S70 2.4 2000r - ECM 3502 (first cylinder), ECM 3503 (first cyl) ECM2120 ECM 9150

lukask · Post by **lukask** » 03 Apr 2026, 00:45

Hi. I've been looking for a solution to my problem on various forums, so I'll give it a try here too.

I have a very old Volvo S70 and I'm having the following issues. ECM 3502 (first cylinder), ECM 3503 (first cyl), ECM2120 (first oxygen sensor) ECM 9150 (throttle position sensor). Is there any chance that all of these issues are caused by a faulty throttle valve, or are they separate problems? The ETS light is on, but there's no “Check Engine” light. The car shakes while driving and has very little power

I don't know if this has anything to do with my problems, but right after the problems started (when I started the car the next day), there was a terrible popping sound under the hood, but now it's stopped and there's no more popping. Also before the problems started, the car had been sitting unused in the garage for two months (+3 months at the mechanic's), and it turned out that a lot of coolant had leaked out. There was a big puddle under the car and the expansion tank was empty, but I topped it off and now it seems like nothing is leaking.

I've already replaced the spark plugs with new ones and swapped the ignition coils, but the ECM 3502 and 3503 is still showing an error on the first cylinder

I would really appreciate any help
Lukasz

Vova585 · Post by **Vova585** » 03 Apr 2026, 08:11

Hello. Our ability to help may depends on your skill level, budget, health ability, time frame for when this car needs to be on the road. Any car with 20+ years will have lots of issues. Looks like this car also has a "story". Currently misfire, coolant leak, sitting at mechanic for 2 month..
Do you have a scanner that able to read OBD 2 codes? It is easier to follow those than volvo specific codes. If I would be on a strict budget I would: remove the coil from cylinder in question(#1 ). Disconnect the connector from 1st cylinder fuel injector. Either put the plug on the engine or ground it through jaw clamp of booster cables(be careful, wiring to the coil is brittle and all insulation may fall off vs wire break). Start the car and verify you have nice spark in this coil/spark plug. If that is present, turn off the car and find yourself a small test light and 2 needles or better t-pins vs needle probes(luxury). If you dont have small test light, see if you have a small bulb from corner light abd attach 2 wires to it. Connect wires to needles(electrical tape vs soldering depending on what you have), verify it work by touching baterry posts(or any positive cable and nearby ground). After that carefully frontprobe woth needles the injector connector for cyl 1. Start the car- intermittent blinking=great, you have control over injector. No blinking- recheck your test equipment. Repeat test on other cylinder to verify it works. If other works and #1 not blinking =broken wire between computer and injector(not sure of it is ground controlled with constant 12v+ on the other leg. Most likely scenario). If you have spark and blinking of injector connector..remove injector rail with injectors. Connect external power supply to injector 1(ideally on positive wire in series install 5 amp fuse) and via t-pins try to "click" injector. Good sound of clicking- swap 1st and 2nd injector with 4 and 5. (In ideal scenario one would be able to see if Injectors are working good via testing its spray cone and how many ml of fluid it can put through on a period of time. Compare to the other injectors and determine if great discrepancy is present. Some local shops might offer injector testing with ultrasonic cleaning and provide you with information about before and after. Compare to the factory spec and recommend further steps if replacement is needed vs all good). Reassemble the car. Put some small ammount of grease on injector o-rings to make them instal better...but 20+ years rubber might be hard and brittle. Luckily those are widely available on aftermarket and are cleap in case needed(likely will be needed). Reassemble the car. Erase all codes. Prime the fuel system several times(ignition on/off) and attempt to start(after injector removal you will have some air in the system so it will likely start hard). Give it couple gas pedal down to stabilize the system via getting air out. Check the work of the engine. Any codes? Vs misfire? If misfire moved to 4, 5=injectors fault. Misfire present in #1 still= most likely compression issue. Here the coolant leak comes in mind and question about any chance of overheating.... remove coils and spark plugs, disconnect injector connectors(you dont want to flood the engine with fuel). And obtain compression tester. Check compression in all cylinders and compare. Weak in 1-there is a mechanical issue within the motor.
Good luck.

Post by **scot850** » 03 Apr 2026, 08:40

For checking injectors there is a simple cheap tool called a NOID test light. It plugs into the injector cable and shows if the injector signal is working as it should pulsing. Vova's alternate is great option for usually easy to access parts to make this tool equivalent.

