HELP! Lexus rx300 check engine lights on

currently have a lexus with the check engine light on. the codes are reading that all 6 coils need to be replaced or all are misfiring, however i checked the coils by unplugging them while the car is running and seem to all be working just fine. I also had the computer repaired by these guys in south carolina for $250. after reinstalling the computer the check engine light turned off for about a day and a half then started sputtering randomly again… does this mean i have a bad computer or something else as far as a sensor thats setting off the coils 1-6?

just spoke with a Toyota dealer as well and he thinks its most likely a bad ECU (computer) or even possibly the wires may need to be replaced… anyone with any suggestions it would be much appreciated!

I suspect you do indeed have ignition misfires

Can you please post the fault codes . . . I highly doubt your codes specifically say "All 6 coils are bad and need to be replaced

I suspect this is what you have

P0301 #1 misfire
P0302 #2 misfire
P0303 #3 misfire
P0304 #4 misfire
P0305 #5 misfire
P0306 #6 misfire

You didn’t post your model year, but I’ll bet you lunch you have the 1MZ-FE V6 . . . correct?

I’m inclined to believe . . . without even seeing your car . . . that you may indeed have a secondary ignition problem. As a matter of fact, Toyota did have some issues with the coils on your engine. So I wouldn’t say it’s impossible they are all bad, but unlikely

That said, how many miles?

Has it ever had an ignition tuneup?

If you’ve got a lot of miles and are still on the original plugs, that may be your problem right there

Can you please explain why you had your computer repaired for $250. If it was to address the misfires, I believe you were in error. That is akin to shooting the messenger, rather than addressing the root cause. Who told you the computer needs to be repaired, anyways?

By the way, is the check engine light flashing?

If so, you have severe misfires which may damage the catalytic converter(s)

Did you pay for any diagnosis, or are you just making educated guesses?

Have you even used a factory-level scanner, or are you just unplugging components at idle, and seeing what happens?


it is a 1999 lexus rx300 with 153,000 miles on it.

Yes those are the codes that it is putting out, which you had mentioned before…

i am not sure about he ignition tune up because i just purchased the car about a month ago. the check engine light does blink sometimes but not all the time, it is very random… the reason i had the computer repaired is because i spoke with multiple mechanics who had mentioned that the computer could be the reason for the rough shifting, riding, etc…

The coils were actually replaced by the previous owner so they may have a total of 100 miles on them if even that.

we did use a factory scanner or at lease my mechanic did.

the odd thing is when i had the computer repaired and re-installed it, the car ran perfect for about two days and the check engine light infact turned off! then a couple days later it turned on and started sputtering again, the people who repaired it refunded me my money and said it is possible that the computer is beyond repairable and may need a new one.

Do you know what in the computer they repaired?

IMHO there’s way too much unknown and questionable history here to be trying to troubleshoot it over the internet.

You purchased a car with an illuminated Check Engine Light that sometimes blinks (meaning you’re damaging the engine) and that’s been exposed to backyard repairs that may or many not have contributed to or exacerbated whatever the original problem was. Speaking with mechanics and getting a “could be” from them is not the same as having a qualified mechanic perform a diagnosis.

If it were mine, I’d perform a compression test. If the compression were good, I’d bring it to a reputable shop and have it diagnosed. You can, if you wish, tell them to just diagnose it as best they can without disassembling it, or to call you before doing any serious disassembly for approval. It’s 18 years old with questionable history. If they say the issues are serious, be prepared to write it off as the cost of an education. The correct answer to the course’s final exam would be “never buy a vehicle with unknown issues”.

If the compression test fails, I’d be prepared to write the cost off and call the junkman to come get it. Or sell it as a “parts car”.

Sorry, but I think this is beyond internet diagnosis and possibly backyard repair. I don’t mean to sound unsympathetic, but I think you’ll need to face reality at some point.


they repaired the ECU computer. When i received the repaired ECU back and installed it, the vehicle ran perfect… no check engine light on and everything working just the way it should.

i honestly do not know… but they said they cleaned and reprogrammed the computer to my specific model and VIN #

Sorry, but I stand by my post. Get the compression checked and post back.

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I agree with the dealership, sounds like there’s still a problem with the ECM. The shop that tried to repair the ECM but when the fix didn’t work they refunded your money, that’s a good shop imo. They put their customer’s first, stand behind their work. Remember that for deciding who you’ll use to replace your starter motor one day in the future.

As far as what’s going on, this may prove difficult to diagnose. Best bet it is to use a well recommended shop with many years of Lexus experience and having the Lexus factory scan tool. Suggest to start with an inde shop if a good one is nearby. You may have to resort to a dealership shop later.

Assuming what the ECM repair shop did actually was the cause of the temporary improvement, there’s several possibilities

  • Crack in a solder joint on the ECM. Temperature changes can cause the crack to open and close & make the symptoms intermittent.
  • Bad/burned/corroded connection on a connector pin that plugs into the ECM. Again thermal changes can make the symptom intermittent. High current pins would be the first suspect, for example the battery power and ground.
  • Some component on the ECM is failing. ECM replacement would be the only way out in this situation usually.

There’s some other possibilities for misfires

  • Faulty crank position sensor, usually this would show up first when the engine is warm
  • Faulty ignition module
  • Faulty coils (seems unlikely however that all would fail, so if it is the coils, more likely a connection or installment problem)
  • Spark plugs
  • Serious engine problems, a compression test would be used for that. The symptoms aren’t consistent with that tho. It could be a problem with the variable valve timing function, if your car has that.

Suggest to ask your shop to check their service data for any recalls, customer interest bulletins, and technical service bulletins that might apply for this make/model/year.

I don’t share your opinion

My gut feeling tells me the control module was never the problem

I suspect a complete and proper diagnosis was never performed

I believe many control modules get needlessly replaced, because the guy’s diagnostic skills were not up to the task