Your Defender idles rough, stumbles on throttle or the check engine light stays on, and the fault memory shows P1171 or P1172. A lot of people go straight for a new O2 sensor. The codes actually say something else: the ECM has corrected the mixture as far as it is allowed to and has run out of room. The real cause is usually somewhere else.
What BRP says about P1171 and P1172
The official BRP fault code list has several wordings for both codes. Depending on the vehicle and the engine control module, they read:
| Code | BRP wordings | In short |
|---|---|---|
| P1171 | Fuel correction reached the maximum limit, additive. Fuel system additive learning reached the maximum limit, on some ECMs on the front cylinder. Additive mixture adaptation exceeds upper limit, mixture too lean. | Too lean |
| P1172 | Fuel correction reached the minimum limit, additive. Additive learning reached the minimum limit, in some cases on the front cylinder. Fuel correction adaptation too rich at idle. Additive mixture adaptation below lower limit, mixture too rich. | Too rich |
BRP adds a note to both codes: if two O2 sensors are present, the code refers to cylinder 1, the front one.
What additive means here
The ECM uses the O2 sensor to check the mixture and learns correction values. There is a multiplicative correction that works as a percentage across the whole load range, and an additive one that adds or subtracts a fixed amount. A fixed amount matters most when the injected quantity is small, so at idle and light load. That fits with BRP listing a too rich at idle variant for P1172. The multiplicative counterparts are P0171 (too lean) and P0172 (too rich), covered in Fault Code P0171: System Too Lean.
Common causes
P1171, too lean:
- An air leak on the intake side: leaking hoses, loose clamps, cracked joints.
- An exhaust leak ahead of the O2 sensor that pulls air into the exhaust.
- Not enough fuel: low fuel pressure, a clogged filter or screen.
- Dirty fuel injectors.
P1172, too rich:
- A leaking or dripping fuel injector.
- Fuel pressure too high.
- A badly clogged air filter, especially after dusty rides.
- A slow or fouled O2 sensor.
Both codes show up especially often after work on the intake or exhaust, for example after a filter change or new gaskets. So ask yourself first what was done to the machine last. The cause is there surprisingly often.
The right check order
- Read every code: if misfire, sensor or voltage codes are stored as well, start there.
- Air filter and intake: filter condition, clamps, hoses, cracks.
- Exhaust ahead of the sensor: look for leaks and soot marks.
- Check live data: lambda value and fuel pressure at idle and under load.
- Fuel: age, water, level.
- Clear and test ride: with live data recording running.
Reading and clearing with the app
With M-JET DIAGNOSTIC you read P1171 and P1172 over Bluetooth in plain language with their status. In live data you pick lambda, fuel pressure, RPM and intake pressure and record a test ride. If the lambda value only goes off at idle, that matches the additive range. How much you can get out of live data is shown in Real-Time Live Data.
Clearing on its own won't help. As long as the air leak or the dripping injector is there, the trim runs back to its limit.
When to go to a shop
If the intake, filter and exhaust check out and the code keeps coming back, you need a fuel pressure test or an injector check with shop equipment. If the Defender also drops into limp mode, see Can-Am Defender in Limp Mode.
Frequently asked questions
Do I need a new O2 sensor for P1171?
Usually not. The sensor only reports what it sees. An air leak or a lack of fuel is the more common cause. Check the intake and fuel side first.
How is this different from P0171 and P0172?
According to the BRP list, P0171 and P0172 are the multiplicative correction and P1171 and P1172 the additive one. The additive part mainly acts at idle and light load.
Is P1171 only a Defender code?
No. It is in the general BRP list and can show up on other models too. The app shows you the exact plain-language text for your machine.
Can I keep riding with P1171?
Short distances are usually fine if the engine runs cleanly and limp mode is not active. A constantly lean mixture is hard on the engine, though, so track down the cause soon.
