Delivering the Goods - December - 2017

Exhausting Problems!

W hen a transmission job comes into your shop, you probably begin work immediately to get everything prepared to diagnose, repair, and deliver the vehicle in a timely manner. Everyone in the shop knows their job, but sometimes the simplest items can fall through the cracks or go unnoticed.

Such as the exhaust system: If you don’t handle the exhaust system correctly and return it to proper operation, it can cause major driveability issues and set codes that weren’t there before.

Let’s look at some obvious — as well as some not-so-obvious — situations that can occur.


A thorough test drive is important for verifying the customer’s complaint and checking vehicle operation… including the condition of the exhaust system.

Exhaust problems can cause hard, late shifts or early, soft shifts, in addition to poor engine performance and turning the check engine light on. Note any noises, exhaust odors, and check the system visually for condition and recent work performed.


Sound travels better through a solid than air. That’s why a screwdriver works so well for pinpointing noises under the hood. Unfortunately, the exhaust system may have many contact points that can distribute noises and vibrations through the vehicle.

Pay attention to any odd noises during the diagnostic procedure. If you don’t note them, they may end up as customer complaints. And make sure you didn’t create any new noises during your repairs. Here are a few areas you should check:

Brackets and heat shields: Never leave brackets or heat shields off. They’re there for a reason. Missing brackets and improperly secured heat shields are prime causes for noise complaints after a transmission job.

Exhaust leaks: Vehicles in rustbelt areas often develop leaks after you remove and reinstall the exhaust system. Handling rotting exhaust components will often create leaks. Again, assess the exhaust and replace any questionable components, including brackets, gaskets, hangers, and clamps.

Physical damage to the exhaust system: If the vehicle was in an accident, shock can cause internal damage to the catalytic converter, muffler, resonator, or pipes, restricting exhaust flow and creating driveability issues and codes.

Make sure all grommets in the firewall and floorboards are in place: Rustbelt vehicles may see missing grommets or rusted-through areas that can cause undercar noises and leak exhaust into the passenger compartment.

Don’t forget to check the steering column: There’s usually a boot that acts as an isolator. It’s possible to dislodge that boot during transaxle R&R on many front wheel drive vehicles.


Few things are more annoying than when a vehicle passes the final test drive and lift checks, only to have a code appear that wasn’t there before. Improper exhaust installation may be the cause. Look for exhaust leaks, rattling noises, and damaged exhaust components.

Let’s look at some real issues that are easy to misdiagnose:

  1. Lean running and oxygen sensor codes:
    • P0171 — Fuel trim lean bank 1
    • P0174 — Fuel trim lean bank 2

    If you didn’t have a lean code prior to working on the vehicle, more than likely you caused it on the exhaust side of the job. First, look at the oxygen sensor switching in graphing mode (figures 1a and 1b) or on a scope.

    Once the vehicle is warm and the computer in closed loop, the pre-catalytic converter sensors (sensor 1, bank 1; sensor 1, bank 2) should switch frequently at idle. The post-catalytic converter sensors (sensor 2, bank 1; sensor 2, bank 2) should just barely switch, or may not switch at all.

    DIAGNOSTIC TIP: Only the pre-cat sensors affect engine performance. Post-cat sensors monitor catalytic converter efficiency.

    With the oxygen sensors in good working order, check short-term fuel trim data. If the fuel trim numbers are +5 or greater, it could indicate an exhaust leak on that bank, either before or just after the sensor 1 location.

    To check for exhaust leaks:

    • Plug the exhaust pipe with a stopper.
    • Apply 5 PSI of air to the exhaust system.
    • Spray soapy water onto joints, sensors, and other exhaust components.

    Bubbles indicate a leak (figure 2).

  2. Catalytic converter efficiency — P0420Code P0420 indicates that the catalytic converter failed or may be empty. When monitoring oxygen sensor activity, the sensor 2 signal will be similar to the signal from sensor 1 (figure 1). Fuel trim values will be frozen.


Late model diesel vehicles must now comply with more strict emission requirements. New diesel fuel refining standards greatly reduced sulfur and particulates, but the government said that wasn’t enough. So manufacturers added emission control devices similar to those on gas combustion vehicles.

In short, there are two major, active components: a catalytic converter and a particulate filter. Pre- and post-catalytic temperature sensors monitor the element temperatures. Urea — also known as diesel exhaust fluid — is added to the filter element to help trap particulates and convert them to ash (figure 3).

The system includes a soot-purging process called regeneration or regen. The system initializes the regen process during an extended drive cycle where the computer allows the filter bed to reach a specified temperature; then it initializes a process to burn off and expel soot from the filter.

Particulate filters on vehicles that aren’t driven on the highway for at least a half hour or more regularly may become restricted, and may need to return to the dealership for service to restore the filter’s integrity. In extreme cases, low diesel exhaust fluid levels or a restricted particulate filter will reduce power output until the system is serviced.

Other faults with system components may cause driveability issues. Being aware of the system functions will aid in initial diagnosis as well the as pre-delivery test drive. Be very careful removing and installing these components.

Code P1451 — Diesel particulate filter system performance

Never erase this code! This code won’t turn the check engine light on, but will light a warning light in the notification center. This is one instance where erasing codes during the verification procedure could cause a problem later.

With the 2014-and-later Dodge diesel applications, code P1451 sets when the particulate filter nears 100% capacity. The code alerts the driver and service technician to initialize a regen process as soon as possible.

The WiTech (OEM) scan tool needs to see this code active to initialize a manual desoot regen process. Erasing this code could keep the system in low power mode and require a more expensive visit to the dealer to restore the particulate filter’s operation.


There are a few issues related to the exhaust system that can cause problems after installation, but may not set a code or check engine light. In most cases, this will initialize low power mode until you correct the condition.

Exhaust backpressure, turbo wastegate, and turbo pressure sensors are a few components to check first. Make sure you didn’t damage these sensors or leave them unplugged. On some models, faulty oxygen sensors may also cause low power problems.

Initializing exhaust components is becoming more popular with late model vehicles, such as the Ford variable geometry turbochargers.

Also, when replacing some late-model GM-application oxygen sensors, you’ll need to perform a relearn process. Skipping this step will cause a new sensor to fail. Always check the engine side of your scan data to search for component reset and initialization.

Refer to information available through resources such as Alldata, Shopkey, Identifix, Mitchell OnDemand, or similar databases to indicate items that require a factory scan tool for reset or initialization.

Attention to detail, from diagnosis to delivery, is key to making sure the exhaust system won’t create an unexpected issue, and exhaust problems in your future!


One important thing to remember is that the exhaust system has some serious legal requirements. And they can affect you, even if you didn’t change anything.

First, the exhaust must be comparable to the original. Any changes, such as a damaged air injection pipe or a missing catalytic converter, must be repaired if you touch the exhaust. And there’s no point asking the customer to sign a waiver: If you touch it, it’s your responsibility.

Second, if you install the cat, it must have been tested. So, if the customer tells you the cat he removed is in the trunk, that’s fine… as long as you can find someone to test it before reinstalling it. If not, you’ll need to replace it with a new or remanufactured and tested cat.

While the EPA probably doesn’t have the resources to go checking every exhaust system you touch, you need to be aware that you are risking stiff fines for failure to abide by the regulations. So be careful, and make sure that, if you touch it, it goes back in factory condition.