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MG MGF Technical - 2 oxygen sensors, 1st one broken?


amber dash light came on in my mums MGF (2001). manual says emission control.

i see there are 2 lambda sensors, one before the CAT, one after. the one before the CAT puts out .445->.455v and that's it, no matter how the car is revved - i.e. seemingly 14.7-1 mixture always! the one after the CAT behaved as i expected, i.e. swings from .1v -> .9v depending on the rev's etc.

so, the car is not even 2 years old with 19k miles so this would be a very young age for an o2 sensor to fail? from what i've mentioned is it pretty obvious that the first sensor has failed or could it be something else? i guess if the CAT had failed the second sensor would return very rich/high values like >.5 always?

steven - lurking from midget board :)


p.s. even if it is the sensor and i replace it will the warning light go out? it is on instantly from starting the car so i guess it is stored in the MEMS/ECU system - therefore will the system need to be reset and how can i do this (will it require use of a rover testbook?)


removed the sensor, heated it up with a blowtorch and the reading went from .1 -> .9v with varying amounts of heat as expected. so maybe it isn't broken. refitted it but still constant when in the car. i'm just worried that if i go to the expense of fitting a new one it is actually something else at fault, i.e. ECU or whatever.

>i.e. ECU or whatever

Disconnect the battery for half an hour and then reconnect.

May be the ECU system *hang up*.
I know it isn't from Micro§oft, but who knows...
May be the system boot does the job.

Dieter K.

cheers dieter - i wasn't sure if it would hold its memory even without power.

anyone know how much rover (or someone cheaper!) sell their o2 sensors for?

MGR prices as at April 2003

RMHK10006 sensor £134.40 + vat
RMEK10060 sensor £10.15 + vat

not mechanical so don't know which one is which but suspect the expensive one is the O2 sensor.

Sorry but HTH
John Ponting


are that official MGR prices for the sensor in the UK ?

They replaced my oxygen sensor lately and i had to pay 300Euro's (about 220GBP) for it. I made a complaint to my garage that I could buy the same sensor in a car shop for about 100€ buy they said that it was the official MGR price in Belgium.

Since I have an 1999 MG, I only have one oxygen sensor so it may not be exactly the same, but still the difference in price is big.

>>removed the sensor, heated it up with a blowtorch and the reading went from .1 -> .9v with varying amounts of heat as expected.<<

Steven, suspect the connection or the loom. If you search through the Archives, you'll find that John Thomas had a similar problem with his Mk1 MGF: the wire to the O2 sensor had melted/broken against the exhaust manifold.

Might be a similar problem with your car (although this is not a common problem) - it is certainly cheaper than a new sensor!!!
Rob Bell


Those were the prices I was charged by an official MGR Dealer in April plus VAT @ 17.5%

Your prices do seem a bit high - we keep being told that it is cheaper to buy cars in Belgium than in UK but it seems we then have to hope nothing breaks.

As a guide, I just bought 2 front brake disks and a set of pads from a parts supplier (not MGR dealer) and they cost me £68 including vat.
John Ponting


yeah, i read the archive on this - took hours! i checked the wiring along the sensor and it seems ok, the loom is of course a different matter!

coincidental or not but the day before a 10AMP fuse went (under instrumentation!) and i couldn't find or repeat the fault. replaced the fuse and didn't blow again...

i think the sensor after the CAT is identical, i'm going to go check that now, and if so swap the 2 around and see what happens with regards to readings etc.

Fingers crossed Steven that you'll be able to track down this irritating problem!

Good luck!
Rob Bell

ok, disconnected the battery overnight and swapped over the lambda sensors and guess difference so i can pretty much rule the sensors out?

oh my god, apart from pulling the loom out i have no idea what to do next. i hate electrical gremlins!!

If one sensor were faulty Steve, then swapping them around would not necessarily result in the MIL being extinguished.

I reckon you are going to need to run the engine, disconnect the leads, and measure the resistance values with a multi-meter. If these check out, then suspect the plugs and the leads themselves.

BTW I hate electrical gremlins too :o(
Rob Bell

For the archives: Geert isn't the only one who paid € 300.00 for a new O2 sensor. BTW, needed to pay for another exhaust manifold as well as they screwed that one up.

rob - what i meant about swapping them round was to see if the first one now worked properly and the rear one would fail (i.e. swapping the sensors = moving the fault) but it made no difference!

Steven, how were you detecting the fault? Probably just me getting confused! ;o)
Rob Bell

rob - as per my first post i was monitoring the lambda sensor output on a DVM, and the readings should average around .5 and swing between .1 and .9 depending on throttle position etc.

the pre cat sensor was returning a VERY stable .5'ish under all conditions, with the rear sensor behaving as expected. when swapped over the rear was still ok and the front one was still stable which leads me to believe something is wrong elsewhere...

Ah, I get you. Mmmm - does sound like a plug/ wire loom fault doesn't it?

You are out of warranty period, I presume? Testbook diagnosis might be helpful.
Rob Bell

another update - just started up the car and the light goes out. fixed itself? ehh? damn cars!

Have an odd memory of a conversation with an MGR techy who impied something like...

Certain warning lights go on with one fault. They do not go out until fault has not been present for X cycles.

This may be rubbish... but it may be worth checking out.

