I've got the dreaded codes P0174 and P0171 still, so ended up changing out the MAF sensor and all four O2 sensors with new, genuine Bosch parts. Unfortunately, it didn't change the codes or the way the engine runs. If anything, I almost think the slight miss at low idle, 650 - 750 RPM, might be a tiny bit worse.

All I have is the Torque Pro app on my tablet connected to a blue-tooth OBDII scanner, but it does have rudimentary O2 sensor voltage graph readout.

What I've noticed is that Bank 1, Sensor 2, does not match the other three. Not that they all match perfectly either, but the voltages go up and down at a pretty good rate, nice peaks and valleys on the graph when driving.
However, O2 voltage at Bank 1, Sensor 2, is much more flat. It will go up and then stay up in a straight line for the equivalent of maybe 3 or 4 peaks and valleys of the other 3 sensors. Then it will drop and stay low for some time before going up again. It does move up and down, sometimes in sync with the other 3, but often just hangs out either high or low way longer than the other sensors do.

Is this normal activity for that sensor, or did I perhaps get a bad sensor from the factory? Would it be worth putting one of the old ones back in and seeing what the output looks like? I have no idea if the old O2 and MAF sensors were actually bad, I just figured at 14 years old, they might be going, but unfortunately they didn't help the codes or the rough idle.

I'm stumped where to go from here. I've gotten other transient codes now, like random misfire and even low MAF sensor voltage code once (yes, with the new MAF), but they seem to come and go, and clear on their own.
P0174 and P0171 always stay, and sometimes P1174 and P1171 stay as well.

Would it be worth it to do the plugs and wires next? I pulled Plug 1 and looked at it - looks good, a Bosch platinum plug, previous owner must have changed them at some point. The Rover just ticked over 92K miles.

Thanks for any suggestions and tips on the O2 sensor readouts or where to go from here with the trouble shooting!

 

You could have bad o2 sensors out of the box. There are a lot of forum members whom have the same issues with multiple brand new Bosch o2 sensors

 

I fought the same codes for months. I ended up solving the issue with a used bosche MAF, and I went with walker O2's. Apparently they are what the U.K. boys are running and they seem to work well.

I read too many stories about the bosche sensors being bad out of the box.

What are your front sensor voltages? They should bounce between .100v and .900v

 

Thanks for the replies! I watched the readout of the O2 voltages today while driving. If I really put my foot in it, I can get them to hit about .9V. At idle they drop to almost zero on the graph, probably around the .1V you specified.
The Bank 1, Sensor 2 is still quite sluggish, though. Does go up and down but hangs out at the top or bottom much longer than the other 3. I will pull it out Tuesday when I get some time and put one of the old ones back in, and re-test.

Something interesting happened with the codes, not sure if anyone has any light to shed on this:
The check engine light went off on its own for the first time in weeks, probably for about 12 miles or so. Then, the next time I drove it, the light came back on after a few miles, so I checked the codes again to see what was up. Now there are a plethora of misfire codes as well:
The usual P0171 and P0174. Then, in addition:
P0300 Random/Multiple misfire
P0303 Cylinder 3 misfire
P0304 Cylinder 4 misfire
P0306 Cylinder 6 misfire
P0308 Cylinder 8 misfire

Then some pending codes: P0307, P1171 and P1174.

Any idea if all the misfire codes are something new I need to chase, or are they possibly part of this original problem causing the P0171 and P0174?
Thanks for any ideas!

 

Your 300 codes sound like your coil packs have an issue. I had multi misfire codes and sluggishness. Replaced the coil packs and did not have again.

 

The P0171 and P0174 codes are, as you likely know, lean codes, which typically points to unmetered air entering the system downstream of the MAF, i.e. a vacuum leak. That causes the ECM to try to compensate by pumping more fuel through the injectors, which can in turn cause misfires.

Since the MAF and the O2s are new I'd try a smoke test to try to locate a vacuum leak. There have also been more than a few instances reported here where P0171 and P0174 were caused by leaky injector O-rings allowing air to enter there.

I read nothing to indicate a problem with the coil packs.

As for the downstream sensors, they should switch more slowly. In fact, ideally they should stay right around 0.45 volts. Faster switching on B2P2 indicates the cat on that sided isn't functioning as it should. But in any case the downstream O2s should have nothing to do with the lean codes nor the misfires.

Focus on the smoke test and the injector O-rings and see what happens.

Good luck.

 

So it's possible that the P0300 misfire codes are being triggered by the possible vacuum leak and/or injector O-rings? I'll have to take it somewhere for a smoke test, not sure who does that around here, but I can ask around. Hope it's not injectors, but if it is, then I'll just have to get busy and tear into it.

If I do end up needing to replace the coil packs, looks like they must be behind the engine, so I would probably do new plugs and wires at the same time. Is it best to put Bosch platinum plugs back in, or anything better for these engines?

Thanks for the tips, guys. Lastly, do you think it's worth it to pull the driver's side downstream O2 sensor (I'm guessing that's Bank 1, Sensor 1) and put an old one back in there and see what the voltage readout looks like? If it's slower like it is now, then that would point to the cat being bad on the passenger side, if I understood your post correctly?

 

I will offer up a suggestion as I had similar issues.

Clean your our gas cap and spout.
Focus on the MAF, if you still have your old one put it back on the truck and see if codes come back (after cleaning gas cap and spout)

 

Many "good" shops should have a smoke machine. Doesn't need to be Rover-specific. If you can't find one convenient to you there's also a DIY method using a cigar (search YouTube).

My friend chased the P0171 and P0174 codes on his '04 for a couple years before he gave up and sold the truck. He replaced the MAF and the O2s. Didn't help. We did the cigar test and found nothing. We'll never know for sure but I ended up convinced it was the injector O-rings. The cigar test probably didn't generate enough pressure to confirm that was the problem.

For now I wouldn't worry about the downstream O2 sensors. Find the vac leak(s) and get the lean codes sorted out first and see if the misfires go away.

 

Take brake cleaner and spray all around vacuum hoses, under intake(fuel injector o rings/UM gasket), Intake hose post MAF while running. If the engine changes when staying at that location, there is a leak.