Check Engine Light

Trouble code P0421 means:Warm up catalyst system efficiency below threshold

When a transmission warning light on any vehicle turns on, it can mean a few things that will spell trouble if not checked right away. One cause is a bad sensor or the fluid is so over used, which is not so serious, and a scheduled fluid/filter change usually fixes the problem. If the color of the fluid is still red and appears to be fine, chances are that slipping is occurring in the transmission. Slipping causes friction and friction generates heat thus boiling the fluid. This is a very serious problem and a replacement of the transmission will be required if this condition is not attended to right away.

Disconnect the battery leads for at least 5 mins., 10 mins. perfered. Reconnect battery leads and your done. Though the light will come back on after a few miles of driving if the engine/emissions problem is not resolved.

where is the plug to conect the code reader on a 1990 Honda accord

if you got a code saying this, then an easy way to check it is to pull out fuse number 58 which is next to battery in a fuse box, a 10 amp fuse, also above coil pack number 5 is the purge unit, disconnect the vacuum hoses and try and blow through the valve, it should be restricted, when you hook the valve to a power source, you should be able to blow through it

A common cause for that code would be a loose gas fill cap or a damaged/missing O ring seal on the cap.

The "Check Engine" light means there is a problem with the emissions system. Take it to a professional for a scan of the ECU with an OBD2 scan tool. The fault code that has been set will help the technician to find the problem. Do not ignore this. Some problems may be a simple fix as in a loose gas cap. Other problems are serious and can cause other damage as in damage to the catalytic converter.

You need to have it checked for codes. You can then diagnose the cause.

There are pages and pages of things that can cause a check engine light. You need to have it checked with a scantool. Then the code can be diagnosed and repaired.

Have vehicle scanned to determine the problem - usually in the emission control system

The light is on because there is a problem with the emissions system. The engine may appear to be running fine but the emissions of the engine are out of spec. It may be as simple as a loose gas cap. Have the ECu scanned with a scan tool to retrieve the code that has been set by the ECU. This will help you find what part is being effected.

You will need to have the engine scanned by a repair shop. That will give them codes to repair it. Once repaired they will clear the light. Clearing the light does not repair the problem.

***See discussion area for my dispute of this above answer****

What engine do you have 1.6 or 1.8? On the 1.8 the first O2 is in the front middle of the engine about 1-2 feet down the exhaust system. I believe the 2nd O2 is after Cat. convertor.

The engine is running lean www.obd-codes.com this will help .

I was told at the dealership to check my gas cap and make sure it clicks when its closed. Sure enough within 24 hours, the lights went out. Hope this helps. -----------------------

A week ago, on my Lexus ES 300 (2001), "Check Engine" and "Trac Off" lights came on. This coincided with me filling the gas tank to the brim to round the dollar to be paid. Some websites suggested that the "Trac Off" is related to the gas tank cap not being tight. I cleaned the gas tank cap and the tank opening. However, both lights persisted. On the same day, when I turned on the car again, there was a hesitance, a stuttering to come on, like the battery was going down. Both "Check Engine" and "Trac Off" lights were still on.

The next day, the car wouldn't come on at all. It sounded as the battery was dead. I couldn't even jump start the car. Usually, if the battery is completely drained, jump starting will not work. So, I got a new battery from Costco ($56.99) and put it in. (There is a $9 fee if you do not turn in the old battery).

It took 15-20 seconds of cranking to get the car started. Once it was started, the engine sounded fine and the RPM was normal. The "Check Engine" light was off. However, the "Trac Off" was still on. I cleaned the gas tank cap one more time.

The next morning, when I turned the car on, there was no problem at all. It came on instantly as it used to be, didn't have any warning lights, and drove smoothly.

The gas tank cap getting dirty and the battery being down are unrelated things, which just happened on the same day. I do not know which one caused which warning light to come on though.

There are pages and pages of things that can cause a check engine light. You need to have it checked with a scantool. Then the code can be diagnosed and repaired.

are you having a check engine light stating "engine too lean?" if so replace all o2 sensors.change spark plugs and especially air filter.rest code and see what happens.

Check fuel pressure when it's bogged down like that. If the pressure drops it could be the fuel filter or the fuel pump. Unfortunately, if it's the filter it won't necessarily drop in pressure until you're driving.

An engine diagram of the Mercedes s420 can be acquired from your local Mercedes dealership. You can also get a diagram at most auto parts stores.

The reduced engine power light comes on to make you get your problem fixed ASAP!

It may be a rich code being picked up by an O2 sensor but whatever it is, the code must be diagnosed and cleared. You may disconnect your battery for 15 minutes to clear the code to see if it resets but if you are not mechanically inclined then you should go to a reputable GM dealership to get it fixed.

The oxygen sensor is "done". It should be replaced.

disconnect the battery for about a minute if that does do it....then u can either take it to a mechanic or diagnose it yourself... there might be a harness under your glove compartment, that's a big multy plug in then there should be a double prong one that u can run a paper clip too.... now turn your ignition but not onto on watch your check engine light it should start blinking.... long blinks i believe where by 10's, and short blinks are by 1's.... so LONG LONG LONG SHORT SHORT SHORT, (33) Hope this gets u started!!

You're going to have to put it on a code reader to get the answer to this one. Could be as silly as a loose gascap or something really serious. Take it to your local mechanic.

Check engine lights come on for several reasons. I own a 98 Monte and my check engine light comes on alot. It could be any thing from a un clipped sensor, fuse, oil needs changed or checked. Take it to Pep Boys or Autozone they check out your check engine light for free.

You need a auto scanner, it reads codes on your onboard computer. If the problem is not fixed the light will keep coming on. After the problem is fixed you can use the scanner to turn off the check engine light. Auto scanner can be expensive but if you like to work on your own vehicles it will pay for it self, if not go to your local dealership and they will read the codes but like i said the problem will have to be fixed.