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The dreaded P0420 code for the Mini One 1.4 R56


TechWhiz

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My son just purchased a 2007 Mini One 1.4. It has around 72000 miles.

It displayed the P0420 code and the engine light a few weeks ago. The exhaust smells like the engine is running rich.

The cam chain is silent (but might be stretched), and the vehicle failed its emissions test a few years ago. I'm not sure how it was fixed.

We replaced the plugs yesterday (as part of a service) and observed the revs dropping periodically at idling. We observed the Lambda output (not sure whether this is before or after cat-assume post) and it plummeted in tandem with the rpm, resulting in the gasoline smell coming from the exhaust. It also said misfire cylinder 2, so we switched the coil packs for 2 and 3, and it now shows misfire cylinder 1.

We've been watching the voltages at the pre and post cat oxygen sensors, but we're not sure what to anticipate. It was roughly 1.45v before and 0.09v post at idle (fluctuating).

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Honestly, at 72,000 miles, I'd be looking at a new (and full) TC kit and probably new valve stem seals and a new rocker cover gasket at the very least. For pollution, these cars run very, very hot, which cooks the seals. To look at necessary and desired cam views, you would need a good scan tool.

@TechWhiz is our resident expert on engine rebuilding, and he suggests starting with the basics to find the cause of the misfire/emissions. They could drop a valve seat if they got too hot, or there could be so much gunk on the valve from burning oil (which would explain why the Cat and O2 sensors are broken) that the valve doesn't sit right. You could use a boroscope or a smoke tester to see if any smoke is coming out of the spark plug hole. Both cylinders 1 and 2 fire close to each other, so a stretched TC could cause either.

If you take off the rocker cover and physically turn the engine so that the flat on the exhaust cam is horizontal, the same flat on the inlet cam should also be horizontal. I don't think it is, but you should check.

Does the engine have any oil or water leaks? These would also need to be dealt with right away.

Just my two cents, since Tech has a lot of experience with these things and can give you much better advice.

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Common things, pcv pipe at rear of engine cracks, exhaust flexi leaks, exhaust valve stems seals allow oil crud to build up on exhaust valve seats and cause misfire as well as dropping unburned air fuel on to upstream 02 sensor, need to scan the ecu for fault codes as well, and live data will provide a clearer picture of what the vehicle is doing. Air escapes must be ruled out prior to examining the timing chain, rocker cover, and camshafts when all four pistons are seated halfway in the bores. This is a simple starting point. I use a smoke tester for much of this, including the valve seats that are escaping due to carbon buildup; I send smoke through the plug opening with all four valves closed.

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We're going to go get a smoke machine (we have an air connection set up). We have one in the office, but nobody can seem to borrow any equipment recently.

In case other people are experiencing the same issues, I will continue to update the article.

Thanks.

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Trouble with these things is that many things will flag that code p420, so its process of elimination ie smoke test proves no unmetered air, then chances are very high its a mechanical issue ie timing related very common, or could be its been using so much oil the cat is now clogged with the crud and thats very very common on them, but going through the process, ie cheaper to test something than replace it, a lot of the time its a number of worn things,, i get them all got one in at mo after refresh, owner put new cat on it never done 02 sensors so we had heater circuit fault code and electronic management fault code, sometimes its just a 02 sensor open short circuit, but on this one it went a stage further and suspect burnt out a driver in the ecu, so ecu been sent off to ECUTESTING.COM found get a good service and life time warranty on repairs from them, and trouble is these engines must run right to stay together,, as running lea

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My Jag XF was giving me trouble for weeks since it kept needing a service because it had oil contamination and was doing re-gens. No error messages. The problem was an EGR valve spindle that had broken. The motor provided the appropriate feedback as it turned, but the valve remained open.

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See, when diesels with dpf get fault codes on the ECU, the regen system shuts down. The egr valve is also part of the regen system. On many cars, if the air conditioner doesn't work right, this will also shut down the regen system. I have a 2015 Honda Civic idtec that is getting its egr and dpf removed and remapped in a couple of weeks. It should make just over 200bhp and still

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