Tackling Check Engine Light +/- Drivability Issues

**swanicyouth** · 10-08-2014, 07:34 PM

So peeps and geeks I thought I would put this together as an OT on the basics how to attack a check engine light (CEL) issue in your car with or without a drivability problem. Pretty much everyone has experienced the knot in their stomach that is brought on from the appearance of the dreaded CEL. Your driving along and the light appears +/- the car doesn't not "run" correctly. I'm guessing the average person just takes their car to the "mechanic" of their choice and sits and waits for the phone call of doom...only really curious about one thing - "how much is this going to cost?".

Well, this is going to be geared towards the average person that doesn't have a lot of experience in this area, but has some basic knowledge of how their car works. Basically, the CEL is your car's computer or engine control manager (ECM) telling you the vehicle is having a fault - usually within the engine. Most/all newer cars cars also have "lights" for vehicle subsystems; like: ABS, transmission, SRS, etc... However, I don't plan on dealing with those here.

The other day I was driving along just fine and my 2001 Pathfinder started hesitating and shaking. The CEL came on as well. As quickly as the problem appeared - it disappeared. However, as is common, the CEL will remain lit telling you the vehicle has experienced an issue. So now what? Well, this is how I attack a problem like this...

Since around 1996 all major car manufacturers are required to use a semi-universal diagnostic and engine monitoring system called On Board Diagnostics II (OBD2). This means that certain errors that can go wrong with your engine that can be detected by the ECM are somewhat standardized (defined) and retrievable by a universal mechanism. This mechanism is the OBD2 Code Scanner / Reader. This is the little device you plug into you vehicles OBD2 port to retrieve diagnostic engine (or other system) codes that the ECM recorded as a fault.

Code Reader:

Generally a "code reader" is an inexpensive device to read / erase diagnostic trouble codes (DTCs) on OBD2 vehicles (>\ 1996). A "code scanner" is usually considered a tool that performs the same function as a code reader, but also has some live or freeze frame capability. This "live capability" is the ability of the tool to retrieve some additional live engine data - such as a particular voltage or reading an engine sensor is sending back to the ECM.

Without getting overly complex, while OBD2 is somewhat universal for most vehicles, there are five or so different "protocols" of OBD2. Most decent newer scanners will support all five. The newest protocol is called Controller Area Network (CAN). CAN is a bit more advanced than previous protocols, as it allows for the ability of the vehicle's different control modules for various systems to communicate with each other and recognize what system/module that communication is coming from.

CAN was somewhat universally adopted in the USA sometime around 2008. This is why many newer scanners are labeled OBD2 / CAN, as older code scanners would not necessarily work correctly on CAN vehicles. Regardless: what year your vehicle is does not affect how one goes about diagnosing and repairing a basic CEL issue. You just need to make sure you are using a code tool that is compatible with your vehicle. Most newer "scanners" will be back compatible with OBD2 vehicles from 1996.

I believe new car dealerships charge $100+ for "diagnostics" to read an engine code. While the software they are using is more advanced than most hand held scanners, 95% of the time this more advanced software will not be required to fix a CEL issue IMHO. Places like Advance Auto used to / still do "read" vehicle engine codes for free, but I would not recommend that. I recommend you simply purchase your own code scanner / reader.

There are a few reasons I recommend you purchase your own code reader / scanner. The first is, you can get a really nice one for about $200. You can pick up one that can test several vehicle systems, work on almost any vehicle, reset DTCs (turn off your CEL), and is updatable as newer software comes out. A cheap "code reader" will cost you as little as $30. Actually, the tool I use is a simple $30 code reader, the Actron CP9125. Code readers/scanners plug into the OBD2 port/plug which is required to be somewhere inside the vehicle within 2 feet of the steering wheel.

