QuestionThe check engine light is on in our 1997 Chrysler Sebring 2.5L coupe. The code being pulled is 174. Any ideas?
AnswerHi Kim,
This code says the front bank of cylinders is operating with too lean a fuel/air mixture for meeting pollution standards.
The first suspect is that the device that measures the mixture, the upstream oxygen sensor mounted on the exhaust manifold of the bank that is closer to the front of the vehicle, is no longer operating accurately. But before changing that out:
Did you run low or out of fuel recently as that can cause this code temporarily? If so try disconnecting the battery for a minute which will erase the code and see if it returns.
Do you notice any leaks in the exhaust system (smell or noise)? Repair those first, then erase code similarly.
There are a number of tests of the various parts of the engine that can be done, but I would believe that investing in a new oxygen sensor and installing it yourself would be more economical as a starting strategy. It changes out just like a spark plug but you need to handle the wires at its electrical connection carefully. Use the anti-locking thread compound when installing, and to remove the old one put some penetrating oil on the threads where it screwn into the manifold and let it sit overnight. It may also help to warm the engine before trying to remove it, use a proper sized box-end wrench, and rock it back and forth to loosen up the rust on the thread gradually.
If that doesn't permanently kill the light, then the following checks are appropriate:
cam or valve timing (in case the timing belt has slipped), ignition coil, fuel filter, spark wires/spark plugs, injector stuck closed, manifold pressure sensor, fuel pressure regulator, fuel pump. Sorry to report such a non-specific list, but most of those can be evaluated before throwing money at parts non-specifically. However the cost of such testing will exceed by far the replacement of the oxygen sensor. A competent mechanic with a diagnostic test tool can probably do some measurements to identify the specific parts that is causing the fault.
Roland
PS: I just notice another question I answered 2 years ago about the 0171 code which pertains to the rear bank of cylinders. It gives more specifics about the various tests, but doesn't really change my overall advice:
One interesting aspect is that the code is specific for the rear cylinder bank and is based upon its upstream O2 sensor not being happy with the mixture. On the coupe there are a pair of sensors on each bank whereas on the convertible there is only sensors on rear bank (ref. 2.5L 1998 powertrain manual). The list of possible causes is legion:
ignition coil, powertrain control module, engine parts tolerance, puel pump, fuel pump inlet filter, ignition secondary wires, low fuel level, injectors stuck closed, MAP sensor, O2 sensor (upstream), fuel pressure regulator, fuel pump relay, spark plugs, wiring harness connectors.
Most of these are not unilaterally inclined so I wonder if one should look at only those that affect just the rear bank or look at all of them?
Interestingly, if you then look at the decision tree it says to take these steps in order:
Check fuel pressure, make sure fuel level is at least 1/4 full (did the car run out of fuel when the code was logged), check out other fault codes first, with engine warmed check for leaks between engine and O2 sensor, check if the 02 sensor is switching between lean and center and rich at least twice in 20 seconds (that is normal), if you have a DRB see what is the long term adaptive memory % value.
Then check secondary ignition with a scope, spray wires and observe scope, disconnect plugs one at a time observe scope, is secondary voltage at least 25kv.
Then check the coolant temp sensor (is it 7 to 14K ohms at 70F and 700 to 1k at 200F).
Then check the MAP sensor and see if it changes
signal and is accurately measuring the vacuum (tee in a vacuum gauge).
Then do typical check for engine mechanical:
read vacuum at idle, valve timing, engine compression, camshaft lobes, check crankshaft sensor slots for debris/deterioration, check exhaust system for restrictions, check PCV system, check drive sprocket timing, check torque converter stall speed, check for power brake vacuum leaks, no contamination of fuel, check for plugged or restricted injector or crossed wires thereto, check the resistance vs temp of intake air temp sensor (spec similar to that of coolant sensor).
I wish it were more specific, but that is how it is treated in the '98 manual for that 2.5L engine. I would be interested to know if you find the correct cause of the code."
