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Tech Tip: 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

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. The OBDII 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 details. 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 include:

Checking if the MIL is illuminated.

Inspecting the fuel filler cap to make sure it is the correct type and seals tightly.

Looking underneath to determine if the catalytic converter (if required) is there.

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 wear.

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 Monoxide

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

After 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

AutoXray EZ-Scan 4000
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.

Eqquus Can OBD2&1
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).

Actron OBD II AutoScanner
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.

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