Paul Nothard

Steven, glad to hear the fault has cleared itself. I wonder what effect disconnecting the sensor has on the mems unit i.e, is it fail safe, does the warning light come on as a precaution. Good luck.
M J Gibbens

Ps just searched for lambda sensor on google, the following link was quite interesting
M J Gibbens

PPs This might help
Technical Information Oxygen Sensor Diagnosis Generic Method

Here are some fast and reliable diagnostic procedures which you can use to check out most oxygen sensors. A great time to do this is when you are performing a tune-up.

The following symptoms will help tip you off to a failed oxygen sensor:

Surging and/or hesitation
Decline in fuel economy
Unacceptable exhaust emissions
Premature failure of the catalytic converter

You will need the following equipment:

A handheld volt meter (digital VOM)
A propane enrichment device
An oxygen sensor socket
The manufacturer's vehicle specific test instructions.

It should take less than 10 minutes to perform a diagnostic check on most vehicles.

1. Verify the basic engine parameters, per the manufacturer's specifications for the following: timing, integrity of the electrical system (supply voltage), fuel delivery mixture performance and internal mechanical considerations.

2. Treat the rich mixture performance as follows:

a. Disconnect the sensor lead to the control unit.
b. Run the engine at 2500 rpm.
c. Artificially enrich the fuel mixture by directing propane into the intake until the engine speed drops by 200 rpm. Or, if you're working on a vehicle with electronic fuel injection, you can remove and plug the vacuum line to the fuel pressure regulator.
d. If the voltmeter rapidly reads .9 volts, then the oxygen sensor is correctly sensing a rich mixture. But, if the voltmeter responds sluggishly, or if it stays below .8 volts, then the sensor should be replaced.

3. Test the lean mixture performance as follows:

a. Induce a small vacuum leak.
b. If the voltmeter rapidly drops to .2 volts or below in less than a second, then the oxygen sensor is correctly measuring the lean mixture. But, if the voltmeter responds sluggishly, or if it stays above .2 volts, then the sensor should be replaced.

4. Test dynamic performance as follows:

a. Reconnect the sensor lead.
b. Set the mixture to specification.
c. Run the engine at 1500 rpm.
d. The sensor output should fluctuate around .5 volts. If it doesn't, replace the sensor.


When performing diagnostic work on your customer's vehicle to determine the cause of a driveability problem or perhaps the reason for failing an emissions test, take the opportunity to check the operation of the oxygen sensor for proper functioning.

Recalling that an oxygen sensor will influence the air fuel mixture preparation only when it has reached proper operating temperature (at least 350oC), it is essential to first ensure that the engine and sensor are warm enough to allow operation in a "closed loop" condition. It may take as long as 2 1/2 minutes after cold start for proper exhaust temperature to be reached (somewhat shorter for heated-type oxygen sensors).

To check the performance of the oxygen sensor, run the vehicle engine at about 2000 rpm (or at normal cruise when working with a dynamometer) to ensure that the sensor remains hot throughout the test procedure. Do not remove or disconnect the sensor lead in order to test it as this will eliminate the "closed loop" signal to the electronic control unit and result in a non-cycling voltage condition. Using a correct electrical impedance test device as found with a laboratory type oscilloscope, connect your test leads so as to read voltage from the signal wire to the electronic control unit. With vehicles that use a heated oxygen sensor (three or four wire), it may be necessary to bridge the connector leads and tap into the signal wire with an appropriate test probe at the connector plug in order to obtain the signal. The oscilloscope will allow you to read the electrical response pattern of the oxygen sensor to changing exhaust gas oxygen content as a measure of its performance.

Before proceeding, be sure that you are using the correct measurement scale for your specific equipment as specified by the test equipment manufacturer. (Invariably, this will be a low voltage scale.)

A properly functioning oxygen sensor will exhibit a rapidly fluctuating voltage signal alternating between approximately .2 and .8 volts in response to varying residual oxygen content in the exhaust stream. Look to your scope's time reference line for a desired lean-to-rich and rich-to-lean time of less than 300 milliseconds. A response time greater than 300ms. means that the sensor should be replaced. It is important to recall that these values are valid only when checking a sensor operating in "closed loop" in a hot exhaust stream (350o-8OOoC). Sensor age degree of contamination, mixture setting, and exhaust temperature all have an effect on response time.

Without this rapid electrical response to changing exhaust composition, the control unit cannot accurately correct the fuel mixture. A sluggish sensor is either contaminated or beyond its intended service life and must be replaced. Additionally, check vehicle manufacturers' service recommendations and suggest replacement of the oxygen sensor at specified intervals.

Oxygen Sensor Installation Tools

In cases where installation position is difficult to access, Bosch recommends using the following tools: OTC 7189 Oxygen Sensor Wrench OR Snap-On 56150 Oxygen Sensor Wrench (Crowfoot type).


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Good luck
M J Gibbens

Great info there Mike, cheers! :o)
Rob Bell

Hi Rob, been to busy lately thrashing around the hills to give this thread any more thought, but lots of interesting posibilities.
For example, a good DVM has on it "Min-Max" recording, might be worth connecting across the sensor and checking the ouput. Could even fix it under the bonnet with a couple of cable ties and road test it.
M J Gibbens

>>Could even fix it under the bonnet with a couple of cable ties and road test it.<<

Sounds like a good idea Mike :o)
Rob Bell

This thread was discussed between 10/07/2003 and 21/07/2003

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