Check engine light and lost power: DTC P0420 on the Mini Cooper R56

Your Mini Cooper started as usual that morning. Halfway through the drive, the check engine light on the instrument cluster came on and stayed on. The engine is not misfiring, the exhaust sounds normal, and there is no strange smell. A few days later, passing on the highway, the car felt a little slower to pull than usual. The scan at the workshop shows one code: P0420.

Many owners read this code as proof that the catalytic converter is dead and has to be replaced. It is not that simple. P0420 is the result of an assessment by the engine computer, not the result of a physical inspection of the converter, and on the R56 generation Mini there are several engine problems that can damage the converter or fool its assessment. Replacing the converter without finding the cause risks the same code coming back, and the new converter getting damaged too. This article explains what the code means, how it ties in with the R56 engines, and a sensible order of checks before you spend serious money.

P0420 is a generic OBD-II code with the title "Catalyst System Efficiency Below Threshold (Bank 1)". The engine computer runs a catalyst monitor that compares the signal of the oxygen sensor before the converter (upstream) with the oxygen sensor after the converter (downstream). A healthy converter stores and releases oxygen, so the rear sensor signal stays fairly steady. When the rear sensor signal starts to mirror the swings of the front sensor signal, the computer concludes that the converter is no longer working as efficiently as expected and stores P0420.

Bank 1 is the side of the engine where the first cylinder is located. An inline four-cylinder engine like the Mini R56 engine has only one bank, so the bank 2 code is not relevant here. It is important to note: this code does not prove the converter is clogged, and it does not prove one of the oxygen sensors is broken. It only states that the monitor result is outside the limit.

The R56 generation Mini (2007 to 2013 models) uses the Prince engine family, developed jointly by BMW and PSA Peugeot Citroën. All of them are inline four-cylinder gasoline engines, but the code differs by variant, and that difference matters for reading P0420.

• Cooper (without turbo): the N12 engine at the start of production, then the N16 in the facelift version.
• Cooper S and John Cooper Works: the N14 turbo engine, then the N18. These turbo engines use direct injection (gasoline sprayed directly into the combustion chamber) and a twin-scroll turbo.

The year of the change from N14 to N18 is given differently by sources (around 2010 to 2011), and the variants sold in any given market are not always the same. So confirm the engine code from the vehicle documents, the label in the engine bay, or the scan result before following advice that is specific to one variant. The things discussed below are most relevant for the Cooper S with the N14 and N18 engines.

A converter rarely fails without a cause. On the N14 and N18 engines, there are two paths that can make the converter lose efficiency or make its monitor fail.

Burning oil. Excessive oil consumption is a known complaint on Minis with the N14 and N18 turbo engines. The cause can be worn piston rings, hardened valve stem seals, a leaking turbo seal, or a faulty crankcase ventilation (PCV) component. Oil that enters the combustion chamber or the exhaust passage is burned, and its combustion residue coats the converter. The list of consequences of oil consumption given by Mini specialists includes converter damage from burned oil.

Misfires from carbon deposits. Because gasoline is sprayed directly into the cylinder, fuel never washes over the N14's intake valves, so carbon builds up on them. The symptoms are rough idle, hesitation and misfire codes. Every misfire sends unburned gasoline to the converter, and the converter can become too hot.

Beyond the R56 specifics, the usual P0420 causes are the same on every make:

• A converter that is really worn, contaminated, overheated, or damaged inside.
• An exhaust leak near the manifold, joints, or oxygen sensor bung, which lets outside air in and disturbs the sensor reading.
• A rear oxygen sensor that is slow, biased, or has wiring and connector problems.
• An engine that runs too rich, too lean, or misfires, including oil or coolant being burned.
• A replacement converter that does not fit the application or whose efficiency is below the needs of the car's calibration.

On the Mini R56, the order of checks should move from the engine to the converter, not the other way around:

1. Read all codes, not only P0420. Misfire, fuel mixture, or oxygen sensor codes that are also stored must be solved first.
2. Measure the oil consumption. Record every oil top-up between oil changes. Bluish smoke during acceleration, spark plugs fouled by oil, and an oil smell from the exhaust strengthen the suspicion that oil is being burned.
3. Check the PCV system and turbo. On the Cooper S, a worn turbo seal can let oil into the intake or exhaust, so watch for signs of oil on the exhaust side.
4. Fix the misfires. On the N14 and N18, carbon deposits on the intake valves are a reasonable suspect. A new converter on an engine that still misfires only postpones the problem.
5. Look for exhaust leaks around the turbo, joints, and sensor bungs before judging the sensor data.
6. Compare the data of both oxygen sensors when the engine is fully warm, following the vehicle service procedure.

Only once the engine checks out healthy and there are no leaks can the catalyst monitor result be trusted to decide on a replacement.

If the check engine light is steady (not flashing) and the engine runs normally, P0420 alone usually does not require you to stop right away. The car can generally still be driven carefully for a short distance. But do not leave it for weeks, because the engine problem behind it can keep damaging the converter.

Stop the car in a safe place and do not continue the trip if the check engine light flashes, the engine misfires badly, power drops sharply, a strong sulfur smell appears, or there is excessive heat around the underbody.

If the engine is healthy, there are no leaks, and the catalyst test according to the manufacturer procedure really fails, the converter needs to be replaced. Choose the correct converter for the vehicle application and of OEM-equivalent quality, because a converter that does not fit or has low efficiency can make P0420 appear again. After the repair, clear the code then drive the car in conditions that make the catalyst monitor run again. The problem is solved when that monitor passes, not when the scan tool shows no codes.