O2 Sensor

Today I pulled the ECU from its cubby hole and bunged a meter probe on all the relevant wires to do some sensor tests. I got text-book readings for engine temp. sensor, air flow meter, air flow temp. sensor. I did the position sensor test and everything was fine with the TPS back in "normal" position. I checked the PCV valve and it seemed fine. I pulled the connector off the ISC and got a click (just one) and the revs lifted. I listened for air leaks and sprayed WD40 around - no smoke so I really, really don't think I have a leak.

Notice I missed out a sensor? The O2 sensor gave funny readings. The value was within the quoted range of 0V to 1V BUT the value swung around at both idle and revs.

So does this mean the O2 sensor is duff? Or could this be a symptom of something else?

The only things on the list left to check are plugs (no stumble at revs though), injectors and compression. And as twiddling the TPS to never report idle to the ECU makes everything work fine I suspect a sensor rather than anything drastic. (1/00)

No, it's supposed to do that. In fact the ECU reports a fault if the values don't change.

The O2 sensor reports oxygen concentration in the exhaust on a second by second basis, and the ECU constantly adjusts the mixture to aim for its targets. It's never exactly right, so it's constantly changing.

I'll have to have a pore over the descriptions of troubleshooting - like I've said before, there are pages - more on curing faulty idle than anything else in the manual, but it's got me beat for now. (1/00)

The O2 sensor should oscillate back and forth around .75V. It will stop doing this and go rich on WOT. My car had a chronic oscillating idle last spring and summer no searching for leaks or checking/adjusting the TPS would cure it. I had my suspicions on the O2 as it got much worse after my CFDF install early last summer (oscillated between 400 and 1200 rpm on a frequency of about 1 sec, yuck!), I had sprayed the sensor mount with WD40 to try and get it out of the exhaust (which it didn't). Anyway installed a new 4 wire O2 sensor with the turbo kit and the idle was a smooth as smooth could be afterwards. Might be worth doing the ground mods on the DA web site (think it's on FM owners page) as well. (1/00)

Forgot to say, rich is above .75 to .76 volts (stoich) and lean below, output should range from 0-1V. Being in that range doesn't mean the sensor is good though. I'd take the car for a drive and whilst keeping an eye on the voltage reading put your foot down and go to WOT. The sensor should initially (for a short time) go lean and then to rich and stay there whilst you are accelerating at WOT. (1/00)

No, it's supposed to do that. In fact the ECU reports a fault if the values don't change.

OK, thanks. I'll have to connect a more permanent wire and see how it behaves when driving. The oscillation was from high (nearly 1V to almost zero) so getting any kind of centrepoint reading wasn't possible. As all the readings from the other sensors were OK it's difficult to know where to go next. I guess checking spark plugs, compression and injectors is worth doing, but the excellent idle with the TPS adjusted suggests the ECU is confused by something. And what else is there but the O2 sensor? (1/00)

This does sound very much like my problem last year. I adjusted my TPS as well, it was reporting closed throttle to the ECU at idle, but I managed to get it to report opening earlier on throttle opening i.e. less throttle opening.

The ECU goes onto closed loop when at idle, relying on the O2 sensor to adjust fuelling, then onto an open loop under acceleration. At steady throttle openings it's again on closed loop, but ISC wouldn't adjust idle speed? This maybe why you get a steadier idle with the TPS not reading closed throttle (anyone got any ideas on this line of thinking?). I didn't try to adjust the TPS to the wrong position, wonder if I'd have had the same results.

The O2 sensors are about 50 to 60, worth replacing if you run out of ideas, I reckon it may be the culprit. If you do go for a new one go for the 4 wire in preference to the single wire one found on pre '94 cars, includes a heated element (comes on-line earlier after cold start up and doesn't drop below 600 deg f during idle) and has a reference ground to give a more steady/reliable signal. (1/00)

get the 4 wire as fitted to 1.8 .. but you should not really need it .. most 1.6 engines live without it (extra wires are for heater element .. but it still goes cool at idle ... even with 4 wire).. hey why not pull O2 sensor off .. this will cause it to go to default settings .. and could isolate it. It does oscillate.(barely readable in O2 v volts). .. I guess you have pulled the plug off the TPS ..(cannot remember now..) (1/00)

hey why not pull O2 sensor off .. this will cause it to go to default settings .. and could isolate it.

