Hey all!

I´m having a bit of trouble with my CTS. My 240 (with a transplanted B230FB from a 940) didn´t pass the emissions part of the inspection, too high CO. At the same time, I´ve been getting code 1-2-3 (coolant temperature sensor) and 2-3-1 (rich/lean), so naturally, I suspected my CTS. I took it out and tested it in some water, by probing the two pins with a multimeter - it read around 7-8k ohms @ room temperature and 600 ohms @ 80 degrees celcius - which is about double of what it should be. I ordered a CTS from a local parts store, which tested badly too, sent that one back. Then I ordered this one : https://www.autodoc.co.uk/stark/15796761 , which tested perfectly within spec in water, but car wouldn´t start with it in. Now I´ve tried this one : https://www.autodoc.co.uk/facet/2180390 , and it´s out of spec in water as well - around 600 ohms @ 80 degrees and 6k @ 20 degrees.
So my question is - am I missing something, testing them wrong, or maybe they´re supposed to be like that specifically in the B230FB? Should I perform the test with the CTS in the car instead, probing at the ECU?
It´s just a bit hard to believe, that 2/3 sensors I´ve ordered are out of spec. Are parts just this bad sometimes? Should I just keep ordering sensors until I find one that reads within spec and starts the car too?
Car runs ok with the old sensor in btw, just need to get the CO down for the inspection, and it makes sense to me that CTS it at fault - telling ECU that engine is colder than it is, making it run rich, which would raise the CO.

 

Note the two sensors you linked - one has blue plastic, the other black. They are not interchangeable - I don't remember what year had each (here in the US) but I do know you have to have the correct one. Is your old sensor from the 240 black or blue? Did you swap the entire fuel system from your old 240 engine to the 940 engine? I guess that would only be the injectors, temp sensor, fuel pressure regulator.(don't really know the difference in a 230 fb, we only had the 230f here in the US) What year is your 240 and what year was the 940 the engine came from?

 

Thanks hoonk, that´s actually great news, because that explains why the blue one (which tested fine) didn´t work in my car. The one in it now (the old sensor) is black
I didn´t do the swap myself - I bought it like this, but I just assumed that they swapped everything over - originally it would´ve had the B230FD. My 240 is a 1993 and the engine I don´t know, I haven´t yet figured out how to date the engine itself

 

The b230 engine from a 240 or a 940 is the same basic block head, pistons, etc. Difference would be a front or cam mounted distributor, the fuel injection components depending on year, and where the alt, ps and ac are mounted. The b230fb is not listed in the parts catalog for any 240 in any market - so hopefully whoever did the swap put all the original stuff in the used engine.

If your CO is too high - in closed loop the O2 sensor should pick that up and try to lean the mixture - You can measure the output from the O2 sensor with a voltmeter to see if it is picking up the rich mixture - (I'm not suggesting buying and replacing any parts without a clear diagnosis that the part IS bad). Just suggesting clues - What voltages do you get from your O2 sensor?

You can artificially lean or enrichen the mixture easily - if you have an exhaust analyzer/CO tester to monitor what you are doing. If you unplug the O2 sensor (yes that will set a code) and ground the green lead - that will tell the ECU it's lean (0 volts) and the ECU will enrichen the mixture (higher CO). Or Touch the green O2 lead with one hand and touch the positive battery terminal with the other hand (to send/simulate a small voltage from the sensor) and watch the mixture go lean.

 

yup, distributor is mounted in the front! I will measure the o2 sensor and get back to you, could I measure it from the ECU or do I have to get under the car, unplug the sensor and measure? I imagine it´s the voltage between the signal and ground, so maybe pin 24 and 5 on the ECU? https://www.davebarton.com/pdf/Harne...ctions2021.pdf

 

Certainly - but it's much simpler from the O2 wire -

Unplug the sensor (green wire) measure voltage between the sensor wire (that was plugged into the green wire) and ground. O2 voltages are usually between .2 and 1.2 volts. .2 or below is lean.

To fool the ECU rich or lean - touch the green wire to ground (to simulate 0 volts or very lean), or as described add voltage through your body from the positive terminal to the green wire in your other hand - to simulate rich.

 

Hi again, I finally got around to testing the O2 sensor. I tested it at the ECU, on the backside of the connector while it was plugged in. I probed terminal 24 and got a ground from a screw of the ECU. I let the car warm up for 15 mins for it to go into closed loop, and it was fluctuating between 50mv to about 700mv. Is that kind of weird? Everything I´ve read says that it should be 100mv-900mv.

I got a reading on the coolant temp sensor in the car, it was 740ohm with coolant warmed up to temp at about 88 degrees.

Also, I ordered and tested 8 different CTS (I will return them of course), almost all of them measured around 600-700ohm at 88 degrees, so I think that´s the standard for this engine. Which is weird to me, because I can´t find any information on it whatsoever, and all texts I can find including several manuals and online, suggest that it should be 300ohm at temp.

 

O2 sensor voltage is ok (close enough, your voltmeter may not be sampling fast enough to get closer readings.)

Years ago - had a 240 with a 123 temp code. Tried everything we could think of. New sensor, wire, etc.

