Vehicle seems to be running absolutely fine with no warning lights or messages but I noticed the exhaust getting a bit black of late and thought it might be running a bit too rich. Plugging in the Nanocom reveals that it is throwing codes P0135 and P0155 which seems to indicate that the upstream O2 sensor heating circuits have both probably gone AWOL (one may have gone AWOL before the other without me really noticing). They seem to be on separate circuits form a common fuse - fuse is OK but one of the sensors appears to be fixed at 4V and the other one is dancing around somewhat. Before I go and order a couple of new O2 sensors can anyone:

a) Confirm my diagnosis or is there something else that could be causing the problem?
b) Recommend makes of sensor to go for (and ones to avoid)?

I've got a lambda heater fault on one of my sensors but I'm getting a P1187 code. My list shows that to be short circuit on the heater yet the Nanocom says it is open circuit. Mine is GEMS so has the 5-0V sensors but a Thor should have 0-1V sensors. On mine, one lambda sensor switches between 0 and 5V as it should while the other stays pinned at 5.04V signifying a lean mixture so the trims go constantly rich. However, as I run a completely stand alone LPG system and hardly ever run on petrol, it doesn't matter, it just runs a bit rich on the odd occasions I do need to run on petrol.

With your sensors, you should never see 4V, are you sure it isn't showing 0.4V? The working one should switch between 0 and 1V roughly every 1-2 seconds. If it is taking longer than that to switch it is a least getting a bit tired. I'd go for either genuine Bosch or NTK sensors.

Well spotted. That should be 0.4V, in fact one of the upstream O2 sensors and both downstream sensors are 'pinned' at 0.44V - the Bank A upstream sensor is wanging about at a range of readings 0.1V and 0.8V changing about 4 times a second!

Bosch it is - the OEM ones look to be a third of the price of the 'genuine' LR ones! Although before I go nap I'll check the price of the NTK ones.

BTW, Nanocom seems to use Bank A & B in its values but Bank 1 & 2 in description of fault codes . . . . . although A does correspond to 1 and B to 2 at least!

As you don't have any downstream sensors (unless you have an imported US spec car as they are the only ones fitted with downstream sensors), what you are seeing there is a simulated reading from the ECU. I suspect what it is doing with the one faulty upstream sensor is it has noticed it isn't working so has pinned it at normal all the time.

Genuine LR will be Bosch in an LR box......

Can't find the correct one on the NTK website, it shows one but the part number cross references to AMR6244, which is the correct one for a '98 GEMS and not a Thor.

Well that explains why three of them appear completely static which struck me as a bit strange! I have put 2 Bosch O2 sensors on order as I couldn’t be sure of the correct NTK ones - may as well change both of them as they are still the originals and the P0155 fault code is intermittent. I can cancel it but it eventually returns after a short while. The P0135 fault code just comes instantly back when cancelled.

Now have to locate one of the O2 sensor connectors - I know it’s up there somewhere but it’s playing hide and seek at the moment!

They should both be in the same area, one either side of the bellhousing where it joins the gearbox. Or just follow the cable....

I just had a quick look under - one is easily visible, t’other not so. Once I get it up on its stilts and grovel properly underneath it will submit and reveal itself I’m sure.

New Bosch O2 sensors have turned up. I have never removed the sensors on this vehicle and assume it’s just a case of disconnecting the existing ones, unscrewing them and reverse for fitment of the new ones making sure not to touch the sensor end itself. However, I have heard stories of them being very difficult to unscrew so any tips on making this easier would be welcome before I get grovelling underneath.

Make sure you have the correct flare nut spanner, they should unscrew easily then.

Thanks, I do not have the correct flare nut spanner. However, I have the correct open ended spanner and the correct ring spanner - the latter will be used if the former is looking flakey, the existing sensors are toast so I’ll just cut the wire and use it if necessary. I also have a length of pipe to wield if necessary! Stilsons will be the tool of last resort!!

You'll find that a ring spanner will just fit over the plug on the end (or it will on a GEMS anyway), so you just thread it along the cable.

and if that don't work, Stilsons......

I’ve only removed O2 sensors a few times and I’ve never needed to use heat but I’ve read that heating the surrounding boss can ease removal.

