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Understanding the Check Engine Light

Originally Published in MotorHome Magazine

Over the years emission controls have become more and more complex, and the ubiquitous (and infamous) check engine light coming on has become one of the most common vehicular problems today. Now that diesels have these dreaded warning lamps too, nearly everyone can join in the fun and frustration of chasing down problems brought to light by these silent sentinels. The check engine or service engine soon lights are officially known as
malfunction indicator lamps, or MILs. These come on to warn the driver that an
emissions-related problem has been detected by the on-board computer (also known as the electronic control module, or ECM). Starting with 1996 and newer models (and a few earlier ones), a more sophisticated and complex system known as On-Board Diagnostics, generation 2, or OBDII, was introduced.

These systems, which are found on many motorhomes and also dinghy
vehicles, constantly perform checks on monitored systems. Depending on the type of
self-test or monitored component, they set a trouble code in memory after an
emissions-related failure is detected a certain number of times. This system also triggers
illumination of the MIL. When a MIL illuminates steadily, it indicates a problem that may
increase emissions, but is not considered immediately damaging to the vehicle or urgent. If
the MIL begins blinking, or the vehicle begins to run rough or poorly, this is an
indication that driving should be limited and the vehicle needs to be checked and the
problem corrected right away or damage may occur. The MIL should not be confused with a
routine maintenance reminder light, and the MIL also does not warn of low oil pressure,
overheating, or other similar problems, so you still need to watch your gauges.

system can also miss certain drivability conditions; these are called “no-code” problems
and can be tough to troubleshoot. Having a MIL illuminated will also cause your vehicle to
fail an emissions or “smog” test, which are used in many areas. Even if your coach is not
required to undergo testing, you still need to know about this subject because problems
that cause emission failures also result in drivability, mileage and performance concerns.
If your coach has a MIL on, or fails a smog test, repairs may be covered by a separate
emission control system warranty. These vary by state and year, but often provide coverage
for a longer period than the chassis warranty and may save you a lot of money. Check the
documents that came with the coach or check with the chassis dealer or manufacturer for

Once out of warranty, the burden for emission repairs becomes your responsibility.
Some smog test programs include waiver provisions that limit how much money owners must
spend on emission repairs. If an emissions failure cannot be fixed within the specified
cost limit, the vehicle may pass even though it is still above limits. Check with the test
facility or state for specifics.

Emissions Testing
Most smog programs
perform a tailpipe test for unburned hydrocarbons (HC) and carbon monoxide (CO), and may
also measure carbon dioxide (CO2) for diagnostic purposes. Areas that have adopted the
inspection and maintenance (I/M) 240 test program check for emissions, including nitrogen
oxides (NOx) emissions, in grams per mile (GPM) at various speeds under load on a
dynamometer (vehicular treadmill) run for 240 seconds. Many inspection programs also
perform visual checks of emissions hardware for evidence of damage or tampering. These

  • Checking if the MIL is illuminated.
  • Inspecting the fuel filler cap to make sure it is the correct type and seals
  • Looking underneath to determine if the catalytic converter (if required) is
  • Checking that the engine has all required emission-control components.

Checking any non-stock (aftermarket) parts on the engine to make sure they are
emissions certified. If an aftermarket part has a California Air Resources Board
Executive Order (E.O.) number, it should be smog-legal in all states. The I/M 240
programs may also check the fuel system for air leaks (evaporative emissions) and test
the charcoal canister purge valve. Emission regulations for new vehicles have become
more stringent over the years, so older vehicles have more lenient emission test
standards than newer ones. Generally, the allowable tailpipe test limits are also
somewhat higher than the standards the engines were designed to meet, to allow for

Tips on Passing a Test
Most engines in good running condition
and properly maintained should pass an emissions test. However, sometimes minor
problems can cause a failure. If a vehicle is only driven a short distance before the
test, it may not be warmed up to the normal operating temperature of the engine and
catalytic converter. This may cause it to fail. Excessive idling while waiting for a
test may also cause the catalyst and/or oxygen sensor to cool enough to cause higher
emissions. If the idle speed is too high or low it can make the difference between
passing and failing. A dirty air filter or restricted air intake can cause
higher-than-normal CO readings. Oil can be contaminated with gasoline if the engine has
been used for a lot of short-trip driving, especially in cold weather. Fuel vapors can
then be sucked in through the PCV system and cause higher CO readings. Some engines
also tend to run dirtier because of their design, or engine controls. Sometimes special
repairs can be found in factory technical service bulletins (TSBs). If your area does
plug-in OBDII emissions testing on 1996 and newer vehicles, a vehicle can fail if all
of the required OBDII readiness monitors (internal tests) have not run. This occurs if
the battery has been disconnected, and may require driving for several hours until all
monitors have run. A vehicle will also fail the test if the MIL is on, or there are
fault codes stored in the PCM. If the OBDII system is working properly, the MIL is not
on and there are no codes, the vehicle should pass the test. Even if you pass, keep a
record of the readings from year to year. This information can be used to spot trends
such as emission system degradation, or an engine becoming worn. Make sure the engine,
battery and charging system are in optimum condition. If the battery is weak, it can
affect the outcome of an emissions test. If the oil is dirty, change the oil before an
emissions test. The oil, coolant and catalytic converter need to be warm. Drive at
least 20 minutes at highway speed before the test to give your engine time to warm up.