Can you get to it from underneath? It's beneath my messy intake bodge on the supercharger. ..

I guess you have pulled the plug off the TPS ..(cannot remember now..)

No, I turned the TPS so the ECU sees WOT but not IDLE. (1/00)

No - you shouldn't pull the pigtail off the sensor - it's VV difficult to get back - it's actually crimped into the sensor inside a rubber sleeve Disconnect the plug (near the back of the inlet side camshaft) (1/00)

Disconnect the plug (near the back of the inlet side camshaft) That sounds quite easy. I'll check Rod Grainger's Enthusiast's Manual (RGEM in future? - it'll make things *much* easier!) and see roughly where the plug is. (1/00)

To get to O2 sensor .. to disable just unplug .. on 1.8 it is the plug above the timing pickup on the back of the exhaust cam .. not sure on a 1.6

To get to the actual sensor go through the passenger's side wheel arch behind the wheel .. may have to take wheel off .. the trouble is as it runs hot it tends to corrode into place and has coarse threads that lock/strip easily .. so get some Plus Gas into it for 24 hours.. even if you are running the car .. it cannot harm ..

Oh and to fit the 4 wire you need add the two extra wires.. earth and 12V.

Car would run without TPS .. just wondering if it is giving a bogus idle reading at a second point just after idle... hence pulling plug. (1/00)

As [above] says sensor plug is above Cam angle sensor, back of drivers side cam though, probably due to change in ignition coil positioning between 1.8 and 1.6.

I'd be cautious on the Plus Gas though, I'm pretty sure the packaging on my new sensor said to avoid touching and getting oil/grease near it (all long thrown away though). Think the Haynes manual (horrible thing) says same. They are a total PITA to remove, mine wouldn't come out for love nor money (and that was with the down pipe out), left in the old exhaust down pipe in the end, just put a new 4-wire in new BEGI down pipe.

May help to warm the engine before removing, not sure whether they have different coefficients of expansion though. (1/00)

Great, but I'm afraid it is the US part number and these don't match the Euro part numbers. Don't ask me why. Isn't the us part number something like 13275 so? Tried that at Bosch a while ago ... they had never heard of it. :( (2/00)

Your right, that's the only number on the box. Sorry (2/00)

Does anyone know of an aftermarket oxygensensor that is suitable for the 5? Our 89 Eunos one is up the swanney, Showing fault code 17 and sensor reading is static at approx 1v (8/00)

All the factors we spoke to said that the MX-5 was not listed, and most were unwilling to supply a universal one. However after a bit of playing around we found the following: Lucas SEB121 Sensor, 24.98 from our local Lucas Automotive stockist. Fits 'spot on' and gives the correct output. (8/00)

Don't Bosch do one? (8/00)

Can someone explain exactly what the O2 sensor does (I sort of know but haven't found a definitive answer) and whether I'll see any benefit fitting a 4 wire one with heater on a low boost turbo application? Can this effect the delay the FPR has causing a brief lean spell on the GReddy kits? Are there distributors for this kind of stuff in the UK or is it another transatlantic order? (2/01)

It measures Oxygen content in the exhaust, and from this decides if your mixture is too weak or lean - feeds back to ECU (closed loop) which adjusts fuel (injector timing) to attain ideal 14.7:1 by mass air:fuel ratio. It's really for emissions and cat protection, but a non-working one can cause problems. Doesn't work anyway for about 2 mins, until it's hot (open loop, using ECU stored data, during that time). And they go sluggish - don't change readings fast enough. If it's working it's OK. If it's not, you get codes 15 and/or 17 in ECU diagnostics. Replaced mine with the Lucas one (from local Lucas agent) ~30 odd - type as posted by Val - can look it up if needed. Don't think you'll get any real benefit from a 4-wire one, even with a turbo - they just heat up quicker so less pollution for a minute or so. (2/01)

