Hi Guys, as this is quite a talked about topic, I thought I'd try and combine all the info on one thread for you, togetther with all the misconceptions that seem to occur.
EMISSION LIGHT This can come on for a number of reasons, either from a signal from the pre -cat O2 sensor OR the post cat O2 sensor. Often the signal is from the Pre-cat sensor ,and although putting the light on (symbol of an engine ,NOT TO BE CONFUSED with the EML light), usually no fault code will be present. It will remain on for a number of drive cycles (usually 2 or 3 days) even if the the over rich mixture is not detected again within that period. On a modded MCS under load the richer mixture is desirable ,therefore for me I WANT the light on! At idle and gentle cruise there is no richening of the mixture over stock so no MOT worries there. It is under WOT conditions that normally triggers the light. Also using 95 Ron fuel and /or giving it beans whilst still warming up ,will make the light more likely to come on. Remember It is only a 'flag'. Its not reducing power or changing the way the car performs.
Ok as I said earlier, the light can also be triggered from the post cat sensor. This sensor has one job and one job only- To compare the outputs of the 2 sensors to monitor cat efficiency. If it detects that the gasses have not been cleaned up enough by the cat ,the Emission light will be triggered BUT this time you get a code ...usually P0420. This is fairly common with a sports cat, which is freeer flowing (bigger holes) for max performance, but in certain driving conditions may not clean up the gas enough to prevent the light BUT will usually be more than adequate to pass the MOT. -That to most people is the important thing.
O2 SENSORS The car uses 2 pretty bog standard narrow band heated 02 Sensors. And Im guessing 95% of the sensors changed under warranty by BMW , (because a light came on) were actually perfectly OK!
Do you need the sensors atall? The upstream one ,yes definatly. The Downstream one, no you dont! Whether you have a cat or not the sensor is not required either for correct running of the car nor for the MOT. My car hasnt had the second sensor fitted for 3 years!! You need to fit a bung in place of it AND disconnect it (remove from car). Does this make the light come on? No it doesnt. All you get is P0036 code in the ecu. The first sensor and the light work as normal. As the sensors are expensive, you can keep the removed one as a spare for the upstream one
FAULT CODES Always get the dealer to list the codes before he clears them. A fault code doesnt necessarily point to the correct faulty item. An air leak is a classic one. The codes point to faulty TB or Map sensor and its a split in the intercooler bellows.!
MOT (UK) this consists of fast idle and natural idle tests (ie no WOT or the like). HC (hydrocarbons) and CO (carbon monoxide) levels are checked. You do not have to have a cat on the car to pass the MOT even if it has one as standard. Obviously the emissions will be higher and if they are above the limit, you fail and driving the car with excess emissions would be illegal. I'm going to try our new R56 MCS (when it comes) to see if it actually needs a cat to pass the mot /be legal. If it doesnt its coming off for good. Cats do rob power, the more power/boost you run the more the restriction. So no cat is a good thing if you dont 'have' to have it.
Best Regards Roland GTT:smile:
EMISSION LIGHT This can come on for a number of reasons, either from a signal from the pre -cat O2 sensor OR the post cat O2 sensor. Often the signal is from the Pre-cat sensor ,and although putting the light on (symbol of an engine ,NOT TO BE CONFUSED with the EML light), usually no fault code will be present. It will remain on for a number of drive cycles (usually 2 or 3 days) even if the the over rich mixture is not detected again within that period. On a modded MCS under load the richer mixture is desirable ,therefore for me I WANT the light on! At idle and gentle cruise there is no richening of the mixture over stock so no MOT worries there. It is under WOT conditions that normally triggers the light. Also using 95 Ron fuel and /or giving it beans whilst still warming up ,will make the light more likely to come on. Remember It is only a 'flag'. Its not reducing power or changing the way the car performs.
Ok as I said earlier, the light can also be triggered from the post cat sensor. This sensor has one job and one job only- To compare the outputs of the 2 sensors to monitor cat efficiency. If it detects that the gasses have not been cleaned up enough by the cat ,the Emission light will be triggered BUT this time you get a code ...usually P0420. This is fairly common with a sports cat, which is freeer flowing (bigger holes) for max performance, but in certain driving conditions may not clean up the gas enough to prevent the light BUT will usually be more than adequate to pass the MOT. -That to most people is the important thing.
O2 SENSORS The car uses 2 pretty bog standard narrow band heated 02 Sensors. And Im guessing 95% of the sensors changed under warranty by BMW , (because a light came on) were actually perfectly OK!
Do you need the sensors atall? The upstream one ,yes definatly. The Downstream one, no you dont! Whether you have a cat or not the sensor is not required either for correct running of the car nor for the MOT. My car hasnt had the second sensor fitted for 3 years!! You need to fit a bung in place of it AND disconnect it (remove from car). Does this make the light come on? No it doesnt. All you get is P0036 code in the ecu. The first sensor and the light work as normal. As the sensors are expensive, you can keep the removed one as a spare for the upstream one
FAULT CODES Always get the dealer to list the codes before he clears them. A fault code doesnt necessarily point to the correct faulty item. An air leak is a classic one. The codes point to faulty TB or Map sensor and its a split in the intercooler bellows.!
MOT (UK) this consists of fast idle and natural idle tests (ie no WOT or the like). HC (hydrocarbons) and CO (carbon monoxide) levels are checked. You do not have to have a cat on the car to pass the MOT even if it has one as standard. Obviously the emissions will be higher and if they are above the limit, you fail and driving the car with excess emissions would be illegal. I'm going to try our new R56 MCS (when it comes) to see if it actually needs a cat to pass the mot /be legal. If it doesnt its coming off for good. Cats do rob power, the more power/boost you run the more the restriction. So no cat is a good thing if you dont 'have' to have it.
Best Regards Roland GTT:smile:
