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Old car, CEL (twice, once flashing), diagnostic codes, runs fine, what's wrong with it?

My first post. A story about a CEL - I can’t seem to tell it in just a few words.

My 1997 Mercury Villager minivan (163K miles) has some problem that has triggered the CEL twice - but not recently. I don’t know what is wrong with it or how to go about finding that out.

About 3 weeks ago I started it and noticed right away the idle was very rough. I drove it about 2 miles, shut it off and then an hour later turned it on and drove it 2 miles home - it was idling rough the whole time.

The next day when I started it the CEL came on and stayed on. I called a mechanic who I had never been to before but who was willing to look at my car that day. He hooked up the diagnostic tool and told me the problem was a misfire in cylinder 1. Based on when I had last done a tune-up (15k miles ago with spark plugs, wires, fuel filter) his guess was a bad fuel injector. I made an appointment to bring it in the next day so he could check it out.

When I started the car the next day the CEL was still on. I dropped the car at the shop and came back 2 hours later and the mechanic said that my car was running fine, the CEL did not come on when he started it and there was nothing he could see or hear wrong with it - he thought it might have just been something in the fuel (a piece of carbon?)

That was on a Tuesday. To complicate matters my car registration was due the coming Friday and required passing an air quality check. So after he said my car was fine I drove to the testing station and my car passed with no problem. During the next week I drove my car around town and on a short trip on the highway (100 miles RT) with no problems and no CEL.

Then on day 8 I was on my way to an appointment and just after I got on the freeway the CEL started flashing. I had no idea at the time what that meant but it was upsetting. I was on the freeway for less than 10 minutes and was so upset about the flashing I don’t remember if it was just a solid CEL when I got off or if it was still flashing. I drove for another 5 minutes, parked the car and went to the appointment. While I was waiting I looked on the internet to see what a flashing CEL means and it was pretty scary since most say it can be dangerous and do lots of damage. I called the mechanic and he said to bring it by.

After my appointment when I went and started up my car the CEL was off. I drove to the mechanic and he could not get his diagnostic tool to work for some reason but the CEL was still off. A different mechanic drove the car and again could not hear or see anything. One of the mechanics thinks it’s a fuel injector and the other was not sure. That was on June 22 - almost 2 weeks ago. I have driven the car around town during the past 2 weeks (but not on the freeway) and the CEL has not come on again since the day it was flashing and the car seems to run fine.

I have read way more about CELs than I ever wanted to know! The other day I bought some fuel additive but have not put it in because the directions say to add it to a near empty tank and I still have about 6-7 gals in my 20 gal tank.

Today I went to O’Reilly’s and had them check for “codes” - even though the CEL is not on I understand they can be stored. Here’s what showed up:
Knock Sensor 1 Circuit (Bank1)

Cylinder 1 misfire detected
(If there was a number with this one I must have missed it)

Not sure what to do next. A new fuel injector is $245 and I don’t know if that will fix the problem. Is the car safe to drive? Is it worth spending any money on? Do fuel additives help anything? Did the time I was on the freeway with the CEL flashing damage my engine? Is there any way to tell? Do the codes tell a mechanic what’s wrong with it?

I don’t know a lot about cars except that this minivan has run GREAT for 60K miles. I bought it used 7 years ago and have had no engine problems and very few repairs. A couple motor mounts, a vacuum hose and the usual maintenance; - the car has never overheated, the A/C works great, and it has been very reliable.

I am planning on buying a newer car soon but in the meantime I’m trying to decide if it is worth trying to get this one repaired especially if I don’t know what’s wrong with it.

Any suggestions, insights or advice much appreciated.

A lot of shops don’t like to deal with issues like this on engines this old because they often portend expensive works that can exceed the value of the vehicle.

The first code, the knock sensor, suggests that you’re experiencing unwanted detonation… but I’ve personally seen a knock sensor triggered by an intermittently banging rod bearing, in essence a worn out engine. The knock sensor only detects a pulse, a spike, typical of preignition, and cannot discern its origin. That’s the mechanic’s job.

The second code, the misfire detected, suggests an anomaly in the crankshaft pulses detected by the crank speed sensor typical of a unwanted or missing combustion.

The codes together suggest that you have unwanted combustion, called preignition, one type of which is pinging. And prolonged pinging can burn a hole in a piston.

For starters, someone is going to need to assess the engine’s ignition system performance, the compression, and the integrity of the valvetrain operation. The first can be assessed with an engine analyzer, the second with a compression test, and the third often with a vacuum gage. Poor valvetrain operation will cause the vacuum gage needle to wobble rather than provide a smooth reading.

I hope you can find a good shop willing to check your engine out. Clearly its condition needs to be assessed before any decisions can be made. It’ll probably cost you a few hours of shop time, but it’s a necessary first step.

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If the CEL light is off and everything seems to be working why not just move up the trade-in date if you are able to do so. Other wise as mountainbike says spend about 100 to 125 dollars and find out what you really need.

1997 isn’t an old car. Both of mine are older than that and still on the road and running fine. So as long as you’ve kept the routine maintenance up to date, whatever problem you have is probably fixable. It might take some time to diagnose is all.

If I had this problem on my Corolla of similar vintage I’d ignore the knock sensor code for now, and focus on the cyl 1 misfire. A misfire isn’t a mysterious thing. A proper firing means the spark plug produces a healthy spark, at the right time, which ignites the proper amount of fuel and air in the cylinder, and the force of that blast – provided the compression is good and there’s somewhere for the exhaust gasses to go – that blast pushes on the crankshaft hard, and accelerates it. A misfire code means that something in that sequence isn’t happening for some reason. The crankshaft isn’t accelerating after number one spark plug should be firing. That’s consistent w/the rough idling too.

Your shop has to test those one by one is all

  • does cylinder one spark plug have a good healthy spark?
  • is the number one injector receiving the proper electronic injector pulses from the computer?
  • is the fuel pressure at the number one injector correct?
  • is the compression correct?

If you’d rather diagnose this by making educated guesses , and that might well work and cost you less money hmmm … here’s some ideas

  • Replace the spark plugs
  • New distributor cap, spark plug wires, and ignition rotor (if that’s your engine configuration)
  • or swap the number one and number two coil (if you have coil on plugs engine configuration), see if the problem follows the swap.
  • If this only happens when the engine is hot, replace the crank position sensor
  • Fuel injector cleaner might fix the problem, or even if it just improves the situation, that’s a clue
  • Temporarily bypass the cat, see if that helps

But if this were my car I’d make sure I had all the codes, then start with a visible check for a good healthy spark at the number one spark plug. Many shops have ignition analyzers which can make this job easier to do. I’d bring out my lab o’scope probably, if I wasn’t sure about how good or constant the spark was compared to the other cylinders.

One more thing, make sure the problem isn’t that you are buying contaminated gas. Even a little water in the tank could cause a symptom like this. The reason I doubt that’s the problem is b/c your misfire is limited to cyl number one. But still be sure to buy quality gas every single time, from brand name station that sells a lot of gas.

First, thank you for this analysis.

I can see from what you said that this is not an easy fix and will require a really knowledgeable mechanic. I really don’t know anyone but I will ask around and see if I can get some recommendations if I decide to pursue fixing it.

I guess I need to decide if it is worth a few hours of shop time - at about $100/hour that could be a few hundred $$. Plus the cost to fix whatever is wrong. I figure that my car in good running condition with no known problems is worth about $1,000. I could spend a few hundred ($500? $600? or more!) and it would still be worth only about $1,000. Selling it unfixed it’s worth very little.

Am I possibly doing damage to the car by continuing to driving it? Is it unsafe for me to drive it?

Thank you for a really well organized set of possible causes and ways to figure it out.
Very encouraging too.

I do not know if the mechanic I took the car to checked the spark plug and spark plug wire for cylinder one. After I went there the second time he seemed less interested in working on the car.

I do not work on cars myself but have a friend who knows a little about it and he has a mechanic who I may go to next. I hope I can get an appointment later this week.

Are the codes enough for a good mechanic to start diagnosing or does the car actually have to be running poorly and showing a problem?

I might try some additive/cleaner to see if it makes any difference. And I had gone to a sketchy gas station a couple times. But right now the car runs fine - no rough idle and no CEL. But I haven’t been going over 35 mph and only drive a few miles per day.