O2 sensor gives a voltage reading which is related to O2 content. Gives an indications of mixture. 4 wire is better because it uses 1 wire as a reference ground, 2 wires for an O2 sensor heater and the other is the O2 signal (present on single wire sensors). Benefits are, better accuracy due to the reference ground and quicker warm up due to the heater (it also stops the O2 sensor falling below its operating temperature at idle etc). For a Link ECU which tunes fuel mixture during closed loop by using O2 sensor feed back the above is very important. For standard ECU it has similar closed loop benefits, although you'd be hard pressed to notice any difference in real life. Lack of auto-tuning also makes it less important.

For your turbo install I would buy a new one, the old ones can be an absolute *PITA* to remove if they've been there along time and spraying them with WD40 etc to remove ruins the sensor (as I found out, should have read the manual *before* spraying liberally;-). It really is worth upgrading IMO, not just for improved accuracy and the sensor being new, but just for ease of install. FM do 4 wire Bosch sensors for $65 which is very cheap and you'll get the right one, but as it's Bosch you can also get it here as well (although I know not where). (2/01)

As ever you are right. Only thing I would add, although this doesn't apply to Gordon (at present;-) is that with a programmable, auto-tuning ECU such as the Link, TEC II etc the 4-wire O2 sensor brings worthwhile improvements in accuracy due to the reference ground and more stable temperature. This is only really important for auto-tuning etc though. One thing that the heated sensor can't help with is the O2 sensors tendency to overheat on high boost, give lower voltage readings and show leaner than actual O2 values. Can cause excessive richness on auto-tuning, especially in summer months when you would expect the mixture to be slightly rich. I think in this case an upgrade maybe worthwhile as the sensors can be a pain to remove and he *may* (power is addictive and all that) decide to upgrade to a Link or similar at a later date. BTW when my sensor started giving bad readings after spraying with WD40 (car wouldn't idle etc) I got no fault codes. Do the fault codes only appear if it dies rather than just starts giving bad readings? (2/01)

I can appreciate the advantages of a four wire sensor if you've got a Link. With the standard ECU though the built in mixture data is probably nearly all you need, and the O2 sensor provides a bit of extra fine tuning to protect the cat and reduce emissions. Actually, performance wise you'd probably get a bit more power if you could run with a *slightly* rich mixture, compared with the 14.7:1, so the O2 sensor might actually reduce performance. It would be interesting to see what would happen if you disconnect the O2 sensor and do some performance testing. Might fail an MoT emissions test or wreck the cat, though! The ECU knows that the sensor is not providing data, and can compensate. It shows code 15 if the sensor continues to provide less than 0.55v, 120 sec after engine revs exceed 1500 rpm, and code 17 if the sensor output is unchanged for 80 sec after engine reaches 1500 rpm. Both codes switch the ECU to open loop operation, ie relying on stored data - which is actually pretty good. I'd think it would err towards running rich - safer for the engine than running lean. But if the sensor is providing a varying voltage, the ECU will act on it, so if it's crap data, the results will be pretty unpredictable. The sensor works by having two platinum electrodes, one in contact with the exhaust gas and one in contact with the air. They are separated by a layer of solid electrolyte - zirconium dioxide. The voltage produced is dependant on the oxygen concentration in the exhaust gas relative to the outside air. (~0.9V at lower exhaust gas O2 concentrations = rich mixture, ~0.05V at higher exhaust gas O2 concentrations = lean mixture with a fairly sharp transition at the stoichiometric point where you've got 14.7:1 mixture) I'd guess that spraying with WD40 would reduce the apparent reference O2 concentration so that the sensor would think you had more O2 in the exhaust than you have/are running lean/will make the ECU increase fuel/go too rich. That's my theory anyway. And WD40 leaves a coating of silicone so it's probably permanent! I've not had a problem removing/replacing my O2 sensor, but a) the mount is stainless steel and b) I use a smear of copper assembly compound on the threads. (2/01)