While a code scanner would give me more information, a have yet to experience a CEL issue I couldn't resolve just obtaining basic codes and performing basic tests - while using my brain. Better and more expensive machines likely have the ability to read and define "manufacturer specific codes" (P1xxx codes) or codes that are unique to a certain vehicle makes as well. Some cheaper machines may have the ability to read common manufacturer specific codes, too. More common (universal / generic) engine codes will begin with P0xxx, P2xxx, or P3xxx. "P" stands for "power train". Other vehicle systems' codes start with other letters that designate which part of the vehicle they are referring to.

Back to my story of my CEL tale of woe. Also, this is related to why I recommend buying your own code retrieval tool. So, whenever you get a CEL, the first thing I'd recommend one do is to read the code(s) with the tool, record them, then erase them (turning off the CEL). Many things can cause the light to come on. Some simple - some complex. But, you want to delete them to see if it's an isolated incident - or there is really something wrong with the vehicle. If the vehicle is not "running right", deleting the codes likely will just make them re-appear. If there is an intermittent problem, deleting the codes will reset your ECM and MAY allow you to obtain additional information for solving the problem. I will explain by my particular example.

Upon checking the codes, I retrieved code P1320. P1320 is a Nissan specific code that is defined as "primary ignition system fault". While this doesn't tell me a whole lot, it tells me the computer THINKS something is amiss with the vehicle's ignition system. I say "THINKS", because codes are an interpretation of what data the computer is retrieving. If the data is somehow incorrect or misleading - the code could be erroneous. In this particular case, I know Nissans use information from the crankshaft position sensor to help determine ignition misfire (along with other sensors' data) - so the possibility exists a problem could exist in one of these subsystems making the computer THINK there is an ignition problem.

The previous scenario is not likely, but possible - so you want to keep it in mind. If your ever looking for what a particular code means, the simplest way to find out is to just Google the code, like "Nissan P1320". Anyway, since the vehicle was now running fine - I knew the failure/issue was intermittent. In my opinion, the simplest thing to do in these situations is to just reset the codes and drive the car. Hopefully, if the problem happens again - you may get additional codes that may point you to the correct fix.

Technically the *correct* thing to do in this case is make a bunch of resistance and voltage tests at various points of the ignition system. Since the problem is intermittent - it's *likely* those tests would not reveal the problem. If those tests are required, I'd recommend the factory service manual (FSM). Often, you can pick up the exact service manual the dealership technician uses on disk or direct download from eBay for next to nothing. I picked up the Nissan one when I bought the vehicle for under $10 via direct download.

The FSM lives on my iPad and it's quite easy to scroll through whatever area of it you need:

Google is your friend here as well. Instead of doing a bunch of tests that are likely to point to nothing (intermittent issue), it's quite easy to just Google your specific code and vehicle and see if there is a common issue that triggers the code(s) you are retrieving. So, in my case, I Googled "Nissan 2001 Pathfinder code P1320" or "Nissan code P1320 3.5l". As I suspected, I got a slew of responses relating to bad ignition coils. The interesting thing was, that most that received this code could not pinpoint the exact coil causing the problem - so ended up replacing all six coils ($400+ for parts). This is why I recommend to reset codes and watch and wait a bit.

First thing is first. Assuming your vehicle is well maintained (mine is), you still need to check the basics. Anything that interferes with the ignition process could possibly trigger such a code related to faulty ignition. So; things like vacuum leaks, air intake leaks, a bad fuel pump, a bad mass air flow sensor, or a bad fuel injector - could cause such an issue. Also, mechanical problems like a burnt or sticky valve, broken piston ring, or clogged exhaust could be the culprit. However, it's less likely for mechanical issues to be intermittent and non duplicatable at will - so I decided to rule those out. Damaged wiring could also be the problem - so performing a careful visual inspection under the hood of the suspected system(s) is free.

After resetting the codes and driving a few longer trips, the CEL came on and the vehicle started to hesitate again. Upon checking the codes this time, I had a P1320; but I also had a P0303. P0303 is a common universal type code for "misfire cylinder 3". Now, I was pinpointed to a particular cylinder (3) and a particular system (ignition). At this point there is a few common non-mechanical things it could be: faulty wiring (visually checked out fine), bad spark plug, bad coil, or bad injector at cylinder 3.