I may just end up selling it or even donating it if it’s too expensive to fix. Even repaired it is only worth about $1000. Should I spend $500 to sell it for $1000? And I probably won’t buy a car from a dealer so there will be no trade-in. Hoping for the best outcome and appreciate all the suggestions on this site!

I would prepare myself for it not being an easy fix… but you never know. It just might be something simple.

Yes. If you’re experiencing preignition, you could burn a hole in a piston.
There are basically two types of preignition.

  • “pinging”, which is a secondary unwanted combustion originating in a corner away from the sparkplug as the plug fires, its wavefront crashing into the sparkplug-initiated flamefront, causing that typical “marbles in a can” sound. This can burn a hole in the top of the piston. And

  • “knocking”, which is the combustion process beginning before the piston reaches the point in its travel where it should. This can not only fry a piston, it can break a connecting rod, destroy the rod bearings and perhaps even main bearings. Basically, the piston is still trying to rise, being pushed by one of the other cylinders even, when the explosion tries to force it backwards. The forces are huge. Connecting rod bearings and even crankshaft bearings are subjected to a lateral pulse (across their axis of rotation) when a piston fires. Normally the design accepts these pulses, but if they happen at the wrong time… it’s like a head-on collision. It ain’t good.

Whether it’s worth more analysis is a personal choice. If your budget can’t support a replacement vehicle without strain, and the rest of the vehicle is otherwise in good shape, it may be worth it.
If you can safely afford to replace at and your family’s safety is at stake, I’d replace it. Or, if you’ve been just looking for an excuse to justify that new Porsche… this may be your opportunity. :stuck_out_tongue_winking_eye:

Yes, it is an old car

It is considerably older than the average car on the road . . . about 11 or 12 years old, I believe

The fact that YOUR vehicles are older, doesn’t suddenly mean that OP’s car is not old

No offense intended, but you . . . and many of us on this website . . . keep cars longer than many people do

Just because a car is well maintained and looks great, doesn’t mean it isn’t old

I fail to follow your logic

I’ll go a little off-topic

I’m 45 years old, middle aged is the term, I believe

If somebody were to mention I look pretty healthy . . . which does NOT happen, by the way . . . it wouldn’t mean that I’m not middle aged


BTW I got my username back - hope the change didn’t confuse anyone


I guess I better stop driving it … just wonder why it doesn’t have any symptoms as you described. There are no odd sounds and no rough idle and no CEL for 2 weeks. But best if I borrow a car while I look for another one. And no, I doubt it will be a Porche!

Great detailed explanations. They are a little technical for me but I understand more about ignition systems that I ever imagined. I do not want to do anything unsafe and all those possibilities sound unsettling. Hoping to take it to a mechanic on Friday.

Thanks again for taking the time to read and reply.

Any coolant loss?

Well …

about a week before the first time the CEL came on I had my oil changed and the mechanic mentioned that my coolant was low. I had some at my house and was going to add it but forgot until the day the CEL came on so added some then. I haven’t checked it again since then.

Is there a connection with low coolant? The car never overheats.

Actually, for a Mecury Villager, it’s ancient.

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Keep an eye on the coolant level because if a head gasket is starting to fail, the coolant could be leaking into that #1 cylinder causing the misfire. Head gaskets are expensive, BTW.

All the posters have pretty good advice. Given your situation, here’s how I’d play it. You had a tuneup 15K ago and the mechanic didn’t tell you that there is a problem with #1. Looking at the sparkplug is a very good way to determine the condition of that cylinder. You could pay them to check it now but first I’d add fuel injector cleaner to the gas and run a tank through to see if that clears things up. While you are doing that, look around for a replacement car. This car is on its last legs.

If it doesn’t clear the problem, list the car for $1300 and take the first offer you get. Any running car, even poorly, is worth that. A trade-in would get you about $500 or less and the dealer would tack that onto the price of the newer car so your net would be $0.


Thank you. You confirmed some things I have been thinking.

I’ll check the coolant level again today. Would that kind of leaking cause the car to overheat? It never has and it’s been really hot the past month.

I hope to get the car to the mechanic on Friday. With today being a holiday it has made scheduling more difficult. I can ask him to check the #1 sparkplug.

Is fuel injector cleaner different than a fuel additive? I bought some Techron complete fuel system cleaner but have yet to add it because I still have 6-8 gallons of gas in my car. How important is it to add it to an empty tank? Should I fill the car with premium gas after I add it? I’m a little lost with how to go about this.