Whenever you need real power, you will be at or close to WOT, which means that the stock ECU goes to open loop and thus the signal of the O2 sensor is ignored. (2/01)

When you remove th O2 sensor the ECU will use a default map to run the engine. From memory I think most will tend to run slightly on the lean side so as not to damage the cat and so performance would be mildly hindered. The risk of damage to the engine from lean running I would have thought was more important than the risk to the cat from rich running, but the manufacturers seem to disagree( I guess a dodgy CAT shows up earlier than engine damage). Most cars (unlike the UK '5) have a warning light on the dash for ECU faults so I guess the manufacturers reckon you will tkae the car straight to a garage. (2/01)

Ah - yes indeed. I'd forgotten that - so really it comes down to some driveability issues at intermediate throttle settings, such as possible stalling on deceleration, and cat life/emissions. (The trouble shooting procedures for driveability problems involve disconnecting the O2 sensor to see if it improves matters. So except for the cat, it's probably more of a hindrance than a help anyway) (2/01)

The trick to clean the O2 sensors given by some in States is to heat with a gas torch to burn off the impurities.. And I think default ECU map is rich ..safer even for cat.. (2/01)

I think a link would be a long way off for me - the upgrade path from what I am fitting will involve an intercooler and uprated fuel pump first, unless I get some more money and a used one comes up for sale it's a couple of years away. I'll need a clutch and tyres this year as well - DOH! After seeing how "welded on" my brothers exhaust nuts were, I may get a cheap lucas 2 wire one to ease installation but will try to re-use the original if it comes off without a problem. Is WD40 the best thing to use on the manifold nuts as it's all coming out in a couple of weeks - everyone on miataforum uses 'liquid wrench' but we don't get it here. (2/01)

After seeing how "welded on" my brothers exhaust nuts were, I may get a cheap lucas 2 wire one to ease installation but will try to re-use the original if it comes off without a problem.

O2 sensors are either 4 wire (signal, signal earth, heater power and heater earth) or just 1 wire (signal) - earthed through the body. You use the original plug/socket - you have to solder the new (longish) wire to the old one (soldered joint by far the best)

Is WD40 the best thing to use on the manifold nuts as it's all coming out in a couple of weeks - everyone on miataforum uses 'liquid wrench' but we don't get it here.

The best penetrating/releasing oil is Plus Gas - dark blue spray can from Halfords (IMHO) But you shouldn't need anything on the manifold nuts - at least the manifold to cylinder head ones - they shouldn't be difficult to remove. Use copper assembly compound when assembling. (I refuse to use the term "Copper grease" although I see that one particular brand calls itself that.) (2/01)

Further to the discussuions on O2 sensors, I don't know if anyone is aware of the newer type of sensor used on some new vehicles which operates at higher voltages and rather than having a switching characteristic like the old ones give a voltage proportional to mixture. These sensors give a much finer mixture control and would probably much better suited to turbo applications. Whilst these are of no use with the stock ECU the aftermarket ones may have facility for using them. (2/01)

Wider band O2 sensors .. still have drawbacks .. I believe they are about $200 and really set up for sensing lean burn not power burns .. I was not aware they were incompatible (you say higher voltages). The real wide band sensors ($1200) used at rolling roads .. the Link has been cross calibrated against these to try and make the cheapo standard O2 sensor more 'wide band'.. there is more about the new types but not prepared to rake for it just now! (2/01)

Not sure what the voltage is on the newer O2 sensors but it is much higher than the max of 1V from the older types (which also have a switch-like characteristic unlike the new linear ones), I need to find out soon though as I am working on a new tester for O2 sensors at work, the old one just does the old types. The Vectra and Omega both use this new sensor I am told. (2/01)