By Larry Carley
Technical Editor

A Check Engine Light that’s illuminated is always a sign that something is wrong. But what? You usually can’t tell until you plug in a code reader or scan tool and read out the code(s) that have turned on the light. At least then you have a starting point for further diagnosis.

Some people think a code is all you need to fix a fault and turn off the Check Engine Light (or Malfunction Indicator Lamp or MIL). A diagnostic trouble code (DTC) certainly helps, but it only tells you that some kind of fault has occurred, and maybe the circuit or component associated with the fault. But no code by itself can tell you which part to replace, because in many instances any one of several causes may have set the code. Keep that in mind as you read about the codes below and the fixes that may solve these faults.

P0300 Random Misfire Code (Ford, GM or Chrysler)
Here’s a code that tells you the engine is misfiring, but it won’t tell you why. The cause could be fuel, ignition or compression, so to diagnose this fault there are number of things you have to check.

A random misfire code means multiple cylinders are misfiring, and in many cases the underlying cause is a vacuum leak. But as we just said, it might also be due to low fuel pressure, dirty fuel injectors, worn or fouled spark plugs, bad plug wires or a weak spark (low coil voltage).

The first thing you do with a P0300 code is to look and listen for possible vacuum leaks while the engine is idling. If you hear any high-pitched whistling noises, the engine may be sucking air though a leaky hose or intake manifold gasket. Plastic intake manifolds with upper and lower plenums are notorious for developing leaks. You can spray carb cleaner or propane at suspected leak points while the engine is running to see if it changes the idle. Or, you can shut off the engine and use a smoke machine to check the intake system for leaks. You should also inspect all the vacuum hose connections, especially the PCV and EGR plumbing, and the large vacuum hose to the brake booster.

No vacuum leaks? It’s possible the EGR valve is not closing due to a buildup of carbon around the base of the valve. It may be necessary to remove, inspect and clean the EGR valve.

You can also plug a scan tool into the data link connector (DLC) and look at fuel trim. Normal fuel trim should usually be plus or minus 8. If fuel trim is running 10 or above, the engine is lean. Rev the engine and hold it at 1,500 to 2,000 rpm and look at fuel trim again. If the value has come back down into a more normal range, it confirms the engine has a vacuum leak because a vacuum leak has more of an impact at idle than at higher speeds. If there is little or no change in fuel trim, and the engine is running lean, the problem is low fuel pressure (weak fuel pump, plugged fuel filter or a leaky fuel pressure regulator) or restricted fuel delivery (dirty fuel injectors).

If fuel is not the issue (no vacuum leaks, fuel trim is in normal range), you should probably remove and inspect the spark plugs. Replace the plugs if they are worn or fouled. Also, you need to inspect the spark plug wires. Replace the wires if they are cracked, burned, have loose boots or terminals, or resistance end-to-end exceeds specifications.

If the plugs and wires are OK, use a scope or a spark plug tester to check the intensity of the spark. Got a weak spark? Check the primary voltage to the coil(s), and check coil primary and secondary resistance with an ohmmeter. If coil resistance is out of specifications, replace the coil(s).

If fuel and spark are both OK, that leaves low compression as the cause of the misfire. The underlying problem might be weak valve springs, incorrect valve lash (too tight or too loose), or maybe even incorrect valve timing due to a slipped timing belt.

Chrysler TSB 09-002-03 describes a random misfire code that can be set by weak valve springs on 1998-2004 300, Concorde, Intrepid and LHS models with 3.2 or 3.5L V6 engines, and 1999 to 2002 Plymouth Prowler. The fix, says Chrysler, is to replace the exhaust valve springs.

Ford P0171 & P0174 Lean Codes
These are very common codes on Fords that indicate something is causing the engine to run lean. There are several possible causes of these codes.

One of the most common causes is a dirty mass airflow (MAF) sensor. The MAF sensor is located in the air inlet tube just ahead of the throttle body.

The MAF sensor should be protected from outside dust and debris by the air filter, but sometimes the air filter doesn’t fit real tight inside the housing and allows unfiltered air into the engine. Dirt can stick to the MAF sensor wire and form a coating that slows the response of the sensor to changes in airflow. The MAF sensor can also be contaminated by fuel vapors that back up through the intake manifold and throttle body when the engine is shut off.