However, I know an intermittent injector failure is relatively rare compared to a coil failure. Also, I know it's a lot EASIER to replace a coil compared to an injector. At this point, since the problem is intermittent, it's best to go with the most common issue - especially if ruling that issue out is relatively easy and CHEAP. Ruling things out by general deduction will usually get you closer to a resolution. If there is nobody reporting on Google their injector went bad on your specific vehicle, it's UNLIKELY (albeit possible), that you are the first one. On the other hand, coils commonly go bad on all distributor-less ignition system vehicles - and symptoms can be intermittent misfires. So, at this point, the most logical thing to do IMHO is simply replace the ignition coil at cylinder 3. I will replace the spark plug as well - since: it's cheap, I'm already there, and a cracked spark plug could cause a misfire.

The last thing you want to do is just guess and hang a bunch of expensive parts on the car. Hanging cheap parts is fine - but nothing is worse than buying a $500 (non-returnable) electrical part only to discover it did not fix the problem. That's analogous to flushing $500 down the toilet plus whatever time you have invested in it as well. It's often the case that whatever sensor the code is pointing you to is bad - but it's not always the case. I'd recommend thinking about it a while, and doing any tests deemed necessary that can rule out anything that can mimic the issue. Here, simply swapping swapping coil 3 and spark plug 3 to another cylinder to see if the code goes to that cylinder could tell if the parts swapped are the the problem or not. However, these parts are cheap - and my time is valuable - so I choose just to replace them.

A few words on parts - especially electrical parts. I recommend buying either factory electrical parts / sensors or what is called an "actual OE part". The later statement simply means whoever made the electrical part for Nissan (Hitachi in this case) makes your replacement part. While a whole slew of companies make something aftermarket like an ignition coil or spark plugs - all electrical parts are not created equal. Numerous people have reported "issues" with various aftermarket electrical engine parts. At that point, it's quite difficult (and quite frustrating) to determine if the part you got doesn't jive with your car for some reason, or the fault lies elsewhere in the system.

If you choose not to go the factory parts route - actual OE is the best way to go. Going into Advance Auto or wherever and asking for an electrical engine part for your specific vehicle will get you a part made by whoever Advance Auto decides to buy from. Also, you will likely pay even more for it than ordering an actual OE part through the mail. Rock Auto stocks or can get parts for ANY vehicle. The reason I like them is, because they give a vast choice of part manufacturers and tell you which part they stock is the actual OE supplier of the factory part. See below:

I also recommend when replacing spark plugs to ALWAYS replace them with the exact same plug the vehicle came with from the factory. This will usually be the most expensive plug - and the best one. It is not uncommon to have a drivability issue when plugs were replaced with another type that was supposed to be interchangeable. Again, Rock Auto can tell you this.

If you notice there are many spark plugs that are supposed to work with this vehicle, but the factory / OE NGK plugs are the most expensive ones towards the bottom of the page:

Since the plugs were recently replaced in this vehicle with the OE NGK plugs, I choose to just change this single plug along with this single coil (#3). This was easy enough and cheap enough. Parts to my door were $75 with 2 day Fed-Ex. This did fix the problem.

Here are the old parts:

Hopefully, this little write up will encourage someone who gets a CEL light in the near future to skip the shop, buy a code tool, and attempt to fix it themselves. This will ensure the correct part is installed and the work is done to your satisfaction. You'll likely save a ton of cash in the process. While not all intermittent drivability/CEL issues are this easy to solve - most are if your vehicle is well maintained.

**damaged442** · 10-09-2014, 07:00 AM

Great writeup!

Like you said, Google is your friend. I've had codes thrown for something as stupid as a spider building a nest in the vent valve for the gas tank, and of course the code for a loose gas cap.

Chances are if you have a problem, someone else has had that problem, and may times it can be an easy, inexpensive fix.

If you run to the stealership with a check engine light on, you can pretty much bet it is not going to be cheap!