Cheating of control probe

For cheating the control probes, the following techniques are used since long ago:

  • mechanical solutions (“spacers” or tubes);
  • electronics based solutions (RC circuit).

Both solutions are intended to avoid the fast fluctuations of the signal of the control probe, which could indicate the insufficient performance of the CO catalytic converters. Unfortunately, a modern DME determines the performance of the CO catalytic converters differently – even theoretically these solutions are not suitable for BMW N series (and even M series) petrol engines.

But, before I get to the experience with MSV70/80, MSD80 and similar DME’s, I will mention several nuances, which are worth to remember, if you have an idea regarding cheating of the control probes:

  • if DME uses the wideband probes as the general ones, control probes perform also trim functions of the general probes, which means – they can both affect the fuel mixture and are critically important for normal functioning of the DME;
  • disabling of the control probes, using NCS, don’t work. You can turn off the service procedures of the control probes, but you can not ”turn off” the status bits of insufficient efficiency of the CO catalytic converters. Accordingly – there will be no new error messages in the DME error message memory, but the engine will not give full power, will consume much more fuel, etc.;
  • the engines, corresponding the Euro 4/5 exhaust norms, even if they are in perfect order, will not be able to pass TI with a dismantled (or not working) CO catalytic converters (if during the TI, the exhaust gasses are measured). For example, in our region the content of the CO should be not more than 0.2 .. 0.3%; the content of HC should be not more than 200ppm. The exhaust gasses of a perfectly working engine, but not working catalytic converters will be 2 .. 3 times higher than the norms, mentioned before. Exactly this is the sense of E4 versus E2 – to force the manufacturers/users to use CO catalytic converters! Instead, if the content of the exhaust gasses is not measured, but only DME error messages and readiness status are red (as far as I know, such order is in the USA), then those, who have played with checkboxes using NCS, can expect problems.

But, let’s assume, that you have some kind of a sports project, or you live in the USA and need a solution, which is fine for TI.


Before we move to cheat the CO catalytic converter itself, remember, that in this solution/problem two components are involved:

  • a control probe;
  • CO catalytic converter.


At first regarding control probe. DME of this generation (most part of functions – already starting with MS42):

  • measures the internal (Nernst) resistance once in a second, based on the results – adapts its heating map and records the error messages regarding incorrect heating/efficiency of the probe if the parameter doesn’t correspond;
  • checks the plausibility of the signal of the probe, it means: in situations, when the fuel mixture parameters (rich or lean) are known for sure and the main probe confirms this status, the signal of the control probe is compared – if the signal doesn’t correspond, the test fails. If large enough amount of tests fail – the error message regarding the plausibility of the probe is recorded;
  • DME checks both delays of the signal and rise/fall times, it means – the reaction ability of the probe to the changes of the fuel mixture. If a too long delay or too slow changes of the signal – the error messages regarding the slow reaction of the probe will be recorded;
  • it goes without saying – no “passing” of any fixed signals to DME will fit – it will recognize this situation very fast, the error messages regarding short circuit to ground or power supply, depending on the voltage of the signal will be recorded;
  • in the situation when the probe is not heated, the Nernst resistance has to be high and the signal of the entrance in DME has to be around 0.40V, significantly lower or higher level of the signal will provoke the error messages regarding short circuit.


Unfortunately, both solutions mentioned at the beginning of the entry fail immediately:

  • mechanical solution – depending on the engine performance mode, the speed and pressure of the exhaust gasses in the exhaust changes very significantly, spacers of fixed length/volume do not provide required retention of gasses. Very quickly the error messages regarding the plausibility of the signal, slow response of the probe, or in case of the shorter tube – if will give nothing;
  • RC filter. In case of RC filter – very soon, the DME will record the error messages regarding inappropriate heating (because the internal resistance will be incorrect) and will also try to overheat the probe (DME, which uses PWM heating maps – will damage the probe quite soon), also the error messages regarding slow response of the probe will be recorded. The reason is similar to previously mentioned – by different RPM, different loads of the engine different parameters of the RC filter are required. If the RC filter (low-pass) is fixed – it will not fit for all performance modes.

To explore the theme more closely, the prototype was created. The schematics:


What does this hub do?