The vapors can leave a waxy coating on the sensor wire. This causes the MAF sensor to under report airflow, which in turn misleads the powertrain control module (PCM) so it doesn’t add enough fuel to maintain a properly balanced air/fuel ratio. As a result, the engine runs lean and sets a P0171 and/or P0174 code (see Ford TSB 98-23-10 for details).

If the MAF sensor is dirty, the fix is easy enough: just clean or replace it. In many instances, the MAF sensor can be successfully cleaned by spraying the sensor element with electronics cleaner. Do not use any other type of cleaner as this may damage the sensor. If the lean codes keep coming back, the MAF sensor may have to be replaced if any of the following is not setting the codes.

Another common cause of the lean codes is an engine vacuum leak. Ford TSB 04-17-4 details procedures for checking fuel trim and looking for vacuum leaks.

On 3.8L Fords with a split-plenum intake manifold, the port gaskets and isolator bolt assemblies for the upper plenum can deteriorate over time and leak air, often as a result of oil being sucked into the intake manifold through the PCV system. Also the vacuum hose that connects the fuel pressure regulator to the intake manifold can swell and leak vacuum where the hose connects to the manifold. Ford TSB 03-16-1 says the fix involves several steps: remove the upper manifold plenum and replace the original gaskets and bolts with revised ones, replace the front valve cover with a revised valve cover that reduces the amount of oil vapor sucked into the PCV system, inspect and replace the fuel pressure regulator hose, and finally, reflash the PCM so it is less sensitive to lean fuel conditions.

The lean codes can also be set by a bad EGR differential pressure sensor (see next code).

Ford P0401 Insufficient EGR Flow
This code is set when the onboard diagnostic system sees less EGR flow than the PCM commands under certain operating conditions. EGR flow on many Fords is monitored by an EGR differential pressure sensor (DPE or DPFE sensor). The sensor is mounted on the engine, and is attached with two rubber hoses to the tube that routes exhaust gas to the EGR valve. The original equipment sensor has a rectangular aluminum housing about three inches long. Corrosion inside the sensor reduces its sensitivity to EGR flow, causing it to under-report EGR flow. The PCM responds by increasing EGR flow, which may keep the EGR valve open longer than usual creating a lean condition in the engine. Thus, a bad sensor may not set a P0401 code, but instead set a P0171 or P0174 code.

The cause of the P0401 code in most cases turns out to be a bad DPFE sensor, not an EGR valve problem or a plugged up EGR port. An aftermarket replacement DPFE sensor costs less than $50 and usually gets rid of not only the P0401 code, but also the P0171 and P0174 codes, too.

Chrysler P1391 Intermittent Loss of Crank or Cam Sensor Signal
This code can pop up if the PCM loses an input signal from the crankshaft (CKP) or camshaft (CMP) position sensors when the engine is running or cranking — but only if it caches 20 failures in two consecutive trips. The cause may be a bad sensor, a loose or corroded sensor connector, a defect in the tone wheel or flexplate, or a fault in the PCM itself.

Inspect the wires for any obvious problems, then connect a scan tool and look for an rpm signal, and/or the CKP and CMP PIDS. Or, you can use a scope to backprobe the sensors to look for a good output signal. If you are not getting a good sensor signal, remove the sensor and inspect the tone ring underneath it for debris or damage. If the sensors are producing good signals, the fault is in the wiring or PCM.

Chrysler P1494 Evap Pump
If the evaporative emission system leak detection pump (LDP) on a Chrysler fails, or the LDP switch solenoid fails to change position when commanded to do so by the PCM, it will set a P1494 code. The pump, which is located next to the gas tank, is checked immediately after a cold start when the outside temperature is between 40° and 90° F. Possible causes of this code include a leak or obstruction in the LDP vacuum supply, a bad LDP pump, or a fault in the LDP switch circuit. The most common cause is a bad switch connector.

GM P1133, P1139 & P1153 O2 Sensor Codes
All three of these codes signify an “insufficient switching condition” of the oxygen (O2) sensor. In most cases, the cause is a bad O2 sensor. But don’t replace the sensor until you have inspected the O2 sensor wiring and connector.

GM P1345 Cam/Crank Sensor Correlation Fault
This code may be set if the camshaft position sensor is not properly synchronized with the crankshaft position sensor. The cause may be damaged wiring, or loose or corroded connectors at either sensor. A sensor tester or multimeter can be used to verify that a proper signal is reaching the PCM. If the distributor has been removed and not reset or reinstalled properly, use a scan tool to monitor the “Cam Counts” and set them as close to zero as possible by adjusting or repositioning the distributor.