Rick's Free Auto Repair Advice

check engine light on Part 2

How to diagnose a check engine light Part 2

If you read part one of this check engine light on series, you already know that the first step in finding the cause of a check engine light on is to read the trouble codes. But then what? How do you interpret them and avoid replacing good parts?

First, understand that vehicle computers NEVER tell you the root cause of the problem. They only tell you which sensors are providing readings that are outside of expected boundaries. Sure, the sensor could be bad, but it can also be telling the truth and the root problem lies somewhere outside of the sensor itself.

Next, understand how the computers tests the sensor. Here’s an example of a throttle position sensor. This sensor tells the computer how much power you’re requesting by pressing the gas pedal. The throttle cable connects to the TPS and rotates it, just like you would adjust the volume on a radio. The TPS contains a variable resistor. As the wiper moves along the variable resistor, it sends a changing voltage back to the powertrain control module (PCM).

TPS sensor wiring diagram, oxygen sensor

TPS sensor wiring diagram

The PCM sends 5-volts into the top of the variable resistor. That’s called a reference voltage. With the car not moving, that reference returns to the computer to complete the circuit. The computer expects to see a certain return voltage. If it doesn’t see any voltage on the reference signal return line, it’ll set a code indicating like these:
•    P0120 Throttle Position Sensor/Switch A Circuit Malfunction
•    P0121 Throttle Position Sensor/Switch A Circuit Range/Performance Problem
•    P0122 Throttle Position Sensor/Switch A Circuit Low Input
•    P0123 Throttle Position Sensor/Switch A Circuit High Input
•    P0124 Throttle Position Sensor/Switch A Circuit Intermittent
If you get those codes and immediately replace the TPS sensor, you’ll fall into the most common DIY trap—replacing the part mentioned in the code. Again, the TPS may be bad. But the key here is the word CIRCUIT, not Throttle Position Sensor. So your job is to use a digital multimeter and check voltages at the TPS connector.

With the connector unplugged and the key turned to RUN, you should see a reference voltage (usually 5-volts) coming from the PCM. If you don’t, you’ve got a broken wire between the PCM and the connector, or a bad connection at the PCM. If you replace the TPS, you’ve just wasted your time and money. Find the wiring problem. Next, plug the connector back into the TPS and backprobe the connector to find reference signal return voltage. Compare it to the specs in the shop manual. If it’s out of spec, chances are the TPS is bad. However, if you get NO reference signal return voltage, chances are, you guessed it, you’ve got a wiring problem.

Here’s another example, the one that messes up most DIY’ers. An oxygen sensor can’t get an accurate reading of exhaust gas oxygen levels until it heats up to operating temperature. To speed up the heating process, car makers make the sensor with a built in heater. Just like the TPS above, the computer checks the circuitry. Here’s how it works:

P0141, P0135, P0155, P0161

Oxygen Sensor with heater

With the Ignition Switch in the RUN position, power flows from the oxygen sensor circuit fuse into the heater. To make sure the heater actually got power, it checks the returning voltage on the oxygen sensor heater ground. The heater itself will use up most of the voltage, so the return voltage will be very low. The computer expects to see that. But if it sees NO voltage, that means there’s a blown fuse, a wiring problem from the fuse to the oxygen sensor heater, a bad heater, or a wiring problem on the heater ground wire. In that case you’d get one of these codes: If you automatically replace the sensor without first checking the fuse and wiring for power and good ground, you’ll make the common mistake of throwing parts at the problem.
•    P0135 02 Sensor Heater Circuit Malfunction (Bank 1 Sensor 1)
•    P0141 02 Sensor Heater Circuit Malfunction (Bank 1 Sensor 2)
•    P0147 02 Sensor Heater Circuit Malfunction (Bank I Sensor 3)
•    P0155 02 Sensor Heater Circuit Malfunction (Bank 2 Sensor 1)
•    P0161 02 Sensor Heater Circuit Malfunction (Bank 2 Sensor 2)
•    P0167 02 Sensor Heater Circuit Malfunction (Bank 2 Sensor 3)

No here comes the killer scenario. You get one of these codes:
•    P0130 02 Sensor Circuit Malfunction (Bank I Sensor 1)
•    P0131 02 Sensor Circuit Low Voltage (Bank I Sensor I)
•    P0132 02 Sensor Circuit High Voltage (Bank I Sensor 1)
•    P0133 02 Sensor Circuit Slow Response (Bank 1 Sensor 1)
•    P0136 02 Sensor Circuit Malfunction (Bank I Sensor 2)
•    P0137 02 Sensor Circuit Low Voltage (Bank I Sensor 2)
•    P0138 02 Sensor Circuit High Voltage (Bank I Sensor 2)
•    P0139 02 Sensor Circuit Slow Response (Bank 1 Sensor 2)
•    P0140 02 Sensor Circuit No Activity Detected (Bank 1 Sensor 2)
•    P0142 02 Sensor Circuit Malfunction (Bank I Sensor 3)
•    P0143 02 Sensor Circuit Low Voltage (Bank I Sensor 3)
•    P0144 02 Sensor Circuit High Voltage (Bank I Sensor 3)
•    P0146 02 Sensor Circuit No Activity Detected (Bank I Sensor 3)
•    P0150 02 Sensor Circuit Malfunction (Bank 2 Sensor I)
•    P0151 02 Sensor Circuit Low Voltage (Bank 2 Sensor I)
•    P0152 02 Sensor Circuit High Voltage (Bank 2 Sensor 1)
•    P0154 02 Sensor Circuit No Activity Detected (Bank 2 Sensor 1)
•    P0156 02 Sensor Circuit Malfunction (Bank 2 Sensor 2)
•    P0157 02 Sensor Circuit Low Voltage (Bank 2 Sensor 2)
•    P0158 02 Sensor Circuit High Voltage (Bank 2 Sensor 2)
•    P0160 02 Sensor Circuit No Activity Detected (Bank 2 Sensor 2)
•    P0162 02 Sensor Circuit Malfunction (Bank 2 Sensor 3)
•    P0163 02 Sensor Circuit Low Voltage (Bank 2 Sensor 3)
•    P0164 02 Sensor Circuit High Voltage (Bank 2 Sensor 3)
•    P0166 02 Sensor Circuit No Activity Detected (Bank 2 Sensor 3)

Ready to install a new oxygen sensor? Whoa! The codes don’t say the sensor is bad. Once again, the key word is CIRCUIT. It could be a bad sensor. But it can also be a wiring or connector problem. If you don’t check the circuit first, you risk wasting your cold hard cash on an oxygen sensor that might be good.
The point of all this is that trouble codes are meant to point you in the right direction to BEGIN your diagnosis. They’re NOT an end point. If you automatically replace the part mentioned in the code, you’ll waste a LOT of money on perfectly good parts.

© 2015 Rick Muscoplat

Posted on by Rick Muscoplat



Custom Wordpress Website created by Wizzy Wig Web Design, Minneapolis MN