Check if the repair shop has access to noid lights.

A cylinder compression check would be a good idea to check if you have a burnt valve.

It would be a suggestion to get the cylinder checked with a boroscope camera to check for cylinder #1 condition. If the piston is nice and clean you may have coolant leaking into that cylinder (head gasket leak). Ideally if the camera is good it may have the reverse looking capability so you can check the condition of the valves. With age it is not uncommon for a valve to burn causing a poor running condition.

Good Luck!

Neil.

lukask · Post by **lukask** » 04 Apr 2026, 04:13

Hello guys . Thank you very much for your time and advices. I managed to get to the injectors and remove the one in the first cylinder. To my eye, it looks fine (all five look identical). The seals are in perfect condition and the nozzles are clean. I did a trick with a battery and two cables. There was a click in the injectors and a spark appeared. If the problem isn't with the fuel injection, what could it be? If coolant has gotten into the engine, will changing the oil help? (I don't have any signs of a blown head gasket, so I doubt it). The water pump is dry (This concerns a coolant leak)

It also shows what my old spark plug used to look like. I have no idea if that’s what a healthy spark plug looks like.

lukask · Post by **lukask** » 04 Apr 2026, 04:22

hey guys small update! The VIDA drained my battery, so I had to recharge it before I started the engine. The ETS light is gone, and the car seems to be running normally (I don't feel any vibrations, and nothing is making strange noises under the hood).Was this related to the fuel injection system (I cleaned it a bit), or is the throttle actually failing, and is the misfire in cylinder 1 related to the throttle ?

lukask · Post by **lukask** » 04 Apr 2026, 05:40

My joy was short-lived. The ignition is still misfiring because the car shakes while driving, just like before. Maybe it works just a little better, or maybe it's just my imagination. I've driven 10 km and the ETS light is still off.

Vova585 · Post by **Vova585** » 04 Apr 2026, 08:16

Sorry, but more work is needed. You just checked the injector is clicking without actually testing it. So not much can be said at this time.
If you have money to dedicate to tools that you might be using once or twice, I would recommend to do compression test in all 5 cylinders as recommended above(compressometer). If your cylinder 1 is low in compression....Honestly I think it will be cheaper to fund someone who is mobile mechanic with good scope equipment. In usa it is commonly known as Pico scope(basically 2-4 channel oscilloscope) in post ussr area it is "oscillograph of postolovsky ". The good diagnostitian with this equipment would be able to perform relative compression test via checking amp draw on started with 2nd channel sync on cylinder 1. Wave form will tell you if there is compression difference. If all looks good- then search continues in air intake, injector system. If relative compression is different- then you problem is mechanical to the engine and more investigation is needed(compression test, cylinder leak test). I am happy to help further, but without some concrete testing, I have nothing to suggest. Also the issue with coolant...If you have time and money and like Lego, this is the car to learn, but if time and money are tight...better find someone reputable and ask him to diagnose engine and figure out if it is financially responsible to invest further funds into this car.

lukask · Post by **lukask** » 06 Apr 2026, 11:50

I think I'll have to take the car to a professional mechanic, but I have the latest update. I swapped the fuel injectors again, moving the injector from cylinder 1 to cylinder 3 and from cylinder 3 to cylinder 1. All the old errors disappeared from the VIDA! The engine is running better but still far from perfection. Twice I even had a little trouble getting the car to start, which has never happened before. ECM 3502 , ECM 3503 , ECM2120 (first oxygen sensor) ECM 9150 (throttle position sensor) have disappeared. What could that mean? Could that loud knocking sound coming from the engine be a faulty fuel injector?

Right now, I don't see any coolant leaks (I sealed up the one pipe that I think was leaking coolant) since 2 weeks.

Vova585 · Post by **Vova585** » 06 Apr 2026, 12:37

It is hard to tell anything about noise without hearing it. You can record video and post it on YouTube and share here. That loud noise can be a misfire vs mechanical issue that causes misfire vs something just not bolted properly...I applaud your decision to take it to a mechanic(Hopefully a good one) and ask him for diagnostic help. In the meantime time, if car has no problems and drives good, you can try to add injector cleaner to the fuel and drive around(will not harm).

Post by **abscate** » 07 Apr 2026, 04:30

I would guess moving the injector reseated a gasket or oring on the injector and stopped your misfire. I lubricate them with a small shot of silicone to help them pop in easily