Newly developed MX5 MK2.5 Immobiliser Emulator

I am seeing more failures in the TRW(ex-Lucas) immobiliser on the MK2.5 cars. These are not available new and it is not possible to reprogram a used immobiliser from another car. Up until now the solution was the same as for a water or otherwise damaged ecu which is to replace all associated components with those from another similar car. Given the rarity of some of the ecu sets and an increasing incidence of immobiliser failure we have developed an emulator to replace the faulty immobiliser.

If your car suddenly becomes a non start and stays that way, it is likely that the faulty component, if from the ecu/immobiliser system will be the ecu. If the car fails to start uncharacteristically on one occasion and perhaps months pass before this occurs again but there is an increasingly recurrent intermittent fault, the immobiliser is likely to blame. In any event always check the ecu for obvious corrosion. Remove the connector plugs and ensure that all connector pins on the ecu are shiny and clean metal. Take the back plate off the ecu(4 philips screws) and visually check as well as doing a sniff test for a possible electrical spike issue(rare on MK2.5).

We require the TRW immobiliser unit and a transponder key from the car. The key is required to test the immobiliser in a harness and confirm fault before progressing. Once fault confirmed, the main chip is then removed from the immobiliser and data read from it and programmed into the emulator replacement. The newly programmed emulator simply plugs into the car, replacing the immobiliser and will resolve any immobiliser issue.

Assuming the incidence of original immobiliser failure continues to increase on these cars, the availability of an immobiliser emulator will future proof this function.

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Wow Rob,

I could have done with one of those a few weeks ago as you know.


Well done you for doing all the hard work so we can profit from it !



Wow, really impressive work there. That’s definitely a case of going above and beyond the call of duty.

It’s going to be the electric side of 80’s onwards cars that will kill a lots of them, fixes like this are what we need to keep them going.

Well done Rob. 

So the new box you will be re programming is a direct replacement for the Lucas box?, but in most cases it can be down to a faulty ECU, is it not worth sending the full system to check before programming a new box in? IE:- key,ECU, Immobiliser box, that way it does not waste your time and costs and the owners if you find out it is not the immobilser at fault.

Out of interest can you program the box to make it unimmobilised so either a after maket system can be fitted or for a track car.


Hi Rich

Given your initiative/knowledge, the fact your ecu set is the most common one available and the price I know you paid, you did the right thing and ended up with other valuable parts as well.

May be worth keeping the old set though as it is potentially of some use. 

This system is really going to be useful for the rarer ecu sets and if/when we reach a crisis point with failing immobiliser boxes.

Up until about 18 months ago all problems I encountered when diagnosis occured were down to failed ecu and/or connector plugs, usually water damage.

Aside from customer issues with failed  immobilisers on the MK2.5, I have had one of mine fail whilst in dry storage and another fail a week after installation.    



Hi Wayne

You are right - most failures with all these MK1 - MK2.5 from my experience are water damaged ecu. Yet to have proven ecu failure where water damage or on rare occasions electrical spike not responsible. It seems that the actual components on these ecus are not failing at the moment without external cause. Fortunately, for diagnosis purposes, in most cases ecu damage stops the car dead and produces very obvious evidence, hence the requirement to check before going the emulator route. The clue to most immobiliser faults I have encountered is the intermittent nature of the fault. Apparently it is the main chip that starts to fail and when removed evidence of moisture ingress seems to be the probable cause. ECU failure may be progressive in its nature but not known an intermittent start. When corrosion/electrical damage on the ecu stops the car that is the end.

The emulator does directly replace the immobiliser but its function is simplified. It holds sufficient data, PIN, key, etc to effectively communicate with the ecu allowing the car to function normally. The immobiliser security function ceases as does the requirement for an antenna or transponder chip. It basically works in the same way as the demob style of ecu/immob replacement with which I am sure you are familiar. Getting rid of the immobiliser function completely is not possible as far as I know. Stock ECU function will always rely on communication with an associated immobiliser or immobiliser emulator. Because of the way the emulator functions I would expect an aftermarket immobiliser/alarm could be fitted. Fitting a programmable Megasquirt/ME221 does away with the immobiliser function on MK1/MK2, reasonably sure they are also available and do the same on the MK2.5. 

The test harness can also check ecu function but given the expected intermittent start issue and a clean ecu, wiring , pins and connectors, sending just the dead or deadish immobiliser and a key with transponder that will no longer be required is the current recommendation. Proving an immobiliser fault is the key to this. If the immobiliser is proven working or the emulator fails to cure the cars starting issue completely we make a charge but refund as much money as reasonable. The emulator can be reused for another car.

There is some issue with the obvious loss of immobiliser function in practical terms. From my experience where a seamless solution for MK2.5 ecu/immob failure with two chipped keys is available for a little extra money, 90% of owners go for the cheaper and more convenient demob solution. Having said that I have a plan to develop a direct replacement for the MK2.5 TRW immobiliser with full functionality.                




Great Work Rob, you are a STAR!!!

Hi how do i get one of these for the wife’s mx5 mk2 thanks dave

My immobiliser has stopped working too, so I may need your device. what do we have to do?

Hi I am having issues with my car it won’t start and ecu/immobiliser has been tested and immobiliser is to blame on a year 2000,
do you still do your fix. Ifso please let me know what you require.

Hello, My immobiliser failed during my MOT causing the mechanic Simon to panic saying I just turned it off honestly. So if this emulator fix is available I would be very grateful if you could tell me where to send it and what it will cost as I don’t really want to put another old immobiliser in it that is probably on borrowed time anyway.

Hello, I understand your point, although Simon should not really be panicking. It’s a well known problem with the MK2.5 immobiliser; you know about it. He should possibly know that disconnecting power or the immobiliser momentarily will probably reset it for one start.
The sad truth of this matter is that the emulator has not been available for some time. Unfortunately it seems that the auto electrician who developed it with me has some kind of issue. His own mainly redundant website says returning on January 15th. No highly qualified auto electrician with knowledge/access/control of emulator pic code on board = no emulator!
Only one good thing to come out of this, his wife, bless her, sent all my developement parts back and also a MK2.5 test harness. I have developed the harness such that now possible to fully test transponder/immobiliser/ecu compatability and operation and now program transponder chips to all relevant MX5 cars, MK1 1.8, MK2 and MK2.5. Strangely the only one I cannot test or program on the harness is the MK1 1.6 = my car!
Given that there is no emulator available, the only viable option I am aware of is to replace the immobiliser and associated components. As you correctly state, this is not a great option as all these units are effectively on borrowed time. For my own part I now offer a three month guarantee with the MK2.5 ecu sets I sell to take some responsibility for this unfortunate situation.
I did send you a private message; perhaps you haven’t seen it?

Hi sorry I got the message but It is my wife’s birthday and so I have been panic buying and wrapping presents the last few days. But she is now with her friends in the back garden drinking, so I can have five minutes to respond. I will have to replace the bits or stick the car on the wall of the garage because it is just a paperweight at the moment.