In this entry – a little about CO catalytic converters and exhaust gases.


Typical content of perfectly performing petrol engine is displayed here:

As you see in the image, expected CO/CH of correctly performing engine (with Lambda 1,00) before catalytic converters is about the following:

a) CO around 0,5 .. 1,5%;

b) HC 100 .. 300 ppm.


Conclusion No.1: if the measurements of CO/HC are highly above characteristics mentioned (CO above 2,0 .. 2,5% and HC above 300 .. 500 ppm), the engine definitely has problems with fuel mixture and it’s burning (regardless the car is equipped with CO catalytic converters or not, they are in work order or not).


Conclusion No.2: if the fuel mixture is not optimal (it is lean and/or enriched), the content of HC before CO catalytic converters increases (even if there is no misfire, which significantly increases HC); if the fuel mixture is enriched, the content of CO in exhaust increases. To evaluate efficiency of CO catalytic converters, at first the fuel mixture has to be evaluated, it mean’s, it’s Lambda (Lambda should be 1,0). Additionally, when the fuel mixture is enriched, the performance of CO catalytic converters decreases. For example, the efficiency of CO catalytic converters decreases 2 .. 3 times, if the Lambda is 0,97 .. 0,98.


In Latvia, for example, following TI norms are settled for petrol engines:

a) if the vehicle is manufactured after 01.01.2001, HC can not exceed 100 ppm;

b) if the vehicle is manufactured after 01.01.2003, CO can not exceed 0,3/0,2 % (idle/increased RPM above 2000).


Conclusion No.3: if the BMW E60/90 and cars of similar generation  don’t have CO catalytic converters (or they are damaged – not working), it will not pass TI (both not for CO, not HC norms).


You can find a large number of articles in the Internet regarding what/how affects the exhaust gases, so I will mention only some specific nuances:

1. to evaluate the exhaust gases of petrol engine, all cylinders have to work with correct fuel mixture (it means, individual long-term trims of each cylinder have to perform correctly); in case of incorrect long-term fuel trims CO/HC before catalytic converters can be so high, that even with brand new catalytic converters CO/CH in the exhaust gases will be close to/above max allowed;

2. the engine has to perform in closed loop mode, it means, it has to regulate exhaust gases according to Lambda values. Lambda has to be 0,99 .. 1,01 (including the tolerable error or measuring gauge, which has to be significantly lower than 0,01). If the Lambda is different from values mentioned before (exception – N43/N53 engines, when/if they perform in Stratified charge), the causes of problem have to be solved, not hurry with replacing the catalytic converter. Obstacle, which characterizes Closed loop: the integrators of fuel mixture short-term trim are working all the time, they don’t go out regular range (typically: out of -15 .. +15 %);

3. before measuring the CO/HC, the work temperature of the engine has to be reached. Normal temperature of N52/N53/N43 engines is above 100 oC. If the temperature of engine is decreased (it is on heating mode or by some other reasons don’t reaches Normal/Eco mode work temperature), the supply of enriched (with Lambda 0,97 .. 0,98) fuel mixture is possible, to forcibly heat up the CO catalytic converters. In this case CO content in wast gases increases significantly;

4. the engines of N43/N53 series in idle (measurement conditions in TI) can perform in Stratified charge with Lambda 2,0 .. 2,6, the same Homogeneous mode. In Homogeneous mode, periodical (short-term) enrichment of fuel mixture is possible. According to TIS: if Lambda is below 1,0 in idle and CO content is increased, wait for a while and repeat the measurements of CO/HC.


In many cases, the CO catalytic converters are defected/replaced unreasonably. For CO/HC to fit in the rules of TI, the exhaust gas content before catalytic converters has to be correct (in numbers mentioned before); the catalytic converters has to be heated to work temperature. Unfortunately, the measuring before catalytic converters is technically complicated, that’s why usually indirect methods are used (values of Lambda, evaluation of misfire – from shivering of the engine, measuring of Lambda in exhaust gases). Without through analysis of such data the evaluation of performance of catalytic converters is not possible.


Before putting the blame on CO catalytic converter:

1. the engine has to be in work temperature (N series: around 100 .. 105 oC);

2. the engine has to perform in normal mode (it means, all Lambda probes has to be heated, all fuel long-term trims have to be completed in all segments);

3. closed loop mode/integrators has to be working;

4. individual long-term trims of cylinders has to be working properly, Rough run menu has to display balanced performance of cylinders, learnbits has to display completed long-term trims of flywheel and idle;

5. control Lambda probes has to be performing, no error messages regarding control and/or regulating Lambda probes, EGR, VANOS, DISA and other basic systems can be present;

6. the Lambda in exhaust has to be 1,0 (excluding N43/N53 Stratified charge);

7. no misfire, engine shivering, vibration;

8. have to make sure, that, when all rules, mentioned above, are fulfilled, both CO and HC waste gas characteristics are proportionally (to perfectly running engine/TI norms) increase.

Only then, if the situation is as described in point 8, there is a reason to believe, that the performance of CO catalytic converters is low.


Related entries:

N43/N53 and CO catalytic converters

Lambda probes. Wide-band

Management of Lambda probes

N43/N53 operation modes. Basics

N43/N53 operation modes. Stratified charge performance check