What If it Fails?
When your motorhome undergoes an emissions test, you
should receive a printout of test results, along with the applicable limits. Using
this, you can determine which specific portions of the test were failed, if any. If you
are mechanically inclined, you may be able to correct some problems, or at least
understand the situation and determine if a service facility is on the right track to
fix it.

Excess Hydrocarbons
A failure due to excess hydrocarbons
(measured in parts per million or PPM), means unburned gas is going through the engine.
The most common causes are ignition misfiring, lean air-fuel mixture misfiring and low
compression. Faulty spark plugs, cables, cap, rotor or coil usually cause ignition
misfire. A lean misfire occurs if there is too much air and not enough fuel, so look
for vacuum leaks, faulty injectors (or carburetion) or a fuel delivery problem such as
low pressure from a faulty fuel pump or clogged filter. HC failures may also be caused
by oil burning from worn valve guides, seals and/or piston rings.

Excess Carbon

Excess carbon monoxide indicates a rich fuel mixture. On older
carbureted engines look for a rich idle mixture, clogged air filter or intake, stuck
choke, or a fuel-soaked or misadjusted float. Vehicles with electronic controls may
also have a restricted air filter or intake, or are not going into closed loop because
of a faulty thermostat or oxygen sensor. Fuel-diluted oil can also cause high CO
readings, as can incorrect ignition timing. Worn rings and valve guides may contribute
also, especially on very worn engines.

Carbon Dioxide
CO2, measured as
a percentage, indicates how complete an engine’s combustion is; the ideal is 14
percent. Valve and ignition timing, compression, fuel mixture, engine condition and
temperature affect CO2 output. Everything must be right to get near the ideal.

Excess CO and HC
If the HC and CO are high, you may have both an
ignition and compression problem. You may also have a bad catalytic converter, or an
air pump problem. Using the above explanations and some logic, you should be able to
solve many emissions failures.

Nitrogen Oxides
Since the exhaust gas
recirculation (EGR) system is mainly responsible for controlling NOx emissions, an
inoperative EGR valve or over-advanced timing are the usual culprits, but a faulty
three-way catalytic converter, hot-running engine, exhaust leaks or an ECM that remains
in open loop can also cause an NOx failure.

Resetting the MIL
you’ve properly diagnosed and repaired a problem that set an OBDII trouble code, you
can reset the MIL and clear the ECM’s memory by these methods. The code and MIL will
usually go away after a certain number of driving cycles, perhaps a few days. Most scan
tools can also clear codes and this is the preferred method; consult the tool’s
instructions for how to do it. It’s possible to disconnect the engine’s battery
negative terminal, wait about 10 minutes and reconnect the battery. Then the coach must
be driven until all monitors have again run before a smog test can be run. You may lose
clock and radio presets that rely on the battery and the engine and transmission may
not run as smoothly until they relearn a few settings. Caution: if the dash radio is
code-protected, it may be locked after disconnecting the battery.

Until recently, diesel engines had relatively few emission
controls and there was little that owners had to do regarding these components and
systems. Engines with EGR valves need to have those checked and cleaned periodically,
as do air filters. Crankcase ventilation systems also need to be inspected regularly.
Some areas perform smoke opacity tests, which measure the amount of soot present in the
exhaust. Since the introduction of the latest generation of clean diesels, operating
procedures have changed and certain steps may be needed by drivers to keep their
emissions systems working properly. Besides using ultra-low-sulfur diesel (ULSD) fuel,
the proper engine oil is also important to ensure that your engine and after-treatment
system perform well. Starting with 2007 models, engines should use a new oil
formulation (CJ-4) that has a low sulfated-ash content to help extend the maintenance
cycles for the diesel particulate filter. Specific operating and maintenance
information varies by brand, year and model and is included in the owner’s manuals and
on manufacturer’s Web sites. Caterpillar, Cummins, Detroit Diesel and Workhorse provide
information on their Web sites and via customer service numbers.