  • simulates correct Nernst resistance (chemical efficiency of the probe) both for cold and warmed up the engine;
  • low-pass filter of 3-rd order is used, it filtrates the swift pulsations of the signal much more efficient, at the same time – keeping relatively small delay and ensure short rise and fall times.

These prototypes were installed on the cars with N43 and N53 series engines, which efficiency of the CO catalytic converters was 0 (the active element – dismantled).

Before installing these emulators: DME immediately (literally several minutes after starting to create the adaptations) identified the insufficient performance of the CO catalytic converters in Homogeneous mode, turned on the Mapped mode, EML lights up in KOMBI.


After installing the emulator:

  • even driving for a long time (several thousands of km, many driving sessions), there were no error messages regarding retention of the signal of probes, plausibility, heating/efficiency problems;
  • ../F5/F2/F6 indicated correct Nernst resistances; heating adaptation maps were not “floating”. But the most important – for a sufficiently long time (500+ km) no error messages regarding CO catalytic converters were recorded. After longer driving, DME although detected the insufficient performance of the CO catalytic converters both in Homogeneous and Stratified charge modes. How so?

In addition to control of swift pulsations of the probe signals, DME of MSV/MSD series performs more advanced tests of CO catalytic converters, read more here.

Due to these advanced tests, each emulator can expect fundamental problems. For example – how to distinguish service procedure of tests of the CO catalytic converters from service procedures of wide-band probe trim? In the first case, the emulator should simulate a good performance of CO catalytic converters, in the second case – send the signal of the control probe without any changes to DME to trim the main probe. Similar problems appear during service procedures of mechanical and chemical efficiency of cylinders. Also – in Stratified charge mode during regenerations, the probe has to “behave” differently as in Homogeneous mode – so: the emulator should be able to distinguish also Stratified charge/Homogeneous lean mode from a “regular” performance mode.

I do not say, that it is impossible to create a correct emulator, but – the problem is more serious and complicated as it seems at the first moment.

If anyone is interested in the control probe adapter of simulation of the performance of CO catalytic converters for old DME (for example, MS42/43), you can try the schematics below.

As these DME use the simple heating strategy of the probe (”on/off”), optocoupler management is simplified. Taking into account, that the basic probes are also narrow-band, LPF filter is modified – its reaction time is created asymmetric, then in case of the Stoichiometric fuel mixture, the “catalytic converter” would stable as if burn the oxygen.

Yes, also this solution has to be tested additionally – if for successful passing of the tests higher retention of the filter would be necessary, but at the same time – quick enough reaction of the probe after “burning the oxygen”, could be, that this solution has to be improved with timer + sample/hold hubs (either in a discrete or MCU form), or the second OPamp should be switched in the comparator mode.

And last note. Possibly, someone will say – it’s overkill! I will oppose. In my experience – in none of the scenarios (both DME re-flash or playing with config) of disconnecting the control probe I haven’t seen a positive result. As if non-existing (disabled) CO catalytic converters are however heated up forcibly, the kick-down mode with enriched fuel is not allowed to the engine, etc. My conclusions are simple – it is not enough to destroy (for example, blocking the procedures of the control probes) the performance of the test procedures of the CO catalytic converters. For DME, the illusion that the CO catalytic converters are in good order has to be created. Only then the DME will work in a mode of full functionality, and the engine – in full power mode. The absence of the error messages (immediately or even in longer time) in the DME error message memory is not the only criteria! Incorrectly modified DME, regularly being not able to pass the tests of the CO catalytic converters or receiving conflicting results, will reduce the max power of the engine for some time, will start to forcibly heat ut the converters, etc. DME will try to fix the problem and will not record the error messages, because:

  • short-term reduction of the performance of CO catalytic converters is allowed (for example, due to low-quality fuel; some leaking injector, reduced the temperature of the injector, etc.);
  • DME has no complete clarity regarding the amount of the problem and 100% confirmation of it.

Respectively – even without the error messages in the error message memory, the engine has all problems, which are typical in case of insufficient performance of the CO catalytic converters. Due to these reasons – it is important, that ALL conditions for a successful test of CO catalytic converters should be fulfilled.