My 2002 1.8is (45,000 miles) is showing a permanent check engine light caused by code P0031 (O2 sensor heater circuit low voltage); after a reset it reappears within a second or so, with the engine cold and hot.
I’ve checked the connector behind the passenger seat - it’s clean - and also the readings on the two wires that I assume form the heating circuit (the two black wires on the O2 side of the connector). The input to this circuit shows 11.9 volts, and the sensor side shows a resistance of 16 ohms.
What I don’t know is whether the voltage & resistance are within spec, or whether anything else might be faulty. That voltage looks a bit low but there are no problems with starting and the battery seems fine. It’s quite pricey to change the O2 sensor and then find the code reappearing once more, as some others have apparently done.
Any ideas anyone?
The 2004 manual states :- O2 sensor resistances front 5.6 ohms. rear 15.7 ohms. signal voltages 0.5v- 1.0v [engine speed rapidly increased to 3000rpm] 0v - 0.5v decreased 3000 -idle.
Hope this is of some help, Geoff Peace
Thanks Geoff, that is really useful. As the resistance is virtually spot on it looks as if the heater circuit is OK.
I guess that the car has two O2 sensors and I’m now wondering if the P0031 code could refer to the front one, which is presumably between the junction of the two down-pipes and the cat itself. Looks like another grope under the car (when the weather improves).
P0031 (HO2S (front) heater control circuit low) is a code for the front HO2S heater (connector under the bonnet). The heater is controlled by a duty signal by the PCM. The black/white wire going the the HO2S should have battery voltage supplied via the 15amp ENGINE fuse as long as the ign is on and the black/orange is the wire that is switched by the PCM. My diagnosis route would be to check for the battery voltage at the B/W wire then check the PCM is grounding the B/O wire. The blue wire and the black/red wire are for the sensor feedback to the PCM, ignore these if you only have DTC P0031. The resistance of the heater circuit should be 5.0 - 6.8 ohms. I’m afraid I can’t tell you which wire is which on the HO2S side of the plug, you’ll have to cross reference the wire colours I have given you on the vehicle side harness.
The code is generated if the PCM doesn’t see voltage at the control terminal (B/O wire).
Brilliant information, Robbie! I’ll check it when the weather warms a bit. A few other questions, if you can bear with me (it’s actually my wife’s car and I’ve not done much work on it before):
There is little room under the bonnet without removing bits (e.g. the strut brace) and I couldn’t spot the location of the front O2 sensor to check the wires to it for damage - can you help?
My code reader (a MemoScan U581) contains the standard code descriptions and I also have a database of code descriptions that I’ve compiled from various sources, but none of them mention that P0031 is specifically the front O2 sensor. Is there a better code list somewhere that I could download?
Finally my wife has at last regained her confidence in the car, after what I eventually discovered to be an intermittent brake servo failure resulted in her crashing it last year. So, as we now plan to keep it, I’m ready to buy a service/workshop manual. Any recommendations?
Thanks once again.
The front HO2S is quite low down in the exhaust manifold, the connector is on the back end of the cam cover, it’s a black plug.
P0037 is for the rear sensor. I can’t help you with regard to downloading a DTC list I’m afraid. Make sure you are searching for (european)E-OBD codes not just american sites.
Again, I can’t offer much advise regarding a workshop manual. NB ones are not published. Depending on what you need one for an NA manual will help in some areas.
edited to add… I have now added an E-OBD DTC list to the Engine FAQ section.
Thanks again Robbie, you’ve saved me a lot of wasted effort in helping to identify the problem. The connector has 12v input to the heater circuit but it shows open circuit on the black wires on the sensor side. Can’t see any obvious problem with the wiring itself so it seems that a new front sensor is needed. Looks to be an awful job, as I cannot even see the sensor, let alone reach it from above!
By the way, the “Download” link (above) didn’t work but I did find the file in another section. Thanks.
Easiest wayto gain access is to remove the nearside front wheel, O2 sensor can then be reached through the inner wing. The wiring is clipped to the exhaust manifold heatshield in two places, could be that the wiring is damaged there, worth checking I think before removing the sensor.
Regards Geoff Peace.
Thanks for the help so far guys.
As far as I can see there’s no way to reach the front O2 sensor through the inner wing on a Mk. 2.5 1.8i Sport. I’ve still not found it, despite removing the exhaust manifold heat shield - which required removal of the front strut brace (a waste of space in my view) the air filter box and the washer bottle & bracket - so that I could get my arm right down the length of the manifold following the cable. Also had to loosen the undertray to retrieve the rogue spanner that had disappeared down there. Seems like I’ll have to get the ramps out and crawl underneath again.
The only useful thing from today’s dismantling was finding that the front discs are ruined, thanks to the inside pad material having broken up in places and detached from its backing plate [:O]. So, I need to order new discs and pads (the back ones don’t look much better).
Or, maybe I’ll swallow a large brandy and ring up for a quote from a dealer.
The front H02S is just in front of the catalyst, just after where the manifold branches merge into one. Best access is from underneath.
Thanks Robbie. I’ll try again when the weather improves (I don’t have a garage).
I’ve given up on the front O2 sensor - there are too many struts and shields to remove from under the car just to find the sensor, and there’s little chance of being able to repair it. So, I’ve cut the two black heater wires and connected a 125 ohm resistor across them. It only dissipates just over 1 watt and seems to have fooled the computer, so the warning light is out. The revs when idling at cold are a bit higher (1500) but it seems to run as normal after a short while once the exhaust gets hot.
If it fails an MOT for exhaust emissions then I’ll have to get it fixed properly. Hopefully the cat itself won’t be harmed by the richer mixture when cold.