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I replaced both the front o2 sensors on my 2000 RX. Codes were cleared all ok, when going for a drive to warm up car for MOT, gearbox would not change into top gear.

Took car to garage for garage owner to take to MOT station, 5 minutes later he phoned to say engine would not rev and appeared to be in limp mode. I managed to get the car home at about 5mph.

I checked and codes P1663 & P1760 are showing. These can be cleared and engine runs ok with diagnostic kit plugged in. I have replaced the 2 VVT oil control valves with no change. Codes still show, and same as before faults can be cleared. 

Can a diagnostic unit take over as a ECU when attached?

If I clear codes engine runs and revs ok but as soon as I disconnect kit engine appears to over fuel, and various things happen on the dash.

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I'd check your Battery is good and the condition of the earthing from the Battery to the body and body to the engine. P1663 is a VVT engine error and P1760 is transmission - these are unrelated and therefore something common such as poor earthing is the likely cause. Also check the connectors on the ECU for corrosion/water ingress.

What errors were you getting to need to replace both O2 sensors - normally only one fails at a time. Was any wiring damaged when the sensors were replaced?

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Thank you, I will check the earths and connections. The car does live outside so there might be corrosion somewhere.

I can not remember the o2 sensor codes but they both related to the heater circuits.

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Were the O2 sensors replaced with genuine or aftermarket parts?

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I knew about the aftermarket sensor problem so used genuine parts.

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If you weren't getting these two codes before replacing the sensors then it's likely to be linked to the new work you've done. Checking connectors etc, as suggested, will help.

If you replaced the sensors with the Battery still connected there's a possibility you shorted something. Disconnecting the Battery for an hour or so ought to reset the ECU.

The engine running fine with the kit attached may well be because the kit is stopping the emissions system from functioning properly. In this case you really want to test the engine with the O2 sensors disconnected. That'll throw other codes but won't give u limp mode.

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