MK2.5 Should I replace the ECU to fix an EGR Valve fault?

  1. My model of MX-5 is: MK2.5
  2. I’m based near: Birmingham
  3. I’m looking for technical help or recommendations on: EGR Valve fault code remedies

My MX5 has recently been suffering from a P1499 “EGR Valve Coil 4 open or shorted” code. I’ve tried replacing the valve with a used one and clearing the code but it just keeps coming back.

It seems like the EGR valve is known to fry part of the ECU on it’s way out, so It’s looking likely that I may have to replace the ECU, immobiliser, and all the locks!

The only thing that gives me pause is if I disconnect the valve entirely I only get P1497 & P1498 codes (according to Torque). But reconnecting still results in only the P1499. I’d assumed that if the ECU had been damaged it would always show the P1499 code?

Is there any test I can do to determine if the fault is actually in the ECU?

Thanks

You can buy a s/h ecu from ebay to substitute and if it doesn’t help simply sell it again for the same price.
If it does help, keep it :slight_smile:
I guess you’d have to buy a complete set of course.

I was thinking of giving it a punt and hoping for the best.
My main worry is with having to change the locks and immobiliser, as there is an after-market Cobra alarm fitted. Not sure how that is linked to the immobiliser but don’t want to end up not being able to start the car, or having the alarm blast non-stop!

The ecu won’t be connected to the after market immobiliser, the cobra will be in the field wiring.

That’s good to hear, I think. I’ll go ahead and order the parts. Thanks for giving me the final push!

Am I right in thinking I will only need to change the ignition barrel to keep the new immobiliser and the Cobra happy, and I can just use my old key on the doors and boot?

Each system ‘can’ be different and I’m unsure how the Mazda one works. Essentially the inductive loop around the ignition barrel is unintelligent, it simply induces a current into the RFID IC in the key, reads the code your key’s RFID tag returns and reports that to the ecu, like a tv aerial collects waves and sends them to the tv. A demodulator in the ecu extracts the RFID data from the incoming waveform and deciphers weather it likes it or not. If it does it tells the processor that all is well and it enables ignition functions. In this case the RFID induction loop would not need replacing. Should the FRID induction loop be ‘intelegent’ and demodulate the data it’s self you will need to replace it. This is less likely as a rule because it’s less secure allowing easier circumnavigating to start the car. It’s also harder to get the electronics shoe horned into the loop. There may however be a demodulator in-line between the two and you will have to replace the lot.

Car keys tend to be passive low frequency tags because they only have about a 100mm range.

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I’ve heard of this fault damaging ECUs and it makes me wonder if there’s a repair service for the ECU since the alternative is a replacement ECU which would necessitate a matched immobiliser (and keys and locks too if you’re using secondhand parts).

I wonder what repair Mazda would do if it was under warranty. Maybe most cars were out of warranty before these faults began to show up.

The chances are that the failure is local to merely the output transistor(s) and maybe the associated protection diodes on the interface to the ECU windings, possibly even a short bit of narrowed ‘fuse-track’.

However, without the circuit of the ECU it takes time to work out where these might be. I would expect (hope) they are on the end of the thicker tracks from the relevant connector, but one never knows.

Having seen some very peculiar circuit-board layouts over the years in a variety of different bits of gear I would not be optimistic on trying an uninformed poke and hope type of repair.

With all the relevant info and suitable test gear, it would be easy to assess damage and decide if a repair is feasible or not.

You do not have to change the mechanical key/locks just because you are changing the ECU, just change the key blade over to the fob matched to the ECU.

Thanks for all the advice.

I had taken it to a couple of garages, including one that specialises in electrical faults like this. Their plan was to replace the EGR valve then either replace the ECU or send it off to somewhere in the hope they could figure it out and fix it (he seemed to think there was a company in Australia that could do it). So I don’t think I stand much of a chance in fixing the current ECU myself as I have neither all the relevant info, suitable test gear, or much know how!

eaa53 I hadn’t even thought of swapping the key blades, though it makes a lot of sense! I imagine it’s something that I could get done at Timpsons fairly easily?

I’ve just had another look at the ECU and it seems to be a BP7PC variant, I’ve not been able to find much difference in the variants. Does anyone know if the more common BP7PA variant would work?

Swapping the key with the ecu is a must, the RFID tag in the key is matched to the ecu. The loop isn’t matched to anything other than having the correct turns ratio for the frequency of tag.

There’s a place in Plymouth called ECU Doctor.
I’ve an idea they do exchanges.
Whether they do MX5 ECUs I don’t know but worth a try.

Would it be possible to disconnect the old antenna ring that goes around the ignition, connect the new one somewhere under the dash and leave the new key tied to it, letting me start the car with my existing keys?
I know this is a horrible hack and means I effectively have no immobiliser but I’d like to get the car fixed and through it’s MOT before I go about replacing all the locks.

Plymouth is a bit out the way for me unfortunately as I’m based near Birmingham.
But I’ll bear it in mind, if they can fix the problem the drive might be worth it.

Yes but you may need to remove the RFID tag from the other key invade the ecu gets confused. I don’t know because I’ve never tried it but I’d imagine that both would send harmonics into the carrier wave and the demodulation wouldn’t be clear. :man_shrugging:

You can easy try it. Just pop the new key behind the steering cowl out of sight and see what happens. There is no need to rewire as such it will all be on the multi plugs at the ECU.

Incase not invade :man_facepalming:t3:

Also, don’t panic about the immobiliser. Your alarm has one that’s insurance approved and just as effective. Ecu immobilisers aren’t hard to bypass if you have your heart set on it, alarm ones neither. They stop the opportunist that’s the main thing, but even chav scum can read and learn how to get past most security online. Anyone wanting your motor would assume the stock immobiliser was in tact anyway, it’s the deterrent you need more than anything.

You would send the ECU to them.
As I said, they may do an exchange, whereby they send a good one.
They do a diagnostic service as well I think, for a fixed fee.

Yeah I imagine the Cobra sticker on the window is probably more useful than the system itself

Yes, but in the case of a Mk2.5 a swapped ECU would not start the engine. Those ECUs are paired to the transponder which communicates with the keys and if the codes don’t match the engine won’t run. You have to change both together.

You don’t need Timpsons or any other key cutting service. The physical key is retained in the fob by a roll pin, change the electronics and swap the physical key to the fob that matches the electronics the rest all stays the same as the physical key still fits the ign and the door and boot.