I think you are right about it being time for a replacement car. I had actually just started looking before these problems came up. I will buy a car from a private party not a dealer. This car has lasted me 7 years - two years longer than I ever expected. I certainly got my money’s worth! I paid $2500 and have had no engine problems until now.

Thanks for the info on pricing and selling. Another plus is that I just put on new tags that are good for 2 years so that must be worth something.

With 7-8 gallons, just pour in 1/2 the bottle. No, you don’t have to use premium. And Injector cleaners can be additives.

As for the coolant, Yes, if you lose enough, it can cause the car to overheat. The reason I suggested you keep track of it is because if a head gasket is starting to fail, it can sometimes “consume” coolant. Basically coolant enters the cylinders and goes out the exhaust. Cars shouldn’t “consume” coolant. that is a hint of a problem. IF the head gasket is starting to fail, there are things that can be added to the coolant to temporarily fix the problem. Normally the posters here would say, “no, don’t do that” but this car is on its last legs.

When you go to your mechanic, don’t tell them to do specific things. Tell them what you told us. Give details on what is happening so that the mechanic can figure it out. After all the car is right there in front of him, not us. Oh, BTW, great buy! $2500 and 7 years? Score!

Good Luck.

Before replacing a fuel injector, try swapping the number 1 and number 2 injectors. If the problem is the injector, the misfire will move to number 2 cylinder. If it stays w/number 1 it’s not the injector and at least you’ve saved yourself the price of a unneeded new injector.

Yes, that’s the place to start. It takes very little time for a mechanic to read the codes. Misfires are not an uncommon problem reported here. Pro mechanics diagnose and fix your car’s problem as a regular course of business. Sometimes it turns out to be a major mechanical problem, but more often than not the problem is something simple, like a loose connection in a wire or a easy to replace part failure, something very fixable like that.

It’s a low risk guess to pour in a can of Techron injector cleaner, but since your problem seems to be intermittent, it’s unlikely to help with this particular problem. But it might clean out the injectors if they’re a little clogged, may not help, but won’t hurt either.

If you’ll allow an editorial comment, I sort of get the sense you want to get this fixed without a major expense. I can understand that; but the problem is that by using a non-pro mechanic and engaging in guess work you may end up spending more than if you just engaged an experienced inde mechanic. For fixing modern cars like yours, the cheapest way into the shop is seldom the cheapest way out of the shop. Best to use someone who has years of experience. Then just tell them the symptoms, any ideas you’ve gleened by your own research , and then stand back & let them solve it. If you have an upper limit on how much $$ you can spend, let the mechanic know that up front. They’ll try to do what will bring the most bang for the buck, given the number of bucks available. The best way to find a shop is to ask your friends, coworkers, church-goers, bar hoppers, anybody you have a trusted personal relationship with, ask them who they use for their cars. From that list interview 2 or 3 that specialize in fixing Ford or at least American cars, and choose the one you like the most. Make sure to tell them who it was that recommended them to you. Best of luck.

Has any consideration been given to the fact that that the No. 1 misfire might be caused by a faulty plug wire or in a worst case scenario; a cylinder dropping compression.

The number for the cylinder misfire should be 301.

It seems a bit premature to suspect a fuel injector or to throw in the towel at this point without considering the plug wire first.

I checked and have not lost any coolant in the past 3 weeks. It’s been super hot here the last couple days and I’ve driven a little and had the AC on full blast and no sign of overheating so hope there is not a gasket problem. Thanks for the info about the additive and for the tips on talking to the mechanic. Had hoped to see the him on Friday but he didn’t have an opening until next week so I will see him on Tuesday.


I am happy to report that I’m taking the car to a trusted and very experienced mechanic, someone a friend of mine has been going to for about 10 years. The appointment is for Tuesday next week.

I will probably add the Techron I bought. Good to know it can’t do any harm.

Your are correct that I don’t want to spend a lot to fix this. That was why I didn’t have the first mechanic I went to put in a new fuel injector - he didn’t seem sure that that was the problem. Your idea of swapping the fuel injectors makes more sense. But I’ll see what this other mechanic thinks.
Thanks for your suggestions. I’ll post an update next week.