How to Read
and Remove Check-Engine Codes Yourself Using Scan Tools
Scan tools were
developed to read trouble codes and related stored information from the ECM. These are
plugged into the diagnostic port, usually under the dash on the driver’s side, and can
also be used to delete the code and reset the MIL. Code readers and scan tools come
with a wide range of prices and features. Some cost thousands of dollars and are
designed for professionals. However, it’s possible to buy an inexpensive tool for about
the same price as it costs to have a shop do a diagnosis. Scan tools and code readers
are available at auto parts stores and online. Before you invest in a tool, make sure
it is compatible with your vehicle and has the features you reasonably expect to use.
If you just want to know what codes are present and then delete them, you need a basic
code-reader unit. Some of the popular scan tools used by do-it-yourselfers are sold by
Actron, AutoXRay, Equus and Harbor Freight Tools.

If you intend to do repairs yourself,
you may need a more complex scan tool. Some of the features these tools come with
include the ability to retrieve generic (P0 and U0) and manufacturer-specific (P1, P3,
and U1) codes, as well as enhanced and pending codes. Some also display emissions
readiness (monitor) status and retain the most recently scanned vehicle’s data and
configuration. Higher priced units may also diagnose ABS, air bags, transmission,
radio, climate control and more. Many of the newest models support CAN-BUS (controller
area network). Advanced models can read the data stream to aid diagnosis and some also
allow you to play back freeze-frame data to scrutinize stored information. Some can
monitor vehicle sensors such as the oxygen sensors, ignition timing, engine and
transmission temperatures and more while the engine runs, to pinpoint a problem. With
live data and freeze-frame capability you can view real-time engine data for even
better diagnosis. These features are very useful when there are intermittent problems.
Combine this with Internet connectivity and the ability to interface with a PC and you
have a pro-quality tool. There are generic trouble codes that apply to all brands, and
specific codes that only apply to certain brands and models. Typical powertrain-related
codes will start with the letter P followed by four digits. For example, P0301 is
Cylinder 1 misfire detected.

Codes can be read using the following
legend: 1st digit P = Powertrain B = Body C = Chassis 2nd digit
0 = Standard 1 = Manufacturer specific 3rd digit 1 = Emission management 2 =
Injector circuit 3 = Ignition 4 = Auxiliary emission 5 = Vehicle speed and idle control
6 = Computer and output circuit 7 = Transmission You can learn more about codes in
service manuals and also online by entering the trouble code into a search engine.

Warning Lights for Diesels

Workhorse Custom Chassis (WCC) recently advised owners of motorhome chassis of the need to respond to new warning lights related to emissions control. Most of this information applies to all motorhome diesel engines built since Jan. 1, 2007, and/or designed to require ULSD fuel, so owners of other diesel chassis brands should read this too.

WCC notes that there is one set of new warning lights for particulate buildup that it is especially important for those driving new diesel Class A’s to monitor, or risk damaging their engine. WCC has noticed instances of driver failure in this regard. Unfortunately, when this happens there is no warranty coverage for repairs and towing, not to mention the potential hazards. So it pays for drivers to read the manual, pay attention and know what all gauges and warning lights mean.

All diesel engines produced after Jan. 1, 2007, must comply with the new regulations requiring the reduction of nitrogen oxide (NOx) and hydrocarbons (HC) by 50 percent and particulate matter (PM) by 90-plus percent more than the previous 2004 emission standards. To reduce particulate matter, a diesel particulate filter (DPF) is used on all WCC Class A diesel motorhomes and many others.

The DPF captures soot and larger sulfate particles in a series of ceramic honeycomb channels as gas passes through the porous material, and the particulates are trapped and accumulate on the channel walls. After thousands of miles, the DPF will eventually become clogged if nothing is done.

To prevent the DPF from clogging, the trapped particulates are burned off, and the filter is cleaned using a high temperature (around 1,700?F in the particulate filter) regeneration process that leaves a harmless ash and residue. There are different regeneration processes for different diesel platforms, including automatic regeneration, manual regeneration and DPF removal for an exchange or off-vehicle regeneration.

For More Information
Actron, (800) 228-7667
AutoXRay, (800) 228-7667
Caterpillar, (309) 675-1000
Cummins, (800) 343-7357
Detroit Diesel, (313) 592-5000
Dodge Sprinter, (800) 992-1997
Equus Products, (714) 433-0121
Ford Power Stroke Diesel, (800) 392-3673
Harbor Freight Tools, (800) 444-3353
Workhorse Custom Chassis, (877) 294-6773

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