Click for Coverage
Due to the EU’s Global Data Protection Regulation, our website is currently unavailable to visitors from most European countries. We apologize for this inconvenience and encourage you to visit for the latest on new cars, car reviews and news, concept cars and auto show coverage, awards and much more.MOTORTREND.COM
  • JP Magazine
  • Dirt Sports + Off-Road
  • 4-Wheel & Off-Road
  • Four Wheeler

Cat Wrangling - DIY Smog Troubleshooting

Posted in How To on September 9, 2013
Share this

Emissions compliance can make a cheap truck either not so cheap or utterly worthless (for street use). Our Cheap Truck Challenge Cherokee is a prime example (Oct. ’13). In California, the seller is responsible for smogging the vehicle prior to a title change, but in reality, emission compliance can be a negotiating point. The low price of our Cheap Cherokee didn’t include smog certification, but the registration was still valid when we bought and ran it.

After the event when the tags became due, the XJ failed its smog test. While it emitted acceptable levels of hydrocarbons (HC) and carbon monoxide (CO), the oxides of nitrogen (NOx) was 649 part per million, just barely exceeding the maximum allowable, 613. Luckily the Check Engine light wasn’t on and no trouble codes were stored on the PCM.

Short story long: We took the failed emissions test printout to a local Jeeper who is also an emissions technician. He helped us decipher the results and pinpoint how we could pass as cost-effectively as possible.

Our expert diagnosis was a bad catalytic converter based on the NOx levels, possibly caused by a defective O2 sensor or age. In California, replacement converters must have a part number approved by the California Air Resources Bureau (CARB), so options are basically limited to an OE replacement—not “high-flow” aftermarket models. Visual and aural inspection revealed that the existing muffler was also tired but the tailpipe was OK.

While we were shopping online, ( emerged as an affordable one-stop source for replacement exhaust parts, both California-compliant and for the other 49 states. The website also has information about how other states are adopting California’s emissions requirements, particularly for ’09 and newer passenger vehicles. It also links to emissions info from Walker, a leading OE supplier of exhaust systems. Walker emphasizes that cats are designed to work with properly tuned engines in vehicles with no exhaust leaks.

Emmissions Chemistry
Here are some factors that elevate levels of the various tailpipe gases.

HC: Unburned fuel, dirty air filter, weak ignition spark, defective O2 sensor, bad fuel.

CO: dirty air filter or a faulty sensor (O2, MAP, TPS, ECT).

NOx: exhaust leak; defective O2 sensor, catalytic converter, or EGR valve; clogged cooling passages; carbon deposits in heads; ignition timing off; lean air-fuel mixture.

Replacing the catalytic converter on our Cheap Cherokee fully corrected failed tailpipe NOx emissions—none registered on the retest. It also dropped the HC from 197 ppm to 47 and the CO from 0.51% to 0.04%. California cat replacement isn’t cheap, but the low purchase price for the Cheap Cherokee helped compensate for the parts’ cost. The Jeep is now a clean machine—emissionswise, at least.

The manufacturer’s defective cat emissions baselines at 2,000 rpm:
HC: 125-380 ppm
CO: >0.3%
O2: >0.4%

How a Cat Works
Geek time: Catalytic converters use exhaust heat to chemically bond oxygen gas (O2) to toxic gases (CO, HC, and NOx), producing less-toxic water (H2O), carbon dioxide (CO2), and nitrogen (N2). Engine exhaust passes through the cat’s honeycomb substrate, which is comprised of specially coated precious metals: notably platinum, rhodium, and palladium. California-spec cats allegedly have a higher precious metal content than “49-state” units. This and the cost of CARB certification make them significantly more expensive. As a result, cat theft is a cottage industry in the Golden State, which gives a whole new meaning to the term “crazy cat lady.”

Converters are designed to start their chemical reactions at about 350 degrees F and are considered “fully lit” at 500 degrees. Normal operating temperature is less than 1,200 degrees. Temperatures exceeding 1,600 degrees can melt the matting that bonds the cat’s substrate to its shell. A marbles-in-a-coffee-can rattle results.

Bronze/blue discoloration on the shell is another indicator of an overheated cat. The cat can also be compromised if the honeycomb gets coated by coolant (e.g., from a blown head gasket) or oil or other chemicals that blow by the piston rings.

PhotosView Slideshow


Madison, WI

Connect With Us

Newsletter Sign Up

Subscribe to the Magazine

Browse Articles By Vehicle

See Results