Left it alone, didn't want to charge the customer any more, car ran fine, and customer accepted the check engine light on.

Couple of years later - at the next major service we decide plug wires were in bad condition and replaced them. Cleared all codes. AND - 123 never came back.

 

Thanks hoonk, I´ll inspect my plug wires. I haven´t been getting a 123 lately, I´m not too sure if the fault is with the CTS anymore.
Would the o2 sensor read like that if it was rich? Car sure smells rich and of course high CO and recurring code 231. I got a very good deal on an original MAF recently, so I will try that as well. I was just hoping to be able to see the rich condition on the o2 sensor, so I could measure whether it disappears after changing MAF

 

so I´ve had a bit of progress - I´ve realized that the CTS for this car has two thermistors, one for each pin. It´s grounded through the body of the sensor. So to measure the resistance of the sensor out of the car, you have to measure from a pin to the body of the sensor, if you measure from one pin to the other, you´re getting the resistance of both thermistors, so essentially double. So that clears up my confusion - the sensors I bought are all right, I just measured both of the thermistors between the pins, therefore getting double the resistance.

The thing is though, while measuring the old CTS that´s in the car, between pin 13 of the ecu and ground, I was also getting about 600 ohms at running temp. So I´m pretty sure that the old sensor is shorted between the two thermistors, or something´s wrong with the wiring. When I get home I´ll take it out again and test, if the sensor has the same resistance between the pins as one pin to the body, the sensor at fault, otherwise I´ll start testing the wiring

 

TLDR: Question - Are you meant to measure the resistance at pin 13 to ground (pin 5) on the ECU with the ignition off only?

I took out the old CTS. It measured just fine, reading what it´s supposed to between a pin and the body of the sensor. I put in a new one (tested good) for good measure anyway. With the CTS unplugged, I inserted a 300ohm resistor into each of the CTS plug/socket pins, and grounded them to the body of the sensor. Then I measured resistance at pin 13 ECU and pin 2 EZK, expecting of course around 300ohms, and that´s what I saw. So continuity between plug and ECU/EZK should be good. I also tested for continuity between pin 13 ECU and pin 2 EZK, there was none.
Then I plugged the sensor in again. What´s happening now, is that I´m getting the correct resistance with ignition off, I got around 4.5K ohms to both EZK pin 2 and ECU pin 13 - which at about 3 degrees celsius seems good. But when I turn the ignition on, or start the car and measure - it gets really weird. It gave me 100k+ resistance (between 13 and 5 ECU), meter jumped around, 15k, then nothing at all. So my question is now, as written above - are you supposed to measure it with ignition on? Or is it meant only to be read with it off? Oh, and I have a spare ECU, so I swapped them, and it was the same.
I´m kind of thinking I might have a ground problem somewhere, but only if the reading is supposed to be the same regardless of ignition on/off.

EDIT: Was poking around, looking at grounds - seems like a bolt for the fuel rail is broken off, isn´t it where there´s usually a ground for the ECU? I suppose it´s been moved to the bolt on top of the manifold, would this make any difference?

 

Measuring resistance of a component in-circuit while current is flowing though the component will give you weird readings. Only measure the resistance of the CTS with the ignition off.

 

gotcha, thanks! well, in that case, CTS should be good. going to keep digging.

 

Have you checked for exhaust leaks upstream of the O2 sensor? These types of leaks will cause rich running and high CO. I haven't mentioned it earlier as these types of leaks alone typically don't throw codes.

Use a piece of tubing as a stethescope around where the manifold bolts to the head, where the y-pipe bolts to the manifold and the y portion of the y-pipe. Take care not to burn yourself.

 

will do, thanks! I changed the exhaust from the cat down, but I guess this shouldn´t really affect O2 sensor

 

Listened for leaks and inspected the exhaust manifold, everything seems to be alright, and didn´t hear any obvious hissing.
Tried another MAF on, everything seemed about the same. The car seems to be running worse though. It´s harder to start, and also bogs down and even stalls going quickly from idle to full throttle. I can get up the RPMs, but only slowly, otherwise it dies. It does this when it´s cold, and gets better as it warms up. I´m thinking that I need to try to check the injectors for leaks - if anybody has any advice for this, it would be greatly appreciated. Other than that, I´m kind of out of ideas. Here´s a recap of what I´ve determined to be good on the car:

coolant temperature sensor is good, connections good - reads about 220ohms in the car at temp
MAF good, swapped in a known good one and the car ran the same
ECU good, swapped in a known good one and the car ran the same
checked fuel pressure regulator, no fuel past the diaphragm
O2 sensor is going from 50-700 mv in closed loop
cleaned grounds on intake manifold, put the one from the ECU on the fuel rail bolt
new air filter

 

Hey again, put an analog scope on my O2 sensor.. hot engine and at around 2000rpm


would you change this sensor? wave seems slow and a bit weird to me, staying in the middle for a long time and then jumping

 

Just wanted to update for anyone in the future troubleshooting, reading this thread - it was the O2 sensor, I put a new one in it, took the car to get emissions tested and the readings were ok. The old O2 sensor was still working, but it was too slow. I got some great help on turbobricks in this thread https://turbobricks.com/index.php?th...g-rich.372597/