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Navara D40 Code P2297- Oxygen Sensor Out of Range During Deceleration

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    Navara D40 Code P2297- Oxygen Sensor Out of Range During Deceleration

    It's a long story and has been an ongoing problem for a while, but here goes:

    My Navara only has one O2 sensor, downstream of both the catalytic converter and DPF.
    There are also 2 temp sensors, one before the DPF and one after and a differential pressure across the DPF measured by 2 pipes, one before and one after the DPF going back to a sensor under the bonnet.

    At about 20k I removed the DPF and put back the standard sensors in a futile attempt to improve the 14 -16 l/100km economy on long highway runs using cruise control, and the even higher fuel usage during towing and other driving.

    No codes were thrown for the next 70k until shortly after the 90k service. At that point I didn't have a code reader, so I took it back to the dealer. I assume all they did was clear the code and send me on my way because it came back 24 hours later. After a few more futile visits they basically told me to piss off and said it was my fault for not having dummy/recalibrated sensors fitted when the DPF delete was done. I thought this explanation was bullshit because it ran for 70k without throwing codes.

    So I got a code reader and found it was P2297 and couldn't see why the temp sensors (they are the only dummy/recal things found on an internet search and they are only simple platinum resistors or similar) would be responsible for this code. A couple of thousand klicks later funny noises were coming from under the bonnet and I took it back and they said it had a leaking exhaust manifold. The manifold was fucked and needed replacing which they did. I thought that fixing the leaking exhaust would stop the code. But no it didn't.

    Since then it has been to another dealer and 3 other independent mechanics and all they do is a reset and the code just comes back on the next day. They all looked for exhaust leaks and said there were none.

    At about 140k the funny noise came back again and this time I took it to a diesel turbo specialist and the exhaust manifold was again fucked, warped and delaminated and needed replacing. They said the heat backing up from the catalytic converter which is connected to the manifold is the problem. They removed the catalytic converter and put a straight through pipe on. This improved the highway running economy to about 13l/100km, not great but getting there, but about a day later the 2297 code came up again.

    During all this crap I've been trying to wade through all the bullshit on various forums and web pages about the poor economy and what functions the various sensors control. I think it works like this:

    The O2 sensor is only there to make sure the O2 level is low enough for the catalytic converter to work, if the O2 level is too high there is post burn injection of fuel which burns in the exhaust to reduce the O2 level. This make sense because the passenger floorwell and the drink holders in the console directly over the exhaust get fuckin hot and always have done since new. On long runs in winter you still need the air con on. It's marginally better now with the catalytic converter gone though. It also accounts for the very poor fuel economy.

    The temp sensors are there to make sure the exhaust is hot enough in the first place to ignite the fuel for a DPF regen or to burn excess O2.

    The diff. pressure sensor for the DPF is pretty well self explanatory.

    So I've had a bit of a play, I thought what if the exhaust temp is always too low for fuel to burn, would the ECU refrain from injecting any fuel post burn? I pulled the temp sensors, they were about 250kohm at ambient and went down to hundreds when I put the hot air gun on them. So I put a 125kohm resistor in series with them. It stopped throwing 2297 but started throwing 0427 and 0428 codes, these are cat too hot and too cold.
    So next I removed the inline resistors from the temp sensors and put some foil over the the O2 sensor to try to stop any varying O2 levels in the exhaust affecting its output. It still throws the 2297 code but more frequently, before it was only 24 hours or more after a code delete, but now it can be only a few hours after a code delete.

    So after all this, is it just a faulty O2 sensor? Or do I need to have the temp sensors reading artificially high so that the ECU thinks it doesn't need to do a post burn injection for the cat or DPF? Or could it be something more sinister like faulty injectors?