Stilsons: I always feel a bit disappointed like it’s a bit if a failure when I have to resort to the Stilsons but I have, all too often, had to apply them and they rarely fail . . . especially when I have my length of scaffold tube over the handle to give that increased and brutal mechanical advantage!

Heat: Yes, good point. I will take my blowtorch with me when I go grovelling and, if needed, hope I don’t set anything on fire . . . particularly me!

Crow's foot spanners are ideal for this. Ok, not 100% necessary but think of the money you're saving by doing it yourself!
Tools are an investment etc etc ;)

A set of those https://tinyurl.com/ycknn6yu and nice big bar.. easy peasy!

I do almost all things myself* including plumbing for which crows foot spanners are invaluable for those tight spaces. Not sure of their utility for this job over and above a standard spanner with nice big bit of scaffold tube?

In an earlier post I did state that Stilsons are the tool of last resort. Not quite true . . . I have been known to undo the odd recalcitrant nut with a cold chisel and lump hammer!

*Apart from removing the gearbox/transfer box on the Rangie - without a suitable vehicle lift I considered this a job just that little bit too far for home DIY!

I've known them to be a right pain to remove requiring their destruction to remove them and be easier than I'd expected and come out very easily. They are more likely to be problematic if they have been out previously I've found rather than fitted once and left alone. Suspect some of that is having the grease on the threads to ease removal

Crows foot benefit is that it goes round and engages with a little bit more of the thread. If you can get a ring spanner over it that is probabbly better still provided its a good fit.

I had issues with the heater circuit on both of mine, which replacing the sensors didn't fix (particually annoying as it involved destroying one of them in the process). Didn't like the look of the drivers side connector so ended up removing it and replacing it with a superseal one. That was the cause of both issues as I didn't do the other side at the same time (As it was a pain to get to due to access) and that did make both sensors start working as they should with no error codes after that.

I just found a crows foot and bar to be ideal, especially as I don't have a length of scaff pole. The crows foot can also go on over the wire, I can't remember why I thought that was useful at the time :shrug:

Lambda probe sockets are available, they're a deep hex socket with a cutout along the length for the wire to stick through, they have a hex at the other end so you can use a ratchet or spanner on them. In the past I've cut the wires off and just used a normal socket. Been a long time since I've changed a probe on a P38 though, and don't remember if there'd be space for any of these options on a P38.

I suppose if heat is an advantage there's an easy way to heat it, run the engine. It'll heat the probe up as much as the surrounding area but it would be difficult to prevent heating the probe if you're trying to heat the boss anyway?

Once when trying to undo a probe from a P38 the lambda boss twisted out of the exhaust but I reckon whoever installed it had somehow cross threaded it, it was blowing and soot etc had probably made removal even more difficult by packing out the crossed threads. I know I've done some on P38's that have come out very easy, others very tight, the cross threaded one wasn't going to come out. I've also known them strip threads in the boss on removal.

I've made various tools for removing probes on various vehicles over the years, e.g. for the XJ8 Jags I made my own crow's foot by cutting a slit out of a ring spanner, cut the spanner short (close to the ring end but longer than a crow's foot) and welded a socket to the cut end to allow use with an extension bar and breaker bar / ratchet. Not as strong as a proper crow's foot (the ring spanner isn't as thick as a crows foot and loses strength with the slit cut through it) so has the disadvantage that if the probe is really tight the ring end could open up and slip.. but with the advantages (for the XJ8) of having a longer length from the probe to the bar and having the socket at an angle to the ring end.. perfect for using a long extension and ratchet/bar working from above in the engine bay, there wouldn't be space to lever near the probe itself or working from below and a conventional crows foot would see the extension bar foul the engine or engine bay components working from above but the DIY tool makes it easy to undo the probe working from above.

Running the engine was a recommended procedure on a previous vehicle I had (Dodge Dakota IIRC). Of course if you could keep the probe cool at the same time it would work even better. For seized bolts etc, heating the whole thing helps, even though it would help even more if you could keep the bolt cool.

I think running the engine will heat the exhaust and sensor in equal measure so may have negligible effect. Externally heating the collar preferentially should yield better results.