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I'm hoping for some advice.
I've got a 2009 207 1.4 petrol. After a recent service it started leaking coolant into the CTS wiring which worked its way onto the ECU terminals and caused misfires resulting in a tow from the RAC.
I've replaced the thermostat housing/sensor and thankfully there's no more liquid dumping onto the ECU.

However, there's a reoccurring high voltage fault code on the downstream O2 sensor, P0138. I've captured some live data while the car is idling, and to my reasonably inexperienced eye most of the time everything looks good, but occasionally there'll be a spike that exceeds 1V.

I've talked to a friend with an electronics background and they said that the voltage can spike like that if there's corrosion effecting the contacts. With that in mind I've wiggled and poked all the cables and connectors that go between the downstream O2 sensor and the ECU and I haven't found anywhere that will make the voltage spike.

I'm hoping that someone here knows what the most likely cause of a spiky voltage reading on a downstream O2 sensor is. My guess is that it's the wiring, but I don't really want to shell out to replace it if it's more likely that the ECU needs to be sent off to be looked at, or that replacing the O2 sensor would do the trick, or some other car magic needs to be done.

My next test will be to set my code reader to record upon a fault condition and hope to get a recording of the engine stats when its bad enough to generate the fault code. But I think that's just going to confirm that the voltage really does go too high rather than provide the giant smoking gun with big neon arrows taped to it that I'm looking for.

Any help would be greatly appreciated!

Pete
 

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The coolant often gets into the ECU causing problems. I had the same issue with my daughters old 207 and ended up changing ecu

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