302, P0303, P0304, or P0172", 2016-2018, March 5, 2019Service Bulletin
Honda, TSB 19-033, "Warranty Extension: 2016-18 Civic 1.5L: Software Update with A/C Control Unit Replacement; DTC P0300, P0301, P0302, P0303, P0304, or P0172; Whirling Noise from Engine", June 18, 2019Service Bulletin
Honda, TSB 19-038, "Warranty Extension: 2016-18 Civic 1.5L: DTC P0300, P0301, P0302, P0303, P0304, or P0172; Whirling Noise from Engine", 2016-2018, October 2, 2019Service Bulletin
emanuals.ai, Query data, "P0172 Honda Civic"First-Party Data

JC Landry

ASE Master Technician

JC spent 15+ years as a certified Master Mechanic before trading the shop for the keyboard. He's worked across dealerships, owned and operated his own independent garage, and consulted on automotive trade school curricula, reviewing technical textbooks used in vocational programs.

He now serves as Editor-in-Chief at emanuals.ai, where he oversees the platform's technical content. He's one of the few voices in automotive publishing who turned wrenches professionally for decades before he